Digestive system of man

Class 12 Biology, unit 1, book pages 7–26

Practise 62 MCQs Take the unit quiz Review 60 cards

The digestive system breaks food into small molecules that the body can absorb and use. This unit follows food from the oral cavity through swallowing, the esophagus, stomach and small intestine, to absorption in the villi and the large intestine, and then covers the liver, bile and pancreas and the hormones that control their secretions. FBISE asks many draw-and-label and explain questions from it.

Learning outcomes (12)

Describe the mechanical and chemical digestion in the oral cavity.
Explain swallowing and peristalsis.
Illustrate with a diagram the structure of the stomach and relate each component with mechanical and chemical digestion in the stomach.
Identify the role of the nervous system and gastrin hormone on the secretion of gastric juice.
Describe major actions carried out on food in the three regions of small intestine.
Trace the absorption of digested products from the small intestine lumen to the blood capillaries and lacteals of the villi.
Describe the component parts of the large intestine with their respective roles.
Correlate the involuntary reflex for egestion in infants and the voluntary control in adults.
Explain the storage and metabolic role of liver.
Describe the composition of bile and relate the constituents with respective roles.
Outline the structure of pancreas and explain its function as an exocrine gland.
Relate secretion of bile and pancreatic juice with the secretin of hormone.

The digestive system and the gastrointestinal tract

Our body needs nutrients from the food we eat to stay healthy and function properly. Nutrients include carbohydrates, proteins, fats, vitamins, minerals and water. The digestive system breaks food down and absorbs the nutrients, which the body uses for energy, growth and repairing cells.

Anatomically and functionally the digestive system is divided into a tubular gastrointestinal tract (GIT) and the accessory digestive organs.

Part	Organs
Gastrointestinal (GI) tract	Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine
Accessory digestive organs	Teeth, tongue, salivary glands, liver, gall bladder, pancreas

The GIT is a continuous tube from the mouth to the anus, about 9 m (30 ft) long. Each region is specialised for a different role. It passes through the thoracic cavity and enters the abdominal cavity at the level of the diaphragm.

Figure 1.4: Human digestive system.

Extra reading material: layers of the digestive tube

The digestive tube has four major layers, present in all areas from the esophagus to the anus: an internal mucosa and an external serosa, with the submucosa and the muscularis in between.

Figure 1.1: Histological layers of the digestive tract (cross-section).

Layer (outside to inside)	Parts shown in the figure
Serosa	Connective tissue layer and peritoneum
Muscularis	Longitudinal muscle layer (outer) and circular muscle layer (inner); myenteric plexus lies between them
Submucosa	Gland in submucosa with its duct; submucosal plexus
Mucosa	Mucous epithelium, lamina propria and muscularis mucosae; lymph nodule

1.1Oral cavityOften examined

The oral cavity (buccal cavity) is surrounded by the lips and cheeks, the tongue and the palate, and includes the space between the palate and the tongue. The tongue has rough projections called papillae that cause friction, which helps in handling the food. The papillae also contain taste buds. The palate forms the roof of the oral cavity. Different teeth are adapted to handle food in different ways. There are three pairs of salivary glands, which secrete saliva containing enzymes. Mechanical and chemical digestion both begin in the oral cavity.

Figure 1.2: Human salivary glands (the three pairs).

Figure 1.3: Location of human taste buds on the tongue.

Mechanical digestion

Mechanical digestion: The physical division of a mass of food into smaller pieces.
Mastication: Chewing of food by the teeth, which breaks large food particles into smaller ones.

Chewing breaks food into small pieces that have a much larger surface area for the action of digestive enzymes. It also breaks the cellulose covering of plant food, which makes digestion more efficient.

Chemical digestion

Chemical digestion in the oral cavity is minor. Two enzymes are secreted: salivary amylase and lingual lipase.

Enzyme	Source	Action
Salivary amylase (ptyalin)	Watery part of saliva (chemically identical to pancreatic amylase)	Breaks the covalent bonds between glucose molecules in starch and other polysaccharides, giving maltose, maltotriose and isomaltose
Lingual lipase	Saliva	Hydrolyses triglyceride ester bonds to form diacylglycerols and monoacylglycerols

Maltotriose is a trisaccharide. Isomaltose is a disaccharide similar to maltose, but it is not joined by the $α$ -(1-4) linkage of maltose. As a result of chemical and mechanical digestion an oval mass called a bolus is formed, ready to be swallowed.

1.2SwallowingOften examined

The act of swallowing involves a set of reflexes. It is divided into three phases.

Figure 1.5: Process of swallowing. UES is the upper esophageal sphincter.

The three phases of swallowing

Phase 1, oral phase. This is the only voluntary part of swallowing. Food is moistened with saliva and the food bolus is formed. The tongue pushes the bolus to the back of the throat. It starts with lip closure.
voluntary
Phase 2, pharyngeal phase. The pharynx is the cavity behind the mouth. This phase starts with stimulation of tactile receptors in the pharynx, and the swallow reflexes are initiated.
reflex
The soft palate lifts to close the nasal airways. The bolus moves over the back of the tongue, and the tongue blocks the oral cavity so that food does not go back into the mouth.
The epiglottis is pushed backward over the larynx. The larynx and vocal folds contract and cover the entry of the trachea to protect the airways, and respiration is temporarily arrested.
The upper esophageal sphincter opens to allow the bolus to pass into the esophagus.
Phase 3, esophageal phase. The bolus is propelled down the esophagus by peristalsis. The larynx moves down to its original position.
involuntary

Phase	Control	Main events
1. Oral	Voluntary	Bolus formed with saliva; tongue pushes it to the back of the throat; lips closed
2. Pharyngeal	Reflex (involuntary)	Soft palate closes nasal passage; epiglottis covers larynx; breathing stops briefly; upper esophageal sphincter opens
3. Esophageal	Involuntary	Peristalsis carries the bolus down; larynx returns to its original position

1.2PeristalsisOften examined

Peristalsis: A wave-like muscular contraction that propels food and fluids through the digestive tract and other tubular organs. It is an involuntary, rhythmic process that moves contents in an anterograde (forward) direction, starting from the esophagus and continuing through the intestines.

Functions of peristalsis

Propulsion: pushes food and fluids through the digestive system, enabling digestion and absorption.
Mixing: in the stomach and small intestine it mixes food with digestive juices and exposes it to the absorptive surfaces.
Waste removal: in the large intestine it drives waste products towards the rectum for elimination.
Other systems: it also occurs in the urinary tract (urine from kidneys to bladder) and in bile ducts (bile from the gall bladder to the duodenum).

Mechanism of peristalsis

Muscle contractions: contraction of the circular and longitudinal muscles in the walls of the digestive tract and other tubes.
Wave-like movement: the contractions create a wave-like motion that pushes the contents along.
Involuntary: controlled by the nervous system, particularly the myenteric plexus in the digestive tract.

Figure 1.6: Peristalsis. (1) A wave of smooth muscle relaxation moves ahead of the bolus, allowing the digestive tract to expand. (2) A wave of contraction of the smooth muscle behind the bolus propels it through the digestive tract.

Steps of one peristaltic wave

A wave of relaxation of the smooth muscle moves ahead of the bolus, so the tube widens and makes room.
A wave of contraction of the smooth muscle behind the bolus squeezes the tube and pushes the bolus forward.
The wave keeps moving along the tube, so the bolus moves on in one direction (anterograde).

Examples of peristalsis

Swallowing: muscles of the esophagus contract in a peristaltic wave to move the bolus to the stomach.
Digestion: in the small intestine peristalsis moves the digested food (chyme) along so nutrients can be absorbed.
Waste removal: in the large intestine peristaltic waves push waste towards the rectum, where it is eliminated as stool.

Worked example: peristalsis and mixing waves

Differentiate between peristalsis and the mixing waves of the stomach in three points.

Point	Peristalsis	Mixing (churning) waves
Where	Esophagus, intestines and other tubes; in the stomach the stronger waves near the pylorus	Smooth muscle of the stomach wall
Effect	Pushes contents forward along the tract	Mixes boluses with gastric juice to form chyme
Strength and frequency	In the stomach, peristaltic waves are less frequent but much more powerful	More frequent and weaker

1.3Esophagus

The esophagus (oesophagus) lies in the centre of the chest in the mediastinum, behind the trachea and in front of the vertebral column. It is about 25 cm long.

At the top there is a ring-shaped muscle, the upper esophageal sphincter (UES). It senses food or liquid coming, relaxes to let it enter the esophagus, and stays closed when there is none.
Peristalsis pushes the food down. The esophagus passes through the diaphragm and joins the stomach.
At the lower end the lower esophageal sphincter (LES) relaxes to let food into the stomach. When there is no stimulus it stays shut to prevent stomach acid and digestive juices from getting back into the esophagus.

Figure 1.7: Esophagus.

1.3Structure of the stomachOften examined

Divisions of the stomach

The stomach has four main anatomical divisions: the cardia, fundus, body and pylorus.

Cardia: Surrounds the superior opening of the stomach, which connects it with the esophagus.
Fundus: The rounded, often gas-filled portion superior to and to the left of the cardia.
Body: The large central portion inferior to the fundus.
Pylorus: Connects the stomach to the duodenum. It is divided into the pyloric antrum, pyloric canal and pyloric sphincter.
Pyloric antrum: The lower or distal portion above the duodenum. A sphincter that regulates the passage of chyme into the duodenum is the pyloric sphincter.

Figure 1.8: Division of the stomach.

Greater and lesser curvatures

The medial and lateral borders of the stomach are curved. They form the lesser curvature (the short inner border) and the greater curvature (the long outer border).

Worked example: identify the parts A to I

Name the parts A to I of the stomach shown in the figure and say which one regulates the passage of chyme into the duodenum.

Parts A to I of the stomach.

Letter	Part
A	Esophagus
B	Cardia
C	Fundus
D	Body
E	Lesser curvature
F	Greater curvature
G	Pyloric antrum
H	Pylorus (with the pyloric sphincter)
I	Duodenum

The pyloric sphincter (H) regulates the passage of chyme into the duodenum.

Figure 1.9: Cutaway section of the stomach reveals the muscular layers and internal anatomy.

Layers of the stomach wall

There are four distinct layers in the stomach wall. From outside to inside:

Serosa: the outermost layer.
Muscularis: three layers of smooth muscle, an outer longitudinal layer, a middle circular layer and an inner oblique layer.
Submucosa.
Mucosa: lined with simple columnar epithelium. It forms numerous tube-like gastric pits, which are the openings of the gastric glands.

Figure 1.10: A section of the stomach wall showing its histology, including several gastric pits and glands.

Epithelial cell	Where	Secretes
Surface mucous cell	Surface, lining the gastric pit	Mucus
Parietal (oxyntic) cell	Gastric gland	Hydrochloric acid and intrinsic factor
Chief (zymogenic) cell	Gastric gland	Pepsinogen
Endocrine cell	Gastric gland	The hormone gastrin into the blood

1.3Functions of stomachOften examined

Digestion in the stomach is of two types: mechanical and chemical.

Mechanical digestion

The mixing action of the stomach walls produces mixing waves, contractions of the smooth muscle. This is more efficient because, unlike other regions of the alimentary canal, the stomach has three layers of smooth muscle. The churning action mixes boluses of food with gastric juice and produces a thick liquid called chyme.

Chemical digestion

Stomach secretions include mucus, hydrochloric acid, gastrin, intrinsic factor and pepsinogen.

Secretion	Cell	Function
Viscous, alkaline mucus	Mucous cells	Forms a thick layer that lubricates and protects the epithelial cells of the stomach wall from the acidic chyme and pepsin
Hydrochloric acid (HCl)	Parietal cells	Gives a low pH (normally 1 to 3, usually close to 2); kills bacteria that enter with food; stops carbohydrate digestion by inactivating salivary amylase; denatures many proteins so proteolytic enzymes reach internal peptide bonds; provides the right pH for pepsin
Intrinsic factor	Parietal cells	A glycoprotein that binds vitamin B12 and makes it ready for absorption in the ileum
Pepsinogen	Chief cells	Inactive enzyme; packaged in zymogen granules and released by exocytosis; converted to pepsin by HCl and by pepsin already formed
Pepsin	Formed in the lumen	Optimum pH 2; cleaves some covalent bonds in proteins, breaking them into smaller peptide chains
Gastrin	Endocrine cells	Hormone released into the blood; stimulates more gastric juice (see next section)

Cells of a gastric gland and what each secretes. Accent arrows show secretions entering the lumen of the stomach.

Activation of pepsin

Chief cells release inactive pepsinogen by exocytosis into the lumen of the stomach.
HCl (and pepsin already present) converts pepsinogen into active pepsin.
Pepsin, working best at pH 2, breaks proteins into smaller peptide chains.
Active pepsin converts more pepsinogen to pepsin, so the supply increases quickly.

Activation of pepsin. Pepsin (accent) also converts more pepsinogen, so the supply of active enzyme increases quickly.

Worked example: what if no HCl were secreted?

Explain how the absence of hydrochloric acid would affect digestion in the stomach.

Pepsinogen would not be converted into active pepsin, because HCl activates it.
The pH would not fall to about 2, which is the optimum for pepsin, so protein digestion in the stomach would be very slow.
Proteins would not be denatured, so pepsin could not easily reach internal peptide bonds.
Salivary amylase would not be inactivated and bacteria taken with food would not be killed.

1.4Role of the nervous system and gastrin hormone on the secretion of gastric juiceOften examined

About 2 to 3 litres of gastric juice are produced each day. Both nervous and hormonal mechanisms regulate gastric secretion. Hormones that regulate stomach secretions include gastrin, secretin, gastric inhibitory polypeptide and cholecystokinin (CCK).

Nervous control

The taste and smell of food, stimulation of tactile receptors during chewing and swallowing, and pleasant thoughts of food stimulate centres in the medulla.
Neuronal stimulation of the stomach mucosa results in the secretion of acetylcholine.
Acetylcholine stimulates the parietal and chief cells and also the secretion of gastrin from the endocrine cells.

Hormonal control by gastrin

Gastrin is released into the circulation and travels to the parietal cells, where it stimulates additional gastric juice secretion.

Control of gastric juice secretion by the nervous system and gastrin.

Presence of food

The greatest volume of gastric secretion is started by food in the stomach. The main stimuli are distension of the stomach and the presence of amino acids and peptides. Peristaltic waves occur less often than mixing waves but are much more powerful and force the chyme near the periphery of the stomach towards the pyloric sphincter. The sphincter usually stays partly closed because of mild tonic contraction, so each peristaltic contraction forces only a small amount of chyme through the pyloric opening into the duodenum.

1.5Small intestine: duodenumOften examined

The small intestine is the part of the digestive system between the stomach and the large intestine. It is the longest part of the GI tract and the place where most digestion takes place. It is about 6 m long and has three parts: the duodenum, the jejunum and the ileum.

Figure 1.11: Small intestine.

Duodenum

The duodenum is the first part of the small intestine, 20 to 25 cm (8 to 10 inches) long. It starts at the pyloric sphincter, is short and C-shaped, and surrounds the head of the pancreas.

Pancreatic juice

The secretion of the pancreas is pancreatic juice. It is poured through the pancreatic duct. It is slightly alkaline (pH about 8) and neutralises the acid chyme from the stomach. Its important enzymes are pancreatic amylase, pancreatic lipase, trypsinogen and chymotrypsinogen.

Enzyme	Action
Pancreatic amylase	The starch-digesting enzyme; hydrolyses polysaccharides to maltose and even to glucose
Pancreatic lipase	The principal enzyme for hydrolysis of fats; breaks fats into mono- and diglycerides, glycerol and fatty acids

Enzyme precursors

Two important enzyme precursors in pancreatic juice are trypsinogen and chymotrypsinogen. Both are inactive forms.

Activation of the protein-digesting enzymes

The intestinal glands secrete an activator enzyme, enterokinase.
Enterokinase converts trypsinogen into trypsin.
Trypsin then activates more trypsinogen, and converts chymotrypsinogen into active chymotrypsin.
Trypsin acts on proteins and converts them into polypeptides.

Activation of the protein-digesting enzymes in the duodenum. Enterokinase from the intestinal glands starts the chain; trypsin (accent) keeps it going.

Bile

Bile is made in the liver but stored in the gall bladder. It emulsifies fat, breaking it into numerous small droplets called emulsion. Emulsification gives the lipase enzyme a relatively large surface area of lipid to act on and so speeds up the digestion of fats and oils.

Worked example: why enzymes are secreted inactive

Give a reason why trypsinogen and chymotrypsinogen are secreted in inactive form, and describe how they are activated.

Trypsin and chymotrypsin digest proteins. If they were made active inside the pancreas they would digest the pancreatic tissue itself.
So they are made as inactive precursors and become active only in the duodenum.
Enterokinase from the intestinal glands converts trypsinogen into trypsin.
Trypsin then activates more trypsinogen and also converts chymotrypsinogen into chymotrypsin.

1.5Jejunum and ileum

The jejunum is about 2.5 m long and the ileum about 3.5 m long. Here the digestion of proteins, carbohydrates and fats is completed. The lining of the jejunum and ileum secretes several enzymes.

Enzyme	Action
Aminopeptidase	Splits polypeptides into dipeptides
Erepsin	Splits peptides into amino acids
Lactase	Converts lactose into glucose and galactose
Maltase	Converts maltose into glucose
Sucrase	Converts sucrose into glucose and fructose

Pancreatic lipase completes the digestion of fats into fatty acids and glycerol. By the action of the enzymes, chyme is turned into a watery emulsion called chyle.

Part	Length	Main work
Duodenum	20 to 25 cm	Receives pancreatic juice, bile and enterokinase; neutralises acid chyme; starts fat emulsification
Jejunum	About 2.5 m	Digestion of proteins, carbohydrates and fats is completed
Ileum	About 3.5 m	Digestion completed; the major site of nutrient absorption

1.6Absorption of digestion productsOften examined

The ileum is the major site of nutrient absorption. Tiny finger-like projections of the mucosa form numerous villi, each 0.5 to 1.5 mm long. Each villus is covered by simple columnar epithelium and contains a blood capillary network and a lymph capillary called a lacteal. Villi and microvilli further increase the surface area for absorption, which makes the small intestine the largest part of the alimentary canal in surface. The internal walls are also folded.

To reach the blood or lymph, a nutrient molecule must pass through an epithelial cell of the intestinal lining and through a cell lining the blood capillary or lymph vessel.

Figure 1.12: Structure of a villus.

Absorption of carbohydrates

Absorption occurs by a combination of simple diffusion and active transport. The monosaccharides are transferred by facilitated diffusion to the capillaries of the intestinal villi and are carried by the hepatic portal system to the liver, where non-glucose sugars are converted to glucose. Glucose enters the cell through facilitated diffusion.

Absorption of lipids

Lipids are digested into fatty acids and glycerol. After absorption by epithelial cells they are recombined into fats within these cells. The fats are mixed with cholesterol and proteins to form small globules called chylomicrons, most of which are transported by exocytosis out of the epithelial cells into the lacteals. The lymph containing chylomicrons eventually drains from the lymphatic system into large veins that return blood to the heart.

Absorption of proteins

Individual amino acids are absorbed in the epithelial cells of the villi and enter the hepatic portal system, which carries them to the liver. They may be modified in the liver or released into the bloodstream and distributed through the body. Most amino acids are used as building blocks for new proteins, but some may be used for energy.

Routes of absorption: glucose and amino acids by the blood, fats by the lymph.

Point	Glucose and amino acids	Fats (fatty acids and glycerol)
Inside epithelial cell	Pass through unchanged (glucose by facilitated diffusion)	Recombined into fats and made into chylomicrons
Enters	Blood capillary	Lacteal (lymph capillary)
Route	Hepatic portal vein to the liver	Lymphatic system, then large veins to the heart

1.7Large intestineOften examined

The caecum (cecum) is the proximal part of the large intestine, where the large and small intestines meet. The junction between ileum and large intestine is the ileocecal junction, guarded by the ileocecal sphincter. Attached to the caecum is a small blind tube about 9 cm long, the vermiform appendix; its walls contain many lymph nodules.

The colon is about 1.5 m long and has four parts: the ascending colon, transverse colon, descending colon and sigmoid colon. The rectum is a straight muscular tube that begins at the end of the sigmoid colon and ends at the anus, which is guarded by a sphincter.

Figure 1.13: Large intestine.

Functions of the large intestine

Absorbing water and electrolytes: no further digestion occurs here. The proximal half reabsorbs some water and electrolytes, making the stool solid. What remains becomes faeces, stored for a time in the rectum.
Absorption of vitamins: bacteria that live in the large intestine make vitamins, mainly vitamin K and biotin (a B vitamin), which are absorbed.
Reducing acidity and protecting from infection: the mucosa secretes bicarbonate to neutralise acidity from fatty acids and other digestive products, and acts as a mucosal barrier against microbial infection.

1.8Defaecation

Defaecation: The act of expelling faeces from the digestive tract through the anus.

Defaecation reflex in infants

In infants the defaecation reflex is involuntary: the anal sphincter relaxes automatically to allow stool to pass, especially shortly after feeding. This reflex is particularly active in the first week of life.

Defaecation reflex in adults

When it is appropriate, a person can start the reflex by holding a deep breath and contracting the abdominal muscles. This raises the internal pressure and forces faeces into the rectum.
When the rectum is filled, its wall is distended and the defaecation reflex is triggered.
Peristaltic waves in the descending colon are stimulated, and the internal anal sphincter relaxes.
The external anal sphincter is signalled to relax and the faeces are forced to the outside.
The reflex lasts only a few minutes and quickly dies. A person can inhibit it voluntarily by keeping the external sphincter contracted.

The defaecation reflex in an adult, in order. The reflex (accent) can be started or inhibited voluntarily.

Point	Infants	Adults
Control	Involuntary reflex	Voluntary control added to the reflex
Anal sphincter	Relaxes automatically	External sphincter kept contracted or relaxed at will
Trigger	Often after feeding	Rectum distended by faeces; person may start it by deep breath and abdominal contraction

1.9Storage and metabolic role of liverOften examined

The liver has two main parts, the larger right lobe and the smaller left lobe. The lobes contain thousands of lobules (small lobes), which connect with many bile ducts, the tubes that carry bile from the liver to the small intestine. Two sources supply blood to the liver: oxygenated blood from the hepatic artery and nutrient-rich blood from the hepatic portal vein.

Figure 1.14: Liver.

The liver regulates most chemical levels in the blood and excretes a product called bile, which helps carry waste away. All blood leaving the stomach and intestines passes through the liver, which processes it, breaks down, balances and creates nutrients, and converts drugs into forms that are easier to use or non-toxic. More than 500 vital functions have been identified.

Some well-known functions

Production of bile, which helps carry away waste and break down fats in the small intestine.
Production of blood plasma proteins, for example serum albumin, globulins and fibrinogen.
Production of cholesterol and special proteins that help carry fats through the body.
Conversion of excess glucose into glycogen for storage (converted back to glucose when needed).
Regulation of blood levels of amino acids.
Processing of haemoglobin to use its iron (the liver stores iron).
Conversion of poisonous ammonia to urea, which is excreted in urine.
Clearing the blood of drugs and other poisonous substances.
Regulating blood clotting.
Resisting infections by making immune factors and removing bacteria from the bloodstream.
Clearance of bilirubin from the breakdown of red blood cells; if it accumulates the skin and eyes turn yellow.

By-products of broken-down harmful substances are excreted into bile or blood. Bile by-products enter the intestine and leave as faeces; blood by-products are filtered by the kidneys and leave as urine.

Storage role	Metabolic role
Glycogen (from excess glucose); iron from haemoglobin; bile stored in the gall bladder	Making plasma proteins and cholesterol, urea formation, detoxifying drugs, regulating amino acids and glucose

1.10Composition and role of bileOften examined

The gall bladder is a sac-like structure on the inferior surface of the liver, about 8 cm long and 4 cm wide. It is connected to the common bile duct by the cystic duct. Bile is continually secreted by the liver and stored in the gall bladder.

Composition of bile

Bile contains no digestive enzymes. It consists of water, bile salts (sodium glycocholate and sodium taurocholate), bile pigment (bilirubin), cholesterol, lecithin (a phospholipid), mucus, cells and cell debris.

Role of the constituents

Constituent	Role
Bile salts	Reduce the surface tension of fat globules and emulsify them into droplets, increasing the total surface area for lipase (emulsification); help absorption of fatty acids from the intestine
Bilirubin	Results from the breakdown of haemoglobin. Intestinal bacteria convert it to pigments that give faeces their brown colour; some pigments are absorbed, modified in the kidneys and excreted, giving urine its yellowish colour
Bicarbonate (added from the liver)	Neutralises acid chyme (see section 1.12)

Emulsification. Bile salts break a fat globule into many small droplets (accent); the large total surface area lets lipase digest the fat faster.

Worked example: bile duct blocked

A person's bile duct is blocked by a stone. Explain what happens to the digestion of fats.

Bile cannot reach the duodenum, so bile salts are missing.
Fat is not emulsified into droplets, so the surface area for pancreatic lipase is small.
Fat digestion is slow and incomplete, and absorption of fatty acids is reduced.
Bile pigments do not reach the intestine, so faeces lose their normal brown colour.

1.11Structure and function of pancreasOften examined

The pancreas is a complex organ with both endocrine and exocrine tissue. The endocrine part consists of the pancreatic islets (islets of Langerhans). The pancreas has a head, a body and a tail, which extends to the spleen.

Figure 1.15: Structure of pancreas.

Head: the widest part, located in the C-shaped curve of the duodenum.
Neck: connects the head to the body.
Body: the main part, behind the stomach and to the left of the superior mesenteric vessels.
Tail: the tapered end, near the splenic hilum (where the spleen connects to other structures).

Pancreas as an exocrine gland

The exocrine secretion is pancreatic juice with two components: an aqueous component (bicarbonate, which neutralises the acid chyme from the stomach) and an enzymatic component, important for the digestion of all major classes of food.

Enzyme	Action
Trypsin, chymotrypsin, carboxypeptidase	Proteolytic enzymes; secreted inactive as trypsinogen, chymotrypsinogen and procarboxypeptidase and activated by removal of certain peptides
Amylase	Continues the polysaccharide digestion started in the oral cavity
Pancreatic lipases	Break lipids into free fatty acids, glycerides and cholesterol
Deoxyribonuclease and ribonuclease	Reduce DNA and RNA to their component nucleotides

Trypsinogen is activated by enterokinase into trypsin. Trypsin then activates more trypsinogen, chymotrypsinogen and procarboxypeptidase. If these enzymes were produced in active forms, they would digest the tissues producing them.

1.12Relation of secretion of bile and pancreatic juice with the secretin of hormoneOften examined

The secretion of bile and pancreatic juice is related to the hormone secretin (and to cholecystokinin).

Hormonal control of bile and pancreatic juice from the duodenum.

Secretion of bile

Fatty acids in the lumen of the duodenum stimulate endocrine cells to release cholecystokinin (CCK). CCK makes the smooth muscle of the gall bladder contract, so bile is released into the duodenum.
Acidic chyme in the duodenum stimulates other endocrine cells to release secretin.
Secretin is carried by the blood to the liver and stimulates the liver to release bicarbonate into the bile.

Secretion of pancreatic juice

Pancreatic juice secretion is regulated by secretin and cholecystokinin, produced by the walls of the duodenum when they detect acidic food, proteins, fats and vitamins. The secretion has an aqueous bicarbonate component from the duct cells and an enzymatic component from the acinar cells: a clear alkaline fluid containing enzymes for proteins, carbohydrates and fats. The main effect of secretin is to stimulate duct cells to secrete water and bicarbonate; as soon as this happens the enzymes made by the acinar cells are flushed through the pancreatic duct into the duodenum.

Hormone	Stimulus	Made by	Effect
Secretin	Acidic chyme in duodenum	Duodenal endocrine cells	Liver adds bicarbonate to bile; pancreatic duct cells release water and bicarbonate
CCK	Fatty acids in duodenum (also proteins, fats)	Duodenal endocrine cells	Gall bladder contracts; pancreatic enzymes released

1.12Action of enzymes in the GIT from mouth to small intestineOften examined

Figure 1.16 of the book is a flow chart of the digestion of carbohydrates, proteins, nucleic acids and fats from the mouth to the small intestine. It is drawn below, and the table gives the same information in the order food meets each enzyme.

Figure 1.16: Flow chart showing action of enzymes in GIT from mouth to small intestine. Enzymes are in accent boxes.

Action of enzymes in the GIT from mouth to small intestine (table form of Figure 1.16)
Food	Mouth	Stomach	Small intestine (pancreatic enzymes)	Small intestine (epithelial enzymes)	End products
Carbohydrates	Salivary amylase: polysaccharides to smaller polysaccharides and maltose	No enzyme (acid stops amylase)	Pancreatic amylases: to disaccharides	Disaccharidases	Monosaccharides
Proteins	None	Pepsin: proteins to small polypeptides	Trypsin and chymotrypsin: to smaller polypeptides; carboxypeptidase: to small peptides	Dipeptidases, carboxypeptidase, aminopeptidase	Amino acids
Nucleic acids (DNA, RNA)	None	None	Pancreatic nucleases: to nucleotides	Nucleotidases give nucleosides; nucleosidases and phosphatases give the final products	Nitrogenous bases, sugars, phosphates
Fats (triglycerides)	Lingual lipase (minor)	None	Pancreatic lipase (with bile emulsification)	None	Glycerol, fatty acids, monoglycerides

Worked example: following a meal

Trace where and by which enzyme the starch of bread, the protein of egg and the fat of butter are digested, and name the end products.

Food	Site and enzyme	End product
Starch	Mouth: salivary amylase (stops in the stomach); small intestine: pancreatic amylase then maltase	Glucose
Protein	Stomach: pepsin; small intestine: trypsin, chymotrypsin, carboxypeptidase, then aminopeptidase and erepsin	Amino acids
Fat	Mouth: lingual lipase (minor); small intestine: bile emulsifies, pancreatic lipase digests	Fatty acids and glycerol

1.12STEAM Activity 1.1

I. Action of amylase on starch

Materials: starch solution, amylase (saliva), water bath, 2 test tubes and iodine. Substrate = starch, enzyme = amylase, product = maltose.

Procedure

Dissolve some starch in water to form a starch solution.
Add equal amounts of starch solution to two test tubes, marked A and B.
Add saliva to test tube A and shake to mix. Do not add saliva to B.
Leave both tubes for ten minutes in a water bath at 37 degrees C (body temperature).
Add a few drops of iodine solution to both tubes.

Content of test tube	Chemical used	Original colour	Final colour
Starch and saliva	Iodine	Yellow	Yellow
Starch	Iodine	Yellow	Blue/black

Starch test with iodine: tube A (with saliva) stays yellow, tube B (no saliva) turns blue-black.

Conclusion: tube A (iodine and saliva) stays yellow and tube B turns blue/black, so amylase digests starch.

II. Qualitative food test for protein

Materials: 2 droppers, test tube, sodium hydroxide, copper sulphate.

Dissolve a sample of food such as yogurt and egg in water.
Add a few drops of sodium hydroxide (colourless). Caution: sodium hydroxide is corrosive, do not let it touch the skin.
Add a few drops of copper sulphate (blue).

Result: if the colour changes from blue to purple, protein is present. Yogurt and egg both give purple, so both contain protein.

Protein test: purple means protein is present.

1.12STEAM Activity 1.2

Observe, identify, draw and label prepared histological slides showing the cross-section of the human stomach, small intestine, liver and gall bladder.

Key terms

Gastrointestinal tract (GIT): The continuous tube from mouth to anus, about 9 m long: oral cavity, pharynx, esophagus, stomach, small and large intestine.
Accessory digestive organs: Teeth, tongue, salivary glands, liver, gall bladder and pancreas.
Mucosa: Innermost layer of the digestive tube.
Serosa: Outermost layer of the digestive tube.
Papillae: Rough projections of the tongue that give friction and contain taste buds.
Taste buds: Sensory structures in the papillae of the tongue.
Salivary glands: Three pairs of glands that secrete saliva containing enzymes.
Mechanical digestion: Physical division of food into smaller pieces.
Mastication: Chewing of food by the teeth.
Chemical digestion: Breakdown of food by enzymes.
Salivary amylase (ptyalin): Enzyme in saliva that digests starch to maltose and related sugars.
Lingual lipase: Enzyme in saliva that hydrolyses triglycerides to diacylglycerols and monoacylglycerols.
Bolus: Oval mass of chewed food formed in the mouth, ready to be swallowed.
Pharynx: Cavity behind the mouth.
Epiglottis: Flap pushed backward over the larynx during swallowing.
Upper esophageal sphincter (UES): Ring-shaped muscle at the top of the esophagus.
Lower esophageal sphincter (LES): Ring-shaped muscle at the lower end of the esophagus that prevents acid returning.
Peristalsis: Involuntary, wave-like muscular contraction that moves contents forward through the tract.
Anti-peristalsis: Backward-moving contractions, associated with vomiting.
Myenteric plexus: Nerve network between the longitudinal and circular muscles that controls motility.
Mediastinum: Area in the centre of the chest where the esophagus lies.
Cardia: Part of the stomach surrounding the opening of the esophagus.
Fundus: Rounded, often gas-filled part of the stomach above the cardia.
Body of the stomach: Large central portion of the stomach.
Pylorus: Part of the stomach joining the duodenum.
Pyloric sphincter: Sphincter regulating passage of chyme into the duodenum.
Gastric pits: Tube-like openings of the gastric glands.
Surface mucous cells: Cells lining the pits that secrete mucus.
Parietal (oxyntic) cells: Gastric gland cells that secrete HCl and intrinsic factor.
Chief (zymogenic) cells: Gastric gland cells that secrete pepsinogen.
Gastrin: Hormone from stomach endocrine cells that stimulates gastric juice secretion.
Mixing waves: Contractions of stomach smooth muscle that churn food.
Chyme: Thick liquid formed when food is mixed with gastric juice.
Intrinsic factor: Glycoprotein that binds vitamin B12 for absorption in the ileum.
Pepsinogen: Inactive precursor of pepsin.
Pepsin: Protein-digesting enzyme of the stomach, optimum pH 2.
Zymogen granules: Granules in which pepsinogen is packaged.
Acetylcholine: Chemical released after nervous stimulation that stimulates parietal and chief cells and gastrin release.
Duodenum: First part of the small intestine, 20 to 25 cm, C-shaped.
Jejunum: Middle part of the small intestine, about 2.5 m.
Ileum: Last part of the small intestine, about 3.5 m; major site of absorption.
Pancreatic juice: Slightly alkaline (pH 8) secretion of the pancreas with digestive enzymes.
Enterokinase: Intestinal enzyme that converts trypsinogen into trypsin.
Trypsin: Active protein-digesting enzyme formed from trypsinogen.
Emulsification: Breaking of fat into small droplets by bile salts.
Chyle: Watery emulsion that chyme becomes in the small intestine.
Villi: Finger-like projections of the small intestine mucosa, 0.5 to 1.5 mm long.
Lacteal: Lymph capillary in a villus.
Chylomicrons: Small globules of fats, cholesterol and protein absorbed into lacteals.
Hepatic portal system: Vein system carrying absorbed nutrients from the intestine to the liver.
Caecum: First part of the large intestine, where ileum joins.
Vermiform appendix: Small blind tube about 9 cm long attached to the caecum.
Defaecation: Expelling faeces through the anus.
Bile: Fluid made by the liver and stored in the gall bladder; contains no enzymes.
Bile salts: Sodium glycocholate and sodium taurocholate, which emulsify fats.
Bilirubin: Bile pigment from the breakdown of haemoglobin.
Secretin: Hormone released when acid chyme enters the duodenum; stimulates bicarbonate secretion.
Cholecystokinin (CCK): Hormone released by fatty acids in the duodenum; makes the gall bladder contract.
Pancreatic islets: Endocrine part of the pancreas (islets of Langerhans).
Acinar cells: Pancreatic cells that make the enzymes of pancreatic juice.

Multiple choice questionsp. 24–25

Select the correct answer.

11 mark

Which among the following is the longest?

Astomach
Besophagus
Csmall intestineAnswer
Dlarge intestine

Why

The small intestine is about 6 m long, far longer than the large intestine (about 1.5 m), the esophagus (about 25 cm) and the stomach.

The esophagus is the tempting choice only for a student who confuses it with the whole tube; in the list it is short, and the stomach is a short bag.

21 mark

Which structure does the appendix come off?

Atransverse colon
Bdescending colon
Cascending colonAnswer
Dsmall intestine

Why

The vermiform appendix is a blind tube attached to the caecum, the first part of the large intestine, at the point where the ileum enters it (Fig. 1.13). The caecum is the start of the ascending colon, so of the four options the appendix comes off the ascending colon region.

The transverse and descending colons lie further along the large intestine, and the small intestine joins the caecum at the ileocaecal junction; neither is where the appendix is attached.

Note. The book gives no key. Strictly the appendix is attached to the caecum (see mcq-q3); the ascending colon is the nearest of the listed options because it begins at the caecum.

31 mark

Which part of the Large intestine attaches to the appendix?

Atransverse colon
Bdescending colon
Cascending colon
DcecumAnswer

Why

The appendix is attached to the caecum (cecum), the proximal pouch of the large intestine where the small and large intestines meet. The walls of the appendix contain many lymph nodules (p. 18).

The ascending colon lies right next to the caecum and is the tempting choice, but it is the next part of the colon, not the part the appendix is attached to.

41 mark

What is the name of the part of the stomach attached to the esophagus?

Afundus
Bpylorus
Cbody
DcardiaAnswer

Why

The cardia surrounds the superior opening of the stomach, which connects it with the esophagus (p. 12, Fig. 1.8).

The fundus is the rounded part above and to the left of the cardia, so it is next to the esophagus but not attached to it. The pylorus is the outlet to the duodenum at the lower end, and the body is the large central part.

51 mark

Where does the pancreatic duct directly join to?

Ajejunum
BduodenumAnswer
Cileum
Dliver

Why

The pancreatic duct carries pancreatic juice through the pancreatic duct into the duodenum, the first part of the small intestine, which curves round the head of the pancreas (p. 15, p. 21).

The liver is a tempting choice because bile also enters the duodenum, but the liver releases bile through the bile ducts and does not receive the pancreatic duct.

61 mark

Where does the body of the stomach lie between?

Afundus and pyloric antrumAnswer
Bpyloric antrum and cardia
Cfundus and cardia
Dcardia and pyloric antrum

Why

The body is the large central portion of the stomach, inferior to the fundus, and it runs down towards the pyloric antrum at the lower end (p. 12, Fig. 1.8). So it lies between the fundus and the pyloric antrum.

The cardia is at the top of the stomach next to the esophagus, beside the fundus, so options B to D put the body in the wrong place.

71 mark

Which one is the largest gland in the body?

AliverAnswer
Bgallbladder
Cpancreas
Dlarge intestine

Why

The liver is the largest gland of the body. The book describes it as a large organ with two lobes and thousands of lobules, and it has more than 500 functions (p. 19 to 20).

The gall bladder is a small sac about 8 cm long, the pancreas is a smaller gland, and the large intestine is part of the gut, not a gland.

81 mark

Pepsinogen is activated to pepsin by

Aactive secretin
Bhydrochloric acid
Cactive pepsin and HCLAnswer
Dgastrin

Why

Pepsinogen is converted to pepsin by hydrochloric acid and by previously formed pepsin molecules (p. 14). So the complete answer is the combination of active pepsin and HCl.

Hydrochloric acid alone (B) is only part of the answer, and secretin and gastrin do not act on pepsinogen.

91 mark

Liver secretes bile into the

AduodenumAnswer
Bileum
Cjejunum
Dascending colon

Why

Bile made by the liver flows through the common bile duct into the duodenum, the first part of the small intestine (Fig. 1.14; p. 20 and 21, where secretin stimulates the liver to release bicarbonate into the bile that enters the duodenum).

The ileum, jejunum and ascending colon are further along the small and large intestine and receive bile only after it has passed the duodenum.

101 mark

Emulsification of fat will not occur in the absence of

Alipase
Bbile pigment
Cbile saltAnswer
Dpancreatic juice

Why

Bile salts reduce the surface tension of fat globules and emulsify them into small droplets (p. 16 and 20). Without bile salts the fat stays in large globules.

Lipase digests fat but does not emulsify it, bile pigment (bilirubin) is a waste product, and pancreatic juice supplies lipase but is not the emulsifier.

111 mark

Fatty acids and glycerol are first absorbed by

Alymph vessel
BvilliAnswer
Cblood capillaries
Dhepatic portal vein

Why

Fatty acids and glycerol are absorbed by the epithelial cells lining the villi (p. 18). Only inside these cells are they recombined into fats, which are then sent into the lacteals.

The lacteal (lymph vessel) receives the fats only after they have been rebuilt inside the epithelial cells; blood capillaries and the hepatic portal vein carry the sugars and amino acids.

121 mark

The hormone responsible for stimulating secretion of hydrochloric acid by stomach cells is

Apepsin
Bsecretin
CgastrinAnswer
Dinsulin

Why

Gastrin is released by the endocrine cells of the stomach, travels in the blood to the parietal cells and stimulates them to secrete hydrochloric acid and additional gastric juice (p. 15).

Pepsin is an enzyme, not a hormone. Secretin acts on the pancreas and liver, and insulin from the islets of Langerhans controls blood glucose; neither acts on the stomach.

131 mark

Enzyme trypsinogen is changed to trypsin by

Agastrin
BenterokinaseAnswer
Csecretin
Dhydrochloric acid

Why

The intestinal glands secrete the activator enzyme enterokinase, which converts trypsinogen into trypsin. Trypsin then activates more trypsinogen (p. 16).

Gastrin and secretin are hormones that control secretion and do not change trypsinogen. Hydrochloric acid activates pepsinogen in the stomach, not trypsinogen in the intestine.

141 mark

Which of the following hormones stimulates the production of pancreatic juice and bicarbonate?

Ainsulin and glucagon
Bcholecystokinin and secretinAnswer
Cgastrin and insulin
Dglucagon and insulin

Why

Secretin stimulates the duct cells of the pancreas to secrete water and bicarbonate, and cholecystokinin (CCK) stimulates the acinar cells to release enzymes. Together they regulate pancreatic juice (p. 21).

Insulin and glucagon regulate blood glucose from the islets of Langerhans and are not the regulators of pancreatic juice. Gastrin acts on the stomach.

151 mark

Secretin and cholecystokinin are secreted in

Apyloric region
Bileum
CduodenumAnswer
Desophagus

Why

Fatty acids in the duodenum stimulate its endocrine cells to release CCK, and acidic chyme in the duodenum stimulates other endocrine cells to release secretin (p. 21).

Gastrin is released in the pyloric region of the stomach, the ileum is an absorptive part of the intestine and the esophagus is only a passage with no digestion.

161 mark

Which of the following is synthesized and stored in the liver cells?

Agalactose
Blactose
CglycogenAnswer
Dinsulin

Why

The liver converts excess glucose into glycogen and stores it in liver cells; the glycogen can later be converted back to glucose (p. 20).

Galactose and lactose are sugars that are absorbed or digested, not stored in the liver. Insulin is made by the islets of the pancreas, not by the liver.

171 mark

Which juice secreted by the alimentary canal plays an important role in the digestion of fats?

Apancreatic juice, saliva
Bsaliva, hydrochloric acid
Cmucus, hydrochloric acid
Dbile juice, pancreatic juiceAnswer

Why

Bile emulsifies fat into small droplets, and pancreatic juice contains pancreatic lipase, which digests the emulsified fat into fatty acids and glycerol (p. 15 and 16). These two juices are the main agents of fat digestion.

Option A contains pancreatic juice but saliva only has a minor role (lingual lipase starts fat digestion). Hydrochloric acid and mucus do not digest fats.

Note. Saliva does contain lingual lipase (p. 9), so option A is not wrong in a strict sense; the book's emphasis and the main answer are bile and pancreatic juice.

181 mark

What is the role of mucus secreted by the stomach?

Ato digest protein
Bto digest fats
Cto kill germs in the food
Dto protect lining of the stomachAnswer

Why

Surface mucous cells secrete a thick, viscous, alkaline mucus that lubricates and protects the epithelial cells of the stomach wall from the damaging effect of acidic chyme and pepsin (p. 14).

Mucus contains no digestive enzymes, so it does not digest protein or fat, and germs are killed by the hydrochloric acid, not by the mucus.

191 mark

The majority of the water from the indigestible food is absorbed in the____?

Astomach
Bfoodpipe
Cpancreas
Dlarge intestineAnswer

Why

The proximal half of the large intestine reabsorbs most of the water and electrolytes, making the stool solid before it is stored in the rectum (p. 18).

The stomach and small intestine absorb only some substances, the foodpipe (esophagus) is a passage, and the pancreas is an accessory gland, not part of the alimentary canal.

201 mark

Which hydrolytic enzymes react in a low pH environment?

Aperoxidases
Bhydrolases
Camylases
DproteasesAnswer

Why

Pepsin is a protease. It is secreted in the stomach and reaches its optimum activity at a pH of about 2 (p. 14), so proteases are the hydrolytic enzymes that act in the low pH of the stomach.

Amylases stop at low pH: the acid of the stomach inactivates salivary amylase (p. 14). Peroxidases are not hydrolytic enzymes at all. Hydrolases is a wider class that includes amylases, so it is less precise than proteases.

Note. The book gives no key. 'Hydrolases' is a class that also contains amylases, which do not work at low pH, so 'proteases' is the answer that fits the question best.

Short answer questionsp. 26

13 marks

What is mechanical digestion?

Solution

Mechanical digestion is the physical division of a mass of food into smaller pieces. No new chemical substances are made, but the food particles get a much larger surface area for the digestive enzymes to act on. It has three main forms:

Mastication: the teeth chew food in the mouth, breaking large pieces into smaller ones, and the tongue mixes them with saliva to form a bolus.
Churning: the three layers of smooth muscle in the stomach wall produce mixing waves that mix the food with gastric juice to form chyme.
Peristalsis: wave-like contractions move and mix the food with digestive juices in the stomach and small intestine.

23 marks

What is chemical digestion?

Solution

Chemical digestion is the breakdown of large food molecules into small, soluble molecules that can be absorbed. It is brought about by hydrolysis, in which digestive enzymes split covalent bonds by adding water. The enzymes are specific, so each one acts on one class of food:

Salivary amylase in the mouth splits starch into maltose, maltotriose and isomaltose.
Pepsin in the stomach, and trypsin and chymotrypsin in the duodenum, split proteins into smaller peptides, which erepsin and amino peptidase then break into amino acids and dipeptides.
Pancreatic lipase, after bile has emulsified the fat, splits triglycerides into fatty acids, glycerol and monoglycerides.

33 marks

Describe peristalsis.

Solution

Peristalsis is a wave-like, involuntary and rhythmic process in which the circular and longitudinal smooth muscles in the wall of the digestive tract contract in sequence. It moves food and fluid forward (anterograde), from the esophagus through the stomach and intestines.

A wave of smooth muscle relaxation forms just ahead of the bolus, so the tube expands in front of it.
A wave of contraction behind the bolus pushes it forward through the tract.
The movement is controlled by the myenteric plexus between the outer longitudinal and inner circular muscle layers, so it does not need conscious control.

43 marks

Name and write the function of epithelial cells of stomach of man.

Solution

The stomach is lined by simple columnar epithelium, which is made of four main cell types:

Cell type	Secretion	Function
Surface mucous cells	Mucus	Thick, viscous, alkaline mucus lines the gastric pits and protects the stomach wall from acid and pepsin.
Parietal (oxyntic) cells	Hydrochloric acid and intrinsic factor	HCl gives the low pH (about 2), kills bacteria and activates pepsinogen. Intrinsic factor binds vitamin B12 so it is absorbed in the ileum.
Chief (zymogenic) cells	Pepsinogen in zymogen granules	Pepsinogen is an inactive precursor that HCl converts to pepsin, which breaks peptide bonds in proteins.
Endocrine cells	Gastrin hormone	Gastrin enters the blood, stimulates the parietal cells to secrete more HCl and gastric juice.

53 marks

Give one reason as to why some enzymes in stomach and intestine are secreted in inactive form?

Solution

Enzymes such as pepsin, trypsin and chymotrypsin are secreted as inactive precursors (pepsinogen, trypsinogen and chymotrypsinogen) so that they do not digest the cells that make them. If they were produced in their active forms, they would digest the tissues of the stomach wall and pancreas that produce them.

They are activated only after they enter the lumen: pepsinogen by HCl and pepsin, and trypsinogen by enterokinase, so the digestion takes place in the food and not in the gland.

63 marks

Name the enzymes involved in protein digestion.

Solution

Protein digestion takes place in three regions, each with its own enzymes:

Region	Enzymes	Action
Stomach	Pepsin (from pepsinogen by HCl)	Splits some peptide bonds, forming smaller polypeptide chains.
Duodenum (pancreatic juice)	Trypsin, chymotrypsin and carboxypeptidase (from trypsinogen, chymotrypsinogen and procarboxypeptidase)	Break the polypeptides into smaller peptides.
Jejunum and ileum (intestinal lining)	Erepsin, amino peptidase and dipeptidases	Split peptides into dipeptides and amino acids, which are absorbed.

Enterokinase from the intestinal glands activates trypsinogen, and trypsin then activates chymotrypsinogen and procarboxypeptidase.

73 marks

How could no secretion of HCl in our stomach affect food digestion?

Solution

Hydrochloric acid has several roles in the stomach, so without it digestion would be affected in these ways:

Pepsinogen would not be converted to pepsin, and pepsin works best at a pH of about 2. Protein digestion in the stomach would therefore stop or be very slow.
Salivary amylase would not be inactivated by the low pH, so starch would go on being digested in the stomach, where the normal process would stop it.
Proteins would not be denatured, so the proteolytic enzymes could not easily reach the peptide bonds, and germs taken in with the food would not be killed, increasing the risk of infection.

83 marks

How the stomach does protect itself from the damaging effect of HCl?

Solution

The stomach protects its own wall from the acid it secretes in three main ways:

Mucus: the surface mucous cells secrete a thick, viscous and alkaline mucus that coats the gastric pits and the epithelium, neutralising acid at the surface.
Lubrication: the thick layer of mucus lubricates the wall, so the acidic chyme moves over it without damaging the epithelial cells.
Inactive enzymes: pepsin is secreted as inactive pepsinogen in zymogen granules, so the protein-digesting enzyme is not active until it reaches the lumen.

93 marks

Why there are villi in the intestine and not in stomach?

Solution

Villi are finger-like projections of the mucosa whose function is to increase the surface area for absorption. Absorption of the digested food takes place in the small intestine, so only the intestine has villi.

The small intestine, especially the ileum, is the major site of nutrient absorption. Each villus contains a blood capillary network and a lacteal to take up the products of digestion.
The stomach stores food and mixes it into chyme. Its job is digestion, mainly of protein by pepsin, and it does not absorb nutrients in the way the intestine does, so it needs no villi.
The villi are covered with simple columnar epithelium and microvilli, which give a very large surface area in the intestine.

103 marks

Trypsin acts at alkaline pH. What provides the alkalinity?

Solution

The alkalinity is provided by bicarbonate ions ( $HCO X_{3} X^{-}$ ) in the pancreatic juice, which has a pH of about 8.

The duct cells of the pancreas secrete an aqueous solution of bicarbonate, which neutralises the acidic chyme that enters the duodenum from the stomach.
The secretion of this bicarbonate is stimulated by the hormone secretin, which the duodenum releases when acidic chyme enters it.
The liver also releases bicarbonate into the bile under the action of secretin, adding to the alkalinity of the duodenum.

113 marks

What would happen to the activity of the intestinal enzymes if the pH in the small intestine remained at 2?

Solution

The intestinal enzymes would lose their activity. The small intestine normally has an alkaline pH, which the bicarbonate of pancreatic juice and bile provides. At a pH of 2:

Pancreatic enzymes such as trypsin, chymotrypsin, amylase and lipase would be denatured or inactive, so digestion of proteins, carbohydrates and fats in the small intestine would stop or slow down greatly.
Enzymes are proteins, so the low pH would change their shape, including the active site, and they could not bind their substrates even if the pH later returned to normal.
Pepsin from the stomach would remain active at pH 2 and could damage the lining of the intestine, which is protected by the alkaline mucus and the bicarbonate.

123 marks

How does absorption of fat differ from absorption of glucose?

Solution

Glucose is absorbed as a monosaccharide directly into the blood, whereas fat is absorbed as fatty acids and glycerol, which are rebuilt into fats inside the epithelial cells and then enter the lymphatic system:

Feature	Glucose	Fat (fatty acids and glycerol)
Form absorbed	Monosaccharide (glucose)	Fatty acids and glycerol, after emulsification and lipase action
Mechanism	Simple diffusion and active transport into the epithelial cells, then facilitated diffusion into the blood capillaries of the villi	Absorbed by the epithelial cells of the villi, then recombined into fats and packed with cholesterol and protein into chylomicrons
Route after absorption	Hepatic portal vein to the liver	Exocytosis into the lacteals, then the lymphatic system and large veins that return blood to the heart

133 marks

Describe defaecation reflex in infants.

Solution

In infants the defaecation reflex is involuntary, so the baby cannot start or stop it at will:

The anal sphincter relaxes automatically to allow stool to pass out of the body.
The reflex is particularly active shortly after feeding.
It is most active in the first week of life, and the stool is passed without any voluntary effort.

143 marks

Describe defaecation reflex in adults.

Solution

In adults the reflex can be started and controlled voluntarily:

A person takes a deep breath and contracts the abdominal muscles. This raises the internal pressure and forces the faeces into the rectum.
The distended rectal wall triggers the defaecation reflex. Peristaltic waves in the descending colon are stimulated, and the internal and anal sphincters relax.
The external anal sphincter is signalled to relax and the faeces are forced out. The reflex lasts only a few minutes, and a person can inhibit it by keeping the external sphincter contracted.

153 marks

Bile juice contains no digestive enzymes, yet it is important for digestion. Why?

Solution

Bile does not digest food, but its salts make the digestion of fats possible and faster:

Emulsification: the bile salts reduce the surface tension of fat globules and break them into small droplets, increasing the total surface area of the fat.
Lipase action: the large surface area of the emulsified fat allows pancreatic lipase to act much faster, so fats are digested more quickly.
Absorption and neutralisation: bile salts help the absorption of fatty acids from the intestine, and the bicarbonate in bile neutralises the acidic chyme.

163 marks

What is the role of hormone gastrin in digestion?

Solution

Gastrin is a hormone secreted by the endocrine cells of the stomach into the blood. Its role is to increase the secretion of gastric juice:

It is released in response to food, mainly the distension of the stomach and the presence of amino acids and peptides.
It travels in the blood to the parietal cells, where it stimulates the secretion of hydrochloric acid and additional gastric juice.
The extra HCl activates more pepsinogen to pepsin, so protein digestion in the stomach is increased.

173 marks

What is the role of hormone secretin in digestion?

Solution

Secretin is a hormone released by the endocrine cells of the duodenum when acidic chyme enters it. It acts on two organs:

On the pancreas it stimulates the duct cells to secrete water and bicarbonate, so the enzymes from the acinar cells are flushed through the pancreatic duct into the duodenum.
On the liver it stimulates the release of bicarbonate into the bile. The bicarbonate neutralises the acidic chyme.
By neutralising the chyme it creates the alkaline pH at which the intestinal enzymes work.

183 marks

Describe the storage role of liver.

Solution

The liver stores substances that the body needs at other times:

Glycogen: excess glucose from the blood is converted into glycogen and stored in liver cells. When blood glucose falls, the glycogen is converted back into glucose.
Iron: the liver processes the haemoglobin released from broken red blood cells and stores its iron for later use.
Amino acids: the liver regulates the blood level of amino acids, the building blocks of proteins, by storing and releasing them as the body needs.

193 marks

What is gall bladder? Write its function.

Solution

The gall bladder is a sac-like structure on the inferior surface of the liver, about 8 cm long and 4 cm wide. It is connected to the common bile duct by the cystic duct. Its function is to store and release bile:

It stores the bile that the liver secretes continually, between meals.
When fatty acids enter the duodenum, CCK is released, and it makes the smooth muscle of the gall bladder contract, releasing bile into the duodenum.
The bile then emulsifies the fats in the small intestine so that they can be digested by lipase.

206 marks

Write the differences between:

Solution

(a) pharynx and larynx

Feature	Pharynx	Larynx
Position	A cavity behind the mouth, above the esophagus	Below the pharynx, at the top of the trachea
Passage	A common passage for food and air	A passage for air to the trachea
Key feature	Tactile receptors start the swallowing reflex and the soft palate closes the nasal airways	Contains the vocal folds; the epiglottis covers its entrance during swallowing

(b) pepsinogen and pepsin

Feature	Pepsinogen	Pepsin
Nature	Inactive precursor (zymogen)	Active proteolytic enzyme
Source	Chief (zymogenic) cells, packed in zymogen granules	Formed from pepsinogen by HCl and by pepsin already present
Action	None until it is converted	Breaks some covalent bonds in proteins into smaller peptide chains
Optimum pH	Not applicable	About 2

Extensive answer questionsp. 26

12026 Annual I, Set B, Q.5 OR8 marks

Describe the process of swallowing in man.

Solution

Overview

Swallowing moves food from the mouth into the stomach. It is a set of reflexes that takes place in three phases: the oral phase, the pharyngeal phase and the esophageal phase. Only the first phase is under voluntary control.

Phase 1: oral phase (voluntary)

The lips close and the food is moistened with saliva. The teeth and tongue form the food into a bolus.
The tongue pushes the bolus back towards the throat (pharynx). This is the only voluntary part of swallowing.

Phase 2: pharyngeal phase (involuntary reflex)

Tactile receptors in the pharynx are stimulated and the swallow reflexes start.
The soft palate lifts to close the nasal airways, so the food cannot go up the nose.
The tongue blocks the oral cavity, so the bolus cannot return to the mouth, and it moves over the back of the tongue.
The epiglottis is pushed backward over the larynx, and the larynx and vocal folds contract to cover the entry to the trachea. Respiration is briefly stopped, so food cannot enter the airway.
The upper esophageal sphincter opens to let the bolus pass into the esophagus.

Phase 3: esophageal phase (involuntary)

The bolus is propelled down the esophagus by peristalsis: a wave of relaxation moves ahead of it and a wave of contraction pushes it from behind.
The lower esophageal sphincter relaxes to let the bolus into the stomach and then closes again, so stomach acid stays out of the esophagus.
The larynx moves back to its original position and breathing resumes.

Figure 1.5: The three phases of swallowing, summarised as a flow

22026 Annual I, Set D, Q.5 (a)8 marks

Describe the human stomach with diagram.

Solution

Position and shape

The stomach is a J-shaped muscular bag in the upper left of the abdomen. It is joined to the esophagus at the top and to the duodenum at the bottom. Its medial border is the lesser curvature and its lateral border is the greater curvature.

Divisions

Cardia: surrounds the superior opening of the stomach, which is joined to the esophagus.
Fundus: the rounded, often gas-filled part above and to the left of the cardia.
Body: the large central portion, below the fundus. Most of the mixing takes place here.
Pyloric region: the lower part, made of the pyloric antrum, the pyloric canal and the pyloric sphincter. The sphincter controls the passage of chyme into the duodenum.

Figure 1.8: Divisions of the stomach

Wall of the stomach

There are four layers, from outside to inside: the serosa (outer layer), the muscularis, the submucosa and the mucosa. The muscularis has three layers of smooth muscle: an outer longitudinal, a middle circular and an inner oblique layer. The three muscle layers allow the strong mixing movements of the stomach. The mucosa is lined with simple columnar epithelium and has many gastric pits, the openings of the gastric glands.

Cells of the gastric glands

Cell	Secretion	Role
Surface mucous cells	Mucus	Lubricate and protect the lining from acid and pepsin
Parietal (oxyntic) cells	Hydrochloric acid and intrinsic factor	Give the low pH, kill bacteria, activate pepsinogen; intrinsic factor helps vitamin B12 absorption
Chief (zymogenic) cells	Pepsinogen	Pepsinogen becomes pepsin, which breaks peptide bonds in proteins
Endocrine cells	Gastrin	Stimulates the parietal cells to secrete more HCl

Mechanical and chemical digestion in the stomach

Mechanical digestion: the smooth muscles produce mixing waves, which churn the food with gastric juice into a thick liquid called chyme. Peristaltic waves push chyme to the pyloric sphincter.
Chemical digestion: hydrochloric acid denatures proteins and stops the action of salivary amylase; pepsin splits proteins into peptide chains.
Protection: the thick layer of alkaline mucus protects the wall from the acid and pepsin.

38 marks

Describe the structure of human small intestine.

Solution

Length and position

The small intestine is the longest part of the gastrointestinal tract, about 6 m long. It runs from the pyloric sphincter of the stomach to the ileocaecal junction, where it joins the large intestine. Most digestion and nearly all absorption take place here.

Regions

Duodenum: the first part, 20 to 25 cm long and C-shaped. It curves around the head of the pancreas, and the pancreatic duct and bile duct open into it.
Jejunum: about 2.5 m long. The digestion of protein, carbohydrate and fat is completed here.
Ileum: about 3.5 m long. It is the major site of absorption, and vitamin B12 is absorbed here with the help of intrinsic factor.

Figure 1.11: Regions of the small intestine (schematic)

Wall

The wall has the same four layers as the rest of the digestive tube: serosa, muscularis, submucosa and mucosa. The inner wall is folded, and the mucosa forms tiny finger-like villi, 0.5 to 1.5 mm long. Each villus is covered by simple columnar epithelium, whose surface cells carry microvilli. Each villus has a network of blood capillaries and a central lacteal. This structure gives the intestine a very large absorptive surface.

Enzymes of the intestinal lining

Enterokinase: activates trypsinogen to trypsin.
Erepsin: splits peptides into amino acids.
Amino peptidase: splits polypeptides into dipeptides.
Lactase, maltase and sucrase: convert lactose, maltose and sucrose into monosaccharides.

48 marks

Explain the absorption of digested products from the small intestine lumen to the blood capillaries and lacteals of the villi.

Solution

Overview

Absorption takes place mainly in the ileum, and partly in the jejunum. A nutrient must cross the epithelial cell of a villus and then the wall of a blood capillary or a lacteal. The folds, villi and microvilli give a very large surface, which makes absorption efficient.

Carbohydrates

Enzymes in the intestinal lining and pancreatic juice convert starch and disaccharides into monosaccharides (glucose, galactose and fructose).
The monosaccharides enter the epithelial cells of the villi by a combination of simple diffusion and active transport.
They leave the cells by facilitated diffusion into the blood capillaries of the villi.
The blood carries them in the hepatic portal vein to the liver, where non-glucose sugars are converted to glucose.

Proteins

Trypsin, chymotrypsin and carboxypeptidase reduce proteins to peptides, and erepsin and amino peptidase reduce them to amino acids and dipeptides.
Amino acids are absorbed by the epithelial cells of the villi and pass into the blood capillaries.
They travel in the hepatic portal system to the liver, which may modify them or release them into the blood, where they build new proteins.

Lipids (fats)

Bile emulsifies the fat into droplets, and pancreatic lipase splits the droplets into fatty acids, glycerol and monoglycerides.
Fatty acids and glycerol are absorbed by the epithelial cells of the villi.
Inside the cells they are recombined into fats, which are mixed with cholesterol and proteins to form chylomicrons.
Most chylomicrons leave the cells by exocytosis into the lacteals, which drain into the lymphatic system and then into large veins that return the blood to the heart.

Figure 1.12: Absorption routes in a single villus (schematic, based on the book's figure)

Nutrient	Enzymes	Absorbed as	Enters	Carried by
Sugars	Maltase, lactase, sucrase	Glucose, galactose, fructose	Blood capillaries	Hepatic portal vein
Proteins	Pepsin, trypsin, erepsin, amino peptidase	Amino acids	Blood capillaries	Hepatic portal vein
Fats	Lipase (after bile emulsifies)	Fatty acids and glycerol, rebuilt as chylomicrons	Lacteals	Lymphatic system, then veins to the heart

58 marks

Describe the large intestine of man. What are the functions of large intestine?

Solution

Structure

The large intestine is about 1.5 m long. It begins at the caecum, where the ileum joins it at the ileocaecal junction, and ends at the anus, which is guarded by a sphincter. It has four main parts:

Caecum: the proximal, blind pouch. The vermiform appendix, about 9 cm long, is attached to it, and its walls contain many lymph nodules.
Colon: the ascending colon, transverse colon, descending colon and sigmoid colon.
Rectum: a straight, muscular tube that begins at the end of the sigmoid colon and holds the faeces until they are passed.
Anus: the outer opening, guarded by a sphincter.

Figure 1.13: Large intestine (schematic)

Functions

Absorbing water and electrolytes: the proximal half of the large intestine reabsorbs water and electrolytes, which makes the stool solid. The remaining material becomes faeces, which are stored in the distal part (the rectum).
Absorbing vitamins: the bacteria that live in the large intestine make vitamin K and biotin (a B vitamin), which the large intestine absorbs.
Reducing acidity and protecting from infection: the mucosa secretes bicarbonate to neutralise the acid formed from the fatty acids and other digestive products, and it acts as a mucosal barrier against microbes and invasions.

68 marks

What is bile? Describe the composition of bile. What is the role of constituents of bile?

Solution

Definition

Bile is a fluid made continually by the liver. It is stored in the gall bladder and released into the duodenum. Bile contains no digestive enzymes, but it is essential for the digestion and absorption of fats.

Composition

Constituent	Examples
Water	Forms the bulk of the bile
Bile salts	Sodium glycocholate and sodium taurocholate
Bile pigment	Bilirubin, from the breakdown of haemoglobin
Cholesterol and lecithin	Cholesterol, and lecithin (a phospholipid)
Mucus, cells and cell debris	Shed from the lining of the bile ducts

Role of the constituents

Constituent	Role
Bile salts	Reduce the surface tension of fat globules and emulsify them into droplets; help the absorption of fatty acids from the intestine
Bilirubin	Bacteria in the intestine convert it into brown pigments that colour the faeces. Some is absorbed, modified in the kidneys and excreted in the urine, which gives urine its yellow colour
Cholesterol	Made by the liver, it helps carry fats through the body (book, p. 20); the book gives no separate role for it in bile
Bicarbonate (released into bile under secretin)	Neutralises the acidic chyme that enters the duodenum

Emulsification

Bile salts gather at the surface of the fat globules and reduce their surface tension.
The large globules are broken into many small droplets (an emulsion). The total surface area of the fat increases greatly.
Pancreatic lipase acts on the surface of the droplets, so the digestion of fat is much faster.

Emulsification: bile salts break one large fat globule into many small droplets, giving lipase a much larger surface to act on

78 marks

How secretion of bile is related to the secretion of hormone secretin?

Solution

Two hormones control bile

Bile secretion is linked to the hormone secretin through its effect on the liver. Secretin is released when acidic chyme enters the duodenum. The hormone cholecystokinin (CCK) controls the release of bile from the gall bladder, and secretin controls the bicarbonate in bile.

Steps

Acidic chyme enters the duodenum. The endocrine cells of the duodenal wall release secretin into the circulation.
The blood carries secretin to the liver, where it stimulates the liver to release bicarbonate into the bile.
The alkaline bicarbonate neutralises the acidic chyme in the duodenum, so the intestinal enzymes can work at their proper pH.
Fatty acids in the duodenum separately stimulate the release of CCK, which contracts the smooth muscle of the gall bladder and sends the stored bile into the duodenum.

Secretin and CCK control the secretion of bile and pancreatic juice (schematic, not in the book)

Hormone	Released by	Stimulus	Target	Effect
Secretin	Endocrine cells of the duodenum	Acidic chyme	Liver and pancreatic duct cells	Bicarbonate into bile; water and bicarbonate into pancreatic juice
Cholecystokinin (CCK)	Endocrine cells of the duodenum	Fatty acids and proteins	Gall bladder and acinar cells of the pancreas	Gall bladder contracts to release bile; enzymes released into pancreatic juice

Note. The book links bile to secretin through bicarbonate. Contraction of the gall bladder is controlled by CCK (book, p. 21), so the answer covers both hormones.

82026 Annual I, Set D, Q.2 (xii)8 marks

Write the functions of liver of man.

Solution

Functions

Production of bile, which carries away waste and breaks down fats in the small intestine during digestion.
Production of certain proteins for the blood plasma, for example serum albumin, globulins and fibrinogen.
Production of cholesterol and special proteins that help carry fats through the body.
Conversion of excess glucose into glycogen for storage. Glycogen can later be converted back to glucose for energy and to balance the blood glucose.
Regulation of blood levels of amino acids, which form the building blocks of proteins.
Processing of haemoglobin for the use of its iron content (the liver stores iron).
Conversion of poisonous ammonia to urea, the end product of protein metabolism, which is excreted in the urine.
Clearing the blood of drugs and other poisonous substances.
Regulating blood clotting.
Resisting infections by making immune factors and removing bacteria from the bloodstream.
Clearance of bilirubin from the breakdown of red blood cells. If bilirubin accumulates, the skin and eyes turn yellow.

Grouped summary

Group	Functions
Metabolism	Glycogen conversion, blood amino acid levels, cholesterol and plasma proteins, ammonia to urea
Storage	Glycogen and iron
Breakdown and excretion	Bile production, bilirubin clearance, clearing drugs and poisons; harmful products go into the bile or blood
Protection and blood	Immune factors, removing bacteria, blood clotting

The liver regulates most chemical levels of the blood and processes the blood that comes from the stomach and intestines. More than 500 vital functions have been identified with the liver.

98 marks

What is the structure of pancreas? Explain the functions of pancreas as an exocrine gland.

Solution

Structure

The pancreas is a complex organ with both endocrine and exocrine tissues. It has a head, which lies inside the C-shaped curve of the duodenum, a neck, a body that lies behind the stomach, and a tail that extends to the spleen. The exocrine part is made of acinar cells, which secrete enzymes, and duct cells, which secrete bicarbonate and water. The endocrine part is made of the pancreatic islets (islets of Langerhans).

Figure 1.15: Structure of the pancreas (schematic)

Exocrine secretion

The exocrine secretion of the pancreas is pancreatic juice. It has two components: an aqueous component of bicarbonate, from the duct cells, and an enzymatic component, from the acinar cells. The bicarbonate neutralises the acidic chyme entering the small intestine.

Enzymes of pancreatic juice

Enzyme	Secreted as	Action
Trypsin	Trypsinogen (activated by enterokinase)	Breaks proteins into polypeptides; activates more trypsinogen, chymotrypsinogen and procarboxypeptidase
Chymotrypsin	Chymotrypsinogen (activated by trypsin)	Breaks proteins into smaller peptides
Carboxypeptidase	Procarboxypeptidase (activated by trypsin)	Splits peptides
Pancreatic amylase	Active enzyme	Breaks polysaccharides into maltose and glucose
Pancreatic lipase	Active enzyme	Splits fats into monoglycerides, diglycerides, glycerol and fatty acids
Deoxyribonuclease and ribonuclease	Active enzymes	Break DNA and RNA into their nucleotides

Why the enzymes are inactive when secreted

Trypsinogen, chymotrypsinogen and procarboxypeptidase are secreted in inactive forms. If they were active, they would digest the tissues of the pancreas that produce them. They are activated only after they reach the duodenum.

Control

Secretin stimulates the duct cells to secrete water and bicarbonate, and the enzymes from the acinar cells are then flushed out through the pancreatic duct. CCK, released by fatty acids and proteins, stimulates the release of enzymes (see the answer to question 7).

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, MCQ 61 mark

HCl and Intrinsic factors are secreted by the stomach cells:

AChief cells
BParietal cellsAnswer
CGoblet cells
DZymogen cells

Why

Hydrochloric acid and intrinsic factor are both made by the parietal (oxyntic) cells of the gastric glands. Chief cells (also called zymogenic or zymogen cells, so options A and D are the same cell type) make only the inactive enzyme pepsinogen. Goblet cells are mucus-secreting cells; the mucus of the stomach comes from its surface mucous cells.

2026 Annual I, Set B, Q.2 (iii)1+2 = 3 marks

Which component of bile helps in digestion? And how? Explain briefly.

Solution

Component (1 mark)

The bile salts (sodium glycocholate and sodium taurocholate). Bile contains no digestive enzymes, so it is the bile salts that help digestion.

How they help (2 marks)

Emulsification: bile salts reduce the surface tension of large fat globules and break them into tiny droplets. The droplets have a much greater total surface area, so pancreatic lipase can act on the fat faster.
Absorption: bile salts also help the fatty acids to be absorbed from the intestine.

2026 Annual I, Set B, Q.2 (viii) OR1.5+1.5 = 3 marks

Why does stomach produce HCl and mucin?

Solution

Why it produces HCl (1.5 marks)

Gives the low pH (about 2) at which pepsin works best.
Kills bacteria that enter with the food.
Stops the digestion of carbohydrates by inactivating salivary amylase.
Denatures proteins so that proteolytic enzymes can reach the peptide bonds inside them.
Changes inactive pepsinogen into active pepsin.

Why it produces mucin (mucus) (1.5 marks)

The mucus of the stomach is viscous and alkaline. It forms a thick layer over the epithelial cells that lubricates the lining and protects it from the acidic chyme and from pepsin, so the acid and enzyme digest the food and not the stomach wall.

2026 Annual I, Set B, Q.5 OR7 marks

Explain the sequence of events that take place during swallowing and peristalsis of food.

Solution

Swallowing

Swallowing moves the bolus of food from the mouth to the stomach. It is a set of reflexes in three phases.

Phase 1: oral phase (voluntary)

The lips close and the food is moistened with saliva, and the tongue forms it into a bolus.
The tongue pushes the bolus to the back of the throat. This is the only voluntary part of swallowing.

Phase 2: pharyngeal phase (reflex)

Tactile receptors in the pharynx are stimulated and the swallowing reflexes start.
The soft palate lifts and closes the nasal airways; the tongue blocks the mouth so food cannot go back.
The epiglottis is pushed back over the larynx, the larynx and vocal folds cover the entrance of the trachea and breathing stops for a moment.
The upper esophageal sphincter opens and the bolus enters the esophagus.

Phase 3: esophageal phase (involuntary)

The bolus is carried down the esophagus by peristalsis.
The lower esophageal sphincter relaxes to let the bolus into the stomach, then closes so acid cannot come back up. The larynx returns to its original position.

Phase	Control	Main events
1. Oral	Voluntary	Bolus formed with saliva; tongue pushes it back
2. Pharyngeal	Reflex	Soft palate closes nose; epiglottis covers larynx; upper esophageal sphincter opens
3. Esophageal	Involuntary	Peristalsis carries bolus to the stomach

Peristalsis

Peristalsis is a wave-like, involuntary muscular contraction that pushes food forward (anterograde) along the digestive tract. It is controlled mainly by the myenteric plexus, which lies between the longitudinal and circular muscle layers.

Peristalsis: a wave of relaxation ahead of the bolus and a wave of contraction behind it.

The circular and longitudinal muscles ahead of the bolus relax, so the tube widens and makes room.
The muscles behind the bolus contract and squeeze it forward.
The wave moves on along the tube, so the bolus travels in one direction only.

Peristalsis moves food down the esophagus, mixes it with juices and moves chyme along the small intestine, and drives waste towards the rectum in the large intestine.

Note. The question joins the book's long question 1 (swallowing) with short question 3 (peristalsis).

2026 Annual I, Set D, MCQ 71 mark

Which enzyme is primarily responsible for protein digestion in the small intestine?

ALipase
BMaltase
CAmylase
DTrypsinAnswer

Why

Trypsin is a protease from the pancreas (released as trypsinogen and activated in the small intestine); it breaks proteins into smaller polypeptides. Lipase acts on fats, and maltase and amylase act on carbohydrates, so none of them digests protein.

2026 Annual I, Set D, Q.2 (viii) OR3 marks

How are fats absorbed in small intestine?

Solution

Fats are digested (after emulsification by bile salts) by pancreatic lipase into fatty acids and glycerol (and monoglycerides).
These products are absorbed by the epithelial cells of the villi in the ileum.
Inside the epithelial cells they are recombined into fats and mixed with cholesterol and protein to form small globules called chylomicrons.
Chylomicrons leave the epithelial cells by exocytosis and enter the lacteal (lymph capillary) in the villus.
The lymph carrying the chylomicrons drains from the lymphatic system into large veins that return blood to the heart.

Routes of absorption: fats enter the lacteal, glucose and amino acids enter the blood capillaries.

2026 Annual I, Set D, Q.2 (xii)1x3 = 3 marks

State any three roles of liver.

Solution

Bile production: bile emulsifies fats in the small intestine and helps carry waste away.
Storage and control of glucose: excess glucose is converted to glycogen for storage and returned to the blood as glucose when needed.
Urea formation: poisonous ammonia is converted to urea, which is excreted in the urine.
Detoxification: it clears the blood of drugs and other poisonous substances.
Plasma proteins: it makes albumin, globulins and fibrinogen (and so regulates blood clotting).

Note. Any three of the functions listed in the book's long question 8.

2026 Annual I, Set D, Q.5 (a)3 marks

Draw labelled diagram of human stomach.

Solution

Human stomach with its divisions and curvatures.

Label: esophagus, cardia, fundus, body, lesser curvature, greater curvature, pyloric region (antrum, canal and sphincter) and the duodenum.

Cardia: around the opening from the esophagus.
Fundus: rounded part above and to the left of the cardia.
Body: large central part where food is churned.
Pylorus: lower part leading to the duodenum; the pyloric sphincter controls the passage of chyme.

Note. Part (a) is the diagram of the book's long question 2.

2026 Annual I, Set D, Q.5 (b)4 marks

Explain chemical digestion process in human stomach.

Solution

Chemical digestion in the stomach is carried out by the gastric juice (about 2 to 3 litres a day), which contains mucus, hydrochloric acid, intrinsic factor, pepsinogen and the hormone gastrin.

Secretion	Cell	Role in chemical digestion
Hydrochloric acid	Parietal cells	Gives pH about 2; kills bacteria; stops salivary amylase; denatures proteins; activates pepsinogen
Pepsinogen	Chief cells	Inactive enzyme released by exocytosis
Pepsin	Formed in the lumen	Active protease, best at pH 2; breaks proteins into smaller peptide chains
Mucus	Mucous cells	Protects the wall from acid and pepsin
Intrinsic factor	Parietal cells	Binds vitamin B12 for absorption in the ileum
Gastrin	Endocrine cells	Hormone that stimulates more gastric juice

Activation of pepsin

Chief cells release inactive pepsinogen into the lumen.
HCl (and pepsin already present) changes pepsinogen to active pepsin.
Pepsin splits proteins into smaller peptide chains.
Active pepsin also converts more pepsinogen, so the supply grows quickly.

Activation of pepsin.

Protein digestion is the main chemical digestion here. Salivary amylase is stopped by the acid, so carbohydrate digestion pauses until the small intestine; the stomach digests very little fat. The churned food becomes the thick liquid chyme, which passes through the pyloric sphincter in small amounts.

Exam focus

Draw a labelled diagram of the human stomach and explain chemical digestion in it (HCl, pepsinogen to pepsin, mucus, gastrin). (long)
Set D Q.5 (a) and (b) (3 + 4 marks) in 2026; book long question 2.
Go to answer
Explain swallowing (three phases) and peristalsis, with the sequence of events. (long)
A 7-mark Section C question in 2026 (Set B Q.5 OR); book long question 1 and short question 3.
Go to answer
Absorption of digested products, especially how fats are absorbed through the lacteal as chylomicrons. (short)
Set D Q.2 (viii) OR in 2026; book long question 4 and short question 12.
Go to answer
State the roles (functions) of the liver. (short)
Set D Q.2 (xii) in 2026 (1 x 3); book long question 8 and short question 18.
Go to answer
Which component of bile helps digestion, and how: bile salts and emulsification. (short)
Set B Q.2 (iii) in 2026; book long question 6 and short question 15.
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Why the stomach makes HCl and mucus, and which cells make each secretion. (short)
Set B Q.2 (viii) OR and MCQ 6 in 2026; book short questions 4, 7 and 8.
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Digestive enzymes: which enzyme digests which food, and where (for example trypsin for proteins). (mcq)
Set D MCQ 7 in 2026; several book exercise MCQs on pepsin, trypsin and lipase.
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Hormonal control of digestion: gastrin for gastric juice, secretin for bile and pancreatic juice. (long)
Not asked in the 2026 papers, but book long question 7, short questions 16 and 17 and MCQs 12 and 14 test it.
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