Respiratory system of man

About 70% of carbon dioxide travels as bicarbonate ions (\ce{HCO3-}), formed inside the red blood cells from \ce{CO2 + H2O}. Carbonate ions (A) and hydroxyl ions (D) are not the main carrier, and free hydrogen ions (C) are buffered by haemoglobin.

Inspiration is the active phase. The diaphragm contracts, its dome flattens and it moves downward, which increases the volume of the thoracic cavity. Options A and B describe relaxation, which happens in expiration, and option C has the movement the wrong way.

Respiratory gases reach the cells and leave the body in the blood, which the circulatory system carries from the lungs to the tissues and back. The skin (A), excretory system (B) and muscular system (D) are not the transport route for oxygen and carbon dioxide.

The respiratory surface is the area where gas exchange actually takes place, and in man this is the alveoli. The larynx (A), trachea (B) and bronchi (C) are air passages that carry air to the alveoli but do not exchange gases.

About 97% of the oxygen is bound to haemoglobin in the red blood cells as oxyhaemoglobin. Only about 3% is dissolved in the plasma (A), and bicarbonate (C) is a way of carrying carbon dioxide, not oxygen.

The side walls of the thorax are formed by the ribs and the intercostal muscles that lie between them. The diaphragm (D) is the floor of the thoracic cavity, separating it from the abdomen, so it is not a lateral wall. Ribs alone (A) or muscles alone (B) do not complete the wall.

The breathing centre responds mainly to the carbon dioxide level in the blood. An excess of carbon dioxide in the blood speeds up breathing most strongly, while a lack of oxygen (A and B) has a much weaker effect.

Diffusion depends on the gradient. Lowering the partial pressure of carbon dioxide in the alveolar air makes the gradient from the blood into the alveoli steeper, so carbon dioxide leaves the blood faster. Holding the breath (A) and damaged alveoli (B) both slow the exchange, and less blood flow through the lungs (D) means less carbon dioxide reaches the alveoli.

Breathing moves air into and out of the lungs, which is ventilation. Cellular respiration (B) is the release of energy inside cells, diffusion (D) is the process that moves gases across the respiratory surface, and counter current exchange (A) is a different mechanism.

Air passes from the pharynx (1) to the larynx (4), then the trachea (3), a bronchus (2), a bronchiole (6) and finally the alveolus (5), which gives 1, 4, 3, 2, 6, 5. Option B puts the alveolus before the bronchiole, and options A and C put the larynx before the pharynx.

During inspiration the rib cage moves up and forward, so the sternum moves ventrally and anteriorly. The thoracic cavity gets larger (not smaller, A), the diaphragm contracts rather than relaxes (B), and the external intercostal muscles contract (C).

Forced exhalation can push out air beyond what is breathed out normally. This extra air is the expiratory reserve volume. Tidal volume (A) is the air moved in a normal breath, and residual volume (B) is the air that stays in the lungs.

Each haemoglobin molecule has four globin chains, and each chain carries one iron-containing haem group, so it has four haem groups and can bind four oxygen molecules. Option A is wrong because haemoglobin also carries carbon dioxide (as carbaminohaemoglobin) and carbon monoxide. Option C is wrong because haemoglobin is inside the red blood cells.

Total lung capacity is the largest volume of air the lungs can hold after a full, forced inhalation. Vital capacity (B) is smaller because it is the most that can be breathed out after the deepest breath. Tidal volume (A) is the air in one normal breath, and ventilation rate (C) is the number of breaths per minute.

Exercise raises body temperature, raises the carbon dioxide level and lowers the pH of the blood (more \mathrm{H^+}). The unit lists these factors as changing how strongly haemoglobin holds oxygen, and a rise in temperature, a fall in pH or a rise in carbon dioxide makes oxyhaemoglobin release its oxygen. A decrease in \mathrm{H^+} (B) raises the pH and holds oxygen more tightly, and a fall in temperature (A) does the same.

Air passes from the pharynx into the larynx, then the trachea, and then the bronchi. Option A places the trachea before the larynx, which is the wrong order, and options C and D start with the larynx, which comes after the pharynx.

The pharynx is the passage behind the nasal and oral cavities, which is commonly called the throat. The windpipe (A) and trachea (B) are the same structure below the larynx, and the voice box (C) is the larynx.

Air moves from the trachea into the bronchi, then the bronchioles, then the alveoli, where gas exchange happens. The pulmonary capillaries surround the alveoli and carry blood, so they are not part of the air path and come last. Option A places the capillaries before the alveoli, which is wrong.

Oxygen diffuses from the alveoli into the blood, so its partial pressure must be higher in the alveoli than in the blood. Option A compares oxygen with carbon dioxide, which is not the relevant comparison, and options B and D would stop oxygen from diffusing in.

The respiratory surface is the area where gas exchange takes place, and in man this is the alveoli. The larynx, trachea and bronchi are air passages only.

The respiratory surface is the area where gaseous exchange with the environment actually takes place. To allow diffusion to work effectively, it must have these properties:

• It is moist and permeable, so that gases can pass through it.
• It is thin, because diffusion is efficient only over a distance of 1 mm or less.
• It has a large surface area, so that enough gas can be exchanged to meet the body's needs.
• It has a good blood supply, which helps to keep a steep diffusion gradient across it.
• It has a good ventilation mechanism, which keeps the gradient steep by bringing in fresh air.

The human respiratory system is divided into two regions.

• Upper respiratory tract: the nostrils, nasal cavities and pharynx.
• Lower respiratory tract: the larynx, trachea, bronchi and bronchioles, alveolar ducts and alveoli, and the lungs.

Gas exchange takes place in the alveoli of the lungs.

The nose and nasal cavity filter the air before it reaches the lungs in three ways:

• Nose hair in the vestibules traps large particles as air enters through the nostrils.
• The nasal cavities are lined with ciliated mucous membrane. The mucus traps dust, microorganisms and other particulate matter.
• The cilia beat and move the trapped substances to the pharynx, where they are removed, so the air reaching the lungs is clean.
• The mucus moistens the air, and blood capillaries under the mucous membrane warm it to about 30^\circ\mathrm{C}.

The pharynx is a cone-shaped passage that leads from the oral and nasal cavities to the oesophagus and the larynx. It is part of both the digestive and the respiratory system.

• It carries inhaled air from the nasal and oral cavities down to the larynx and on to the lungs.
• It is divided into three sections: the nasopharynx, the oropharynx and the laryngopharynx.
• Because it is shared with the digestive tract, it lets air and food pass through the same region, and the epiglottis and larynx keep them apart.

• The larynx is an enlargement of the airway at the top of the trachea, below the pharynx. Its cartilages and muscles keep the airway open.
• The two vocal cords inside the larynx are responsible for vocalisation, which is how voice is produced.
• The epiglottis is a cartilage flap that is pressed down during swallowing. It partly covers the opening into the larynx (the glottis), so food and liquid do not enter the air passages.

Structure:

• Alveoli are tiny air-filled chambers at the ends of the alveolar ducts. There are over 700 million of them in the lungs.
• The wall is only 0.1\,\mu\mathrm{m} thick and is made of very thin, flat squamous epithelium.
• A dense network of blood capillaries surrounds each alveolus. Collagen and elastin in the walls let the alveoli expand and recoil easily.
• A film of surfactant lines the alveolus. It lowers the surface tension, which stops the alveolus from collapsing.

Function: the alveoli are the sites of gas exchange. Oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air.

The lungs have no muscles of their own. They are expanded and compressed by the diaphragm and the intercostal muscles, which change the volume of the thoracic cavity.

The chloride shift (also called the Hamburger phenomenon) is the exchange of bicarbonate and chloride ions across the membrane of the red blood cell. It keeps the ion balance on both sides of the membrane.

• In the tissues, carbonic acid dissociates inside the red blood cell into \mathrm{H^+} and \ce{HCO3-}. The negatively charged \ce{HCO3-} diffuses out into the plasma.
• To balance the charge, chloride ions (\ce{Cl-}) diffuse into the red blood cell in the opposite direction. A bicarbonate-chloride carrier protein in the membrane moves the two ions in opposite directions.
• In the lungs the reverse happens: \ce{HCO3-} enters the red blood cell and \ce{Cl-} leaves it.

• Large surface area: the millions of alveoli give a very large surface, so enough oxygen and carbon dioxide can be exchanged to meet the body's needs.
• Short diffusion distance: each alveolar wall is only 0.1\,\mu\mathrm{m} thick, and diffusion is efficient over distances of 1 mm or less.
• Good blood supply and moisture: a dense capillary network surrounds each alveolus, which keeps the gradient steep, and surfactant keeps the small alveoli from collapsing.

The two processes are linked: the oxygen taken in by external respiration is used in internal respiration, and the carbon dioxide made in internal respiration is removed by external respiration.

Cigarette smoking affects the respiratory system in these ways:

• It causes about 87% of lung cancers.
• It is a major cause of cancer of the mouth, larynx and oesophagus.
• It causes other lung diseases, such as chronic bronchitis and emphysema.
• Chemicals in the smoke irritate the air passages and lungs, which causes an early morning cough.
• Smoke damages the cilia, so they can no longer protect the lungs, and smokers are more likely to get pneumonia.
• Smoking makes breathing hard within a short time and can worsen asthma and allergies.

Introduction

The upper respiratory tract is the first part of the air passage. It consists of the nostrils, the nasal cavities and the pharynx.

Nose

• The external openings of the nose are the nostrils. The inner hollow spaces are the nasal cavities, two of them, separated by the nasal septum.
• The anterior part of each nasal cavity, near the nostrils, is the vestibule, which contains hair.
• The nostrils and nasal cavities are lined with ciliated mucous membrane.
• The nose is made of bone, cartilage and fatty tissue.

Functions of the nose

• Filtering: nose hair, mucus and cilia trap dust, microorganisms and particulate matter. The cilia move the trapped material to the pharynx.
• Warming: blood capillaries under the mucous membrane warm the air to about 30^\circ\mathrm{C}.
• Moistening: the mucus moistens the air before it reaches the lungs.
• Defence: the mucus and cilia prevent harmful pathogens from entering the respiratory system.

Pharynx

The pharynx is a cone-shaped passage that leads from the oral and nasal cavities to the oesophagus and the larynx. It is part of the digestive system and also of the respiratory system. It is divided into three sections:

• Nasopharynx: behind the nasal cavities.
• Oropharynx: behind the mouth.
• Laryngopharynx: above the larynx.

Introduction

The lower respiratory tract includes the larynx, the trachea, the bronchi and the lungs. Air passes down through these structures, and gas exchange takes place in the alveoli.

Larynx

• The larynx is an enlargement of the airway at the top of the trachea and below the pharynx.
• It is made mainly of cartilages and muscles. The largest cartilage, the epiglottis, partly covers the opening of the larynx during swallowing, so food does not enter the airway.
• The opening of the larynx is called the glottis. Two vocal cords inside the larynx are responsible for vocalisation.
• It is lined with mucous membrane.

Trachea

The trachea (windpipe) is a membranous tube made of dense regular tissue and smooth muscle. It is reinforced with 15 to 20 C-shaped pieces of cartilage, which keep it open.

Bronchi and bronchioles

• The trachea divides into two primary bronchi, one for each lung.
• Within each lung the primary bronchi give rise to secondary bronchi: there are two secondary bronchi in the left lung and three in the right lung.
• The secondary bronchi give rise to tertiary bronchi, and the branching continues to bronchioles, which are less than 1 mm in diameter.
• The bronchioles divide further into terminal bronchioles.
• In the secondary bronchi the C-shaped cartilages are replaced by cartilage plates. The bronchioles and their terminal branches have no cartilage.

Alveolar ducts and alveoli

Each terminal bronchiole divides into alveolar ducts, and these end in tiny air-filled chambers called alveoli. There are over 700 million alveoli in the lungs.

• Each alveolar wall is only 0.1\,\mu\mathrm{m} thick and is made of thin, flat squamous epithelium, which keeps the diffusion distance short.
• A dense network of blood capillaries surrounds each alveolus.
• Collagen and elastin in the walls let the alveoli expand and recoil during breathing, and surfactant keeps them from collapsing.

The lungs

• Each lung is conical in shape. Its base rests on the diaphragm and its apex reaches just above the clavicle.
• The lungs are separated in the middle by the mediastinum, the region that contains the heart.
• The left lung has two lobes (superior and inferior), and the right lung has three (superior, middle and inferior).
• The hilum is a triangular depression in each lung where the blood vessels and airways enter.

Muscles involved

• The diaphragm is a large dome of skeletal muscle that separates the thoracic cavity from the abdominal cavity.
• There are two sets of intercostal muscles between each pair of ribs: the external intercostals and the internal intercostals. Their fibres run diagonally in opposite directions.

The lungs do not move by themselves. The diaphragm and the intercostal muscles change the volume of the thoracic cavity, and the lungs follow. Breathing has two phases: inspiration and expiration.

Inspiration (active)

1. The diaphragm contracts and flattens, so it moves down. The external intercostals contract and the internal intercostals relax.
2. The rib cage moves up and forward (outward).
3. The volume of the thoracic cavity increases, so the pressure in the thorax and lungs falls below atmospheric pressure.
4. Air flows in from the atmosphere and the alveoli are inflated.

Expiration (passive)

1. The diaphragm relaxes and rises into a dome. The external intercostals relax and the internal intercostals contract.
2. The rib cage moves down and backward (inward).
3. The volume of the thoracic cavity decreases, so the pressure in the lungs rises above atmospheric pressure.
4. Air is forced out of the lungs.

Breathing is normally controlled involuntarily. A breathing centre in the medulla of the brain carries out this control.

The breathing centre

• The inspiratory centre is the ventral portion of the breathing centre. It increases the rate and depth of inspiration.
• The expiratory centre is formed by the dorsal and lateral portions. It inhibits inspiration and stimulates expiration.

Conscious and emotional control

• Through the cerebral cortex, a person can consciously or unconsciously increase or decrease the rate and depth of breathing.
• A person can also stop breathing voluntarily. If the blood partial pressure of oxygen falls low enough, the person loses consciousness, and then the breathing centre resumes automatic control.
• Emotions act through the limbic system of the brain and can also affect the breathing centre.

Overview

About 97% of the oxygen is carried by the red blood cells as oxyhaemoglobin. The remaining 3% is dissolved in the plasma.

Haemoglobin

• Haemoglobin has four globin protein chains, and each chain is associated with one haem group, which contains iron.
• Each haemoglobin molecule can bind four oxygen molecules. The binding is reversible.
• One gram of haemoglobin combines with 1.34 mL of oxygen, and a 100 mL sample of blood contains about 15 g of haemoglobin.

Loading and unloading

1. In the lungs, the partial pressure of oxygen is high. Oxygen diffuses from the alveoli into the blood and binds to haemoglobin.
2. Binding is reversible and forms oxyhaemoglobin. The equation is shown below.
3. In the tissues, the partial pressure of oxygen is low, so oxyhaemoglobin releases its oxygen, which diffuses into the cells.
4. Oxygen carrying capacity is sensitive to temperature, to the pH of the blood and to the partial pressures of carbon dioxide and oxygen. A rise in temperature, a drop in pH or a rise in carbon dioxide makes oxygen release easier.

Amounts carried

Each 100 mL of blood therefore releases 19.4 - 14.4 = 5.0\ \mathrm{mL} of oxygen to the tissues.

Carbon dioxide is carried in the blood in three ways, and most of it travels as bicarbonate ions.

Bicarbonate ions (70%)

1. Carbon dioxide diffuses from the tissues into the plasma and then into the red blood cells.
2. Inside the red blood cell, carbon dioxide combines with water to form carbonic acid. The enzyme carbonic anhydrase speeds up this reaction.
3. Carbonic acid is unstable and dissociates into hydrogen ions and bicarbonate ions.
4. The hydrogen ions bind to haemoglobin, which buffers them and keeps the pH near 7.4. This also releases oxygen from oxyhaemoglobin.
5. The bicarbonate ions diffuse out into the plasma, and chloride ions move in the opposite direction (the chloride shift).
6. In the lungs the reactions run in reverse: bicarbonate re-enters the cell, combines with hydrogen ions to form carbonic acid, which breaks down into carbon dioxide and water. The carbon dioxide diffuses into the alveoli and is breathed out.

Carbaminohaemoglobin (23%)

Carbon dioxide combines with the globin part of haemoglobin. The reaction depends on the partial pressure of carbon dioxide. In the tissues, where PCO₂ is high, carbaminohaemoglobin forms. In the lungs, where PCO₂ is lower, it releases its carbon dioxide.

Dissolved carbon dioxide (7%)

Only about 7% of carbon dioxide dissolves directly in the plasma. This is an inefficient method, but it does occur.

Respiratory pigments are coloured molecules that carry oxygen by binding to it reversibly. Every known respiratory pigment has a coloured, non-protein part, the haem group. In man the two respiratory pigments are haemoglobin and myoglobin.

Haemoglobin

• Haemoglobin transports oxygen from the lungs to the tissues. It is found in the red blood cells.
• It has four globin chains, each with one haem group, so each molecule binds up to four oxygen molecules.
• At high oxygen concentrations it combines with oxygen, and at low oxygen concentrations it quickly releases the oxygen.

Myoglobin

• Myoglobin has one polypeptide chain with one iron-containing haem group, so it binds one oxygen molecule.
• It is found in skeletal muscle and is the main reason meat looks red.
• It stores oxygen in the muscle and releases it when the partial pressure of oxygen falls below 20 mmHg. It is an intermediate that passes oxygen from haemoglobin to the aerobic processes of the muscle cells.

Definition

Sinusitis is an inflammation of the nasal sinuses, which are air spaces in the skull between the facial bones. It may be acute, when the symptoms last 2 to 8 weeks, or chronic, when they last much longer.

Causes

• Cold and wet climate.
• Atmospheric pollution, smoke and dust.
• Overcrowding.
• Dental infections and viral infections.

Symptoms

• Fever and nasal obstruction.
• A raspy voice and a pus-like nasal discharge.
• Loss of the sense of smell.
• Facial pain or a headache, which is sometimes made worse by bending over.

Treatment

• If the infection is bacterial, antibiotics or sulpha drugs are usually prescribed.
• The physician may also prescribe nebulisation, which reduces the inflammation in the sinuses and nose and speeds up recovery.

Definition

Otitis media is an inflammation or infection of the middle ear.

Causes

Middle ear infections usually result from a malfunction of the eustachian tube, which connects the middle ear to the throat. When the tube does not work properly, fluid cannot drain from the middle ear, and fluid builds up behind the eardrum. Bacteria and viruses can then grow in this fluid and cause acute otitis media. The main reasons are:

1. A cold or an allergy, which can cause swelling and congestion of the lining of the nose, throat and eustachian tube. The swelling blocks the normal drainage of fluid from the ear.
2. A malformation of the eustachian tube.

Symptoms

Each child may experience the symptoms differently. The most common symptoms include:

• unusual irritability and difficulty sleeping or staying asleep
• tugging or pulling at one or both ears, and ear pain
• fever, especially in infants and younger children
• fluid draining from the ear, loss of balance and hearing difficulties

Treatment

• Antibiotic medication, taken by mouth or as ear drops.
• Medication for pain and fever.
• A combination of the above.

Definition

Pneumonia is an infection that inflames the air sacs (alveoli) in one or both lungs. The air sacs may fill with fluid or pus, which causes coughing with phlegm or pus, fever, chills and difficulty in breathing.

Causes

A variety of organisms can cause pneumonia: bacteria, viruses and fungi.

Symptoms

• Chest pain when breathing or coughing, and a cough that may produce phlegm.
• Confusion or changes in mental awareness (in adults aged 65 and over).
• Fatigue, fever, sweating and shaking chills.
• Lower than normal body temperature (in adults over 65 and people with weak immune systems).
• Nausea, vomiting or diarrhoea, and shortness of breath.

Treatment

1. Antibiotics: pneumonia caused by bacteria is treated with an antibiotic.
2. Antifungal medication: pneumonia caused by a fungal infection is treated with antifungal drugs.
3. Antiviral medication: viral pneumonia is usually not treated with medication and can go away on its own.
4. Draining of fluid: if there is a lot of fluid between the lungs and the chest wall (pleural effusion), a physician may drain it with a catheter or by surgery.

Definition

Pulmonary tuberculosis (TB) is a highly contagious, chronic bacterial infection of the lungs.

Cause

It is caused by Mycobacterium tuberculosis.

How the tubercle forms

Symptoms

• A low-grade fever, usually in the evening, and night sweats.
• Weight loss, anorexia, depression and weakness.
• A dry cough with sputum, and a dull ache in the chest caused by inflammation of the pleura.

Treatment

Taking medicines regularly for 9 months can cure TB. This is called Daily Observed Treatment Short Course (DOTS). The patient takes the medicines under supervision, so that the full course is completed.

Definition

Emphysema is a lung disease that results from damage to the walls of the alveoli. A blockage (obstruction) may develop that traps air inside the lungs. With fewer alveoli, less oxygen passes into the bloodstream.

Causes

• Smoking, which is the main cause.
• Marijuana, vaping and e-cigarettes.
• Cigar smoke, air pollution and dust.

Symptoms

• Long-term coughing (smoker's cough) and wheezing.
• Shortness of breath, especially during light exercise such as climbing steps.
• A constant feeling of not being able to get enough air, and a tightness in the chest.
• Increased mucus production, with abnormal mucus colour (yellow or green).
• Ongoing fatigue and heart problems.

Treatment

1. Quitting smoking.
2. Bronchodilators.
3. Inhaled and oral corticosteroids.
4. Antibiotics and anti-inflammatory medications.

Definition

Chronic obstructive pulmonary disease (COPD) is the name for a group of lung conditions that cause breathing difficulties. It includes emphysema, which is damage to the air sacs, and chronic bronchitis, which is long-term inflammation of the airways.

Causes

• Smoking, which is the main cause. The lungs become inflamed, damaged and narrowed.
• Long-term exposure to harmful fumes or dust.
• A rare genetic problem that makes the lungs more vulnerable to damage.

Symptoms

• Shortness of breath.
• A persistent chesty cough with phlegm.
• Frequent chest infections.
• Persistent wheezing.

Treatment

1. Stopping smoking.
2. Inhalers and medicines to make breathing easier.
3. Pulmonary rehabilitation: a specialised programme of exercise and education.
4. Surgery or a lung transplant, which is an option for only a very small number of people.