Thermoregulation and homeostasis

8.1ThermoregulationOften examined

Humans, like most mammals, can maintain a constant body temperature despite changes in the surroundings. The core temperature of the human body stays reasonably constant at $37^{\circ }\mathrm{C}$. It can fluctuate by a degree or so, more during fever, but it is generally very stable.

Heat is produced inside the body as a byproduct of metabolic reactions. Heat production occurs more generally in the organs of the body. Since heat is a byproduct of metabolism, the amount of heat produced depends on the metabolic rate.

Thermoregulation and its needs

Thermoregulation

The maintenance of internal temperature within a range that allows cells to function efficiently.

The body works to maintain a stable internal temperature. A temperature colder or warmer than the optimum range of the enzymes changes the shape of the active site, and the chemical reactions stop. The main purpose of thermoregulation is therefore to keep the enzyme systems of the body working properly.

8.1.2Classification of animals on the basis of temperatureOften examined

Animals can be classified on the basis of the ability to thermoregulate and on the source of body heat.

a. Classification on the basis of the ability to thermoregulate

Animals can be classified, on the basis of their ability to keep a constant body temperature, as poikilotherms and homeotherms.

Poikilotherms

All non-vertebrates, fishes, amphibians and reptiles. They are unable to maintain their body temperature within narrow limits using physiological mechanisms.

Homeotherms

Birds and mammals, which are able to maintain a fairly constant body temperature by using physiological mechanisms.

b. Classification on the basis of the source of body heat

Animals are also classified on the basis of the source of the body's heat, as ectotherms and endotherms.

Ectotherms

Animals that produce metabolic heat at a low level, which is also exchanged quickly with the environment. They rely more on heat derived from the environment to raise their body temperature. Examples are most invertebrates, fishes, amphibians and reptiles.

Endotherms

Animals that produce their own body heat as a byproduct during metabolism in muscles, or by the action of hormones that increase metabolic rate. Examples are birds and mammals.

8.1.3Thermoregulatory strategies in manOften examined

The thermoregulatory centre in the human body is located in the hypothalamus, which acts as a thermostat. It can detect the temperature of the blood that passes through it. If the temperature increases or decreases even slightly, the hypothalamus initiates corrective responses such as sweating or shivering.

When a person meets a particularly warm or cold environment, temperature receptors in the skin inform the hypothalamus. They also stimulate the higher, voluntary centres of the brain. This means that the person "feels" the change and changes clothing or turns the heating up or down. Often this behavioural response corrects the situation without the need for any physiological response.

I. Physiological responses to heat

There are two main physiological responses when the hypothalamus detects an increase in blood temperature: vasodilation and sweating.

a. Vasodilation

One way to increase heat loss is to supply the capillaries in the skin with a greater volume of blood, which then loses heat to the environment by radiation. Arterioles have muscles in their walls that can relax or contract to allow more or less blood to flow through them. During vasodilation these muscles relax, causing the arterioles near the skin to dilate and allowing more blood to flow through the capillaries.

b. Sweating

Sweat is a salty solution made by sweat glands. Evaporation of sweat from the skin's surface leads to cooling. As the sweat evaporates, it absorbs heat from the body, cooling the skin and helping to regulate internal body temperature. The efficiency of sweating depends on humidity. In dry air, humans can tolerate a temperature of $65^{\circ }\mathrm{C}$ for several hours. In humid air, however, when the sweat cannot evaporate, a temperature of only $56^{\circ }\mathrm{C}$ causes overheating.

II. Physiological responses to cold

There are four main physiological responses when the hypothalamus detects a drop in blood temperature: shivering, vasoconstriction, piloerection and increased metabolic rate.

a. Shivering

Shivering happens when muscles contract and relax rapidly. Shivering muscles give out four or five times as much heat as resting muscles.

b. Vasoconstriction

One way to decrease heat loss is to supply the capillaries in the skin with a smaller volume of blood, minimising the loss of heat to the environment by radiation. During vasoconstriction the muscles in the arteriole walls contract, so the arterioles near the skin constrict and less blood flows through the capillaries. The blood is diverted through shunt vessels, which lie further down in the skin and therefore do not lose heat to the environment. Vasoconstriction reduces heat loss from the blood as it flows through the skin.

c. Piloerection

Piloerection means literally "erection of hairs" and involves a reflex. In most mammals it makes the fur "thicker", so that it traps more air to provide extra insulation. In humans the erector pili muscles in the skin pull the tiny hairs upright and succeed in creating goose bumps.

d. Increased metabolic rate

The body secretes the hormone adrenaline in response to cold. This raises the metabolic rate and therefore increases heat production.

Thermoregulation as a feedback loop

Figure 8.3 puts the responses together as a loop: a stimulus (a change in body temperature) is detected by receptors, the message travels along the afferent pathway to the control centre, which sends instructions along the efferent pathway to the effectors, and the response brings the temperature back so that the stimulus ends. The parts of such a loop are explained in section 8.2.1.

8.2HomeostasisOften examined

Animals have two environments in their lives: an external environment in which the organism is situated, and an internal environment in which the tissues live. The external environment consists of varying conditions of atmosphere, marine water and fresh water. The internal environment is formed by the interstitial fluid (tissue fluid) that surrounds and bathes all the tissues, and by circulating fluids such as lymph and the plasma, the liquid part of the blood.

Homeostasis

The process by which organisms keep internal conditions relatively constant despite changes in the external environment. It means "keeping things in balance".

How organ systems contribute

The human organ systems work constantly to maintain a controlled, stable environment.

• The digestive system takes in and digests food, providing nutrient molecules that enter the blood and replace the nutrients that are constantly being used by the body cells.
• The respiratory system adds oxygen to the blood and removes carbon dioxide. The amount of oxygen taken in and carbon dioxide given off can be increased to meet the needs of the body.
• The liver and kidneys contribute greatly to homeostasis. Immediately after glucose enters the blood it can be removed by the liver and stored as glycogen. Later the glycogen is broken down to replace the glucose used by the blood cells, so the glucose composition of the blood stays the same. The hormone insulin, secreted by the pancreas, regulates glycogen storage.
• The kidneys are also under hormonal influence as they excrete wastes and salts, substances that can affect the pH of the blood.

8.2.1Components of homeostatic mechanismsOften examined

Adjustment of physiological systems within the body is called homeostatic regulation. It involves three parts or mechanisms: (1) the receptor, (2) the control centre and (3) the effector.

Receptor

The receptor receives information that something in the environment is changing. It responds to changes in the environment, either external or internal. Receptors include thermoreceptors and mechanoreceptors.

Control centre

The control centre or integration centre receives and processes information from the receptor. The control centres include the renin-angiotensin system (RAS).

For your information: the renin-angiotensin system

The renin-angiotensin system (RAS), also known as the renin-angiotensin-aldosterone system (RAAS), is a critical hormonal system that regulates blood pressure and fluid balance. It is the system of hormones, proteins, enzymes and reactions. It regulates your blood pressure by increasing sodium (salt) reabsorption, water reabsorption (retention) and vascular tone (the degree to which your blood vessels constrict, or narrow). The RAAS consists of three major substances: renin (an enzyme), angiotensin II (a hormone) and aldosterone (a hormone).

Effector

The effector responds to the commands of the control centre by either opposing or enhancing the stimulus. This ongoing process continually works to restore and maintain homeostasis. For example, during body temperature regulation, temperature receptors in the skin communicate information to the brain (the control centre), which signals the effectors: blood vessels and sweat glands in the skin. As the internal and external environments of the body are constantly changing, adjustments must be made continuously to stay at or near a specific value: the set point.

8.2.2Control of homeostatic mechanismOften examined

Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals.

Feedback

Feedback mechanism

A physiological regulation system in a living body that works to return the body to its normal internal state, commonly known as homeostasis. In a living system the feedback mechanism takes the shape of a loop, which aids in maintaining homeostasis.

Negative feedback mechanism

In negative feedback loops a change in a given direction causes a change in the opposite direction. For example, an increase in the concentration of a substance causes feedback that ultimately causes the concentration of the substance to decrease.

Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down; conversely, if a level is too low, the body does something to raise it. Hence the term negative feedback.

Example 1: blood glucose

When an animal has eaten, blood glucose rises, which is sensed by the nervous system. Specialised cells in the pancreas (part of the endocrine system) sense the increase and release the hormone insulin. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system. However, if an animal has not eaten and blood glucose levels decrease, this is sensed by a different group of cells in the pancreas, and the hormone glucagon is released, causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected from the use of the word "negative".

Example 2: blood calcium

If calcium levels decrease, specialised cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys. The effect of PTH is to raise blood levels of calcium. Negative feedback loops are the predominant mechanism used in homeostasis.

Positive feedback mechanism

In positive feedback loops a change in a given direction causes additional change in the same direction. An increase in the concentration of a substance causes feedback that produces continued increases in concentration.

Example 1: blood clotting

When a wound creates bleeding, the body responds by clotting the blood and preventing blood loss through a positive feedback loop. The wounded blood vessel wall releases substances that start the clotting process. Platelets in the blood begin to adhere to the wounded area and produce substances that attract more platelets. As the platelets continue to accumulate, more chemicals are released, and more platelets are drawn to the clot location. The clotting process is accelerated by the positive feedback until the clot is large enough to halt the bleeding.

Example 2: uterine contractions in childbirth

The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.

8.3 and 8.3.1Osmoconformers and osmoregulators

Osmoregulation

The maintenance of constant osmotic conditions (water and solute concentration) in the body.

With respect to their external environment, animals may be either osmoconformers or osmoregulators.

Osmoconformers

Osmoconformers

Animals that change the osmotic concentrations of their body fluids according to that of the surrounding medium. They are isotonic to their external environment.

These include all marine invertebrates, some freshwater invertebrates and some marine vertebrates such as Myxine (hagfishes) and elasmobranchs (sharks and rays).

Osmoregulators

Osmoregulators

Animals that can maintain an internal osmotic concentration different from that of the surrounding medium. Such animals are hypotonic or hypertonic to their environment.

Almost all of the freshwater animals and most of the marine animals are osmoregulators.

The unusual, higher osmotic concentration of marine sharks and rays than of other vertebrates of the marine habitat is maintained by high levels of urea and trimethylamine oxide (TMAO) in the blood. These organic substances are called osmolytes because they increase the osmotic (solute) concentration.

8.3.2Problems faced by osmoregulators

Each group of osmoregulators lives in a different osmotic situation, so each faces a different problem.

Freshwater animals

Since freshwater animals live in a hypotonic environment, water constantly enters the body and they also face a deficiency of salts. So they have to lose the excess water and maintain a higher salt concentration than their environment.

Marine bony fishes

On the other hand, most of the marine teleosts (bony fishes) are hypotonic to sea water. So these fishes have the problem of losing water to the environment, especially across the gill epithelium. They also have the problem of excess salts in the body due to drinking sea water.

Terrestrial animals

Terrestrial animals are also hypotonic to the outer environment. Evaporation of water, which leads to dehydration, is the major problem faced by these animals.

8.3.3Osmoregulatory adaptations in animals

Each habitat needs its own solution to the problem described in section 8.3.2. The book describes the adaptations of freshwater animals, marine animals and terrestrial animals.

Freshwater animals

Almost all freshwater animals are osmoregulators. They are generally hypertonic to their environment. They deal with the problems of too much water and too few salts in three ways.

• They produce a large volume of dilute urine, which removes the excess water.
• Their kidney reabsorbs the salts that are required.
• Salts are also obtained from the food they eat, and the animals actively transport salts from the external dilute medium with the help of special salt cells called ionocytes. Ionocytes are found in the amphibian skin and in the gills of fishes.

Ionocytes

Special salt cells that actively transport salts from the external dilute medium into the body. They are found in the amphibian skin and in the gills of fishes.

Marine animals

Teleosts (bony fishes) are osmoregulators in the marine environment and are hypotonic to their surroundings. They tend to lose water to the environment, especially across the gill epithelium. To replace the water loss, these fishes usually drink a large amount of water, unlike freshwater fishes.

Drinking sea water brings the problem of excess salts in the body. Among the excess salts, $\mathrm{Na}{}^{+}$, $\mathrm{Cl}{}^{-}$ and some $\mathrm{K}{}^{+}$ are removed across the gill epithelium, while the divalent ions $\mathrm{Mg}{}^{2+}$ and $\mathrm{Ca}{}^{2+}$ are excreted by the kidney. Some fishes also have special salt-secreting glands in the wall of the rectum, called rectal glands, that remove salts into the digestive tract, from which they are eliminated from the body during egestion.

Rectal glands

Special salt-secreting glands in the wall of the rectum of some marine fishes. They remove salts into the digestive tract, and the salts are eliminated during egestion.

Terrestrial animals

The successful groups of land animals are the arthropods among the invertebrates and the reptiles, birds and mammals among the vertebrates. The chitinous exoskeleton of arthropods and the dead keratinised skin of vertebrates are adaptations that reduce water loss from their bodies.

Desert mammals are very resistant to water loss. They can tolerate a strong degree of dehydration by special metabolic and behavioural adaptations. This characteristic is called anhydrobiosis. These animals feed on the seeds of desert plants, in which a large amount of carbohydrate is stored. When these compounds are broken down, water is produced as a by-product and is used by the animals.

Anhydrobiosis

The ability to tolerate a strong degree of dehydration, shown by desert mammals through special metabolic and behavioural adaptations.

The best example of such an animal is the kangaroo rat. It avoids the heat of the day and emerges at night. Ninety percent of the water that it uses is metabolic water derived from cellular oxidation.

Key terms

Thermoregulation

The maintenance of internal temperature within a range that allows cells to function efficiently.

Poikilotherms

Animals (all non-vertebrates, fishes, amphibians and reptiles) that cannot maintain their body temperature within narrow limits using physiological mechanisms.

Homeotherms

Birds and mammals, which maintain a fairly constant body temperature by using physiological mechanisms.

Ectotherms

Animals that produce metabolic heat at a low level, which is exchanged quickly with the environment, so they rely more on environmental heat to raise their body temperature.

Endotherms

Animals that produce their own body heat as a byproduct of metabolism in muscles, or by the action of hormones that increase metabolic rate.

Hypothalamus

The thermoregulatory centre of the human body. It acts as a thermostat and detects the temperature of the blood passing through it.

Vasodilation

Relaxing of the muscles in the walls of the skin arterioles so that they dilate and more blood flows through the skin capillaries, increasing heat loss by radiation.

Vasoconstriction

Contraction of the muscles in the walls of the skin arterioles so that they constrict, less blood flows through the capillaries and heat loss is reduced.

Sweating

Production of a salty solution by sweat glands; evaporation of sweat absorbs heat and cools the skin.

Shivering

Rapid contraction and relaxation of muscles, which give out four or five times as much heat as resting muscles.

Piloerection

Literally the erection of hairs: a reflex in which the erector pili muscles pull the hairs upright, trapping air for insulation (goose bumps in humans).

Erector pili

Muscles in the skin that pull the tiny hairs upright in piloerection.

Shunt vessels

Vessels lying further down in the skin through which blood is diverted during vasoconstriction, so that it does not lose heat to the environment.

Adrenaline

The hormone secreted in response to cold that raises the metabolic rate and increases heat production.

Homeostasis

The process by which organisms keep internal conditions relatively constant despite changes in the external environment.

External environment

The varying conditions of atmosphere, marine water and fresh water in which the organism is situated.

Internal environment

The interstitial (tissue) fluid and circulating fluids such as lymph and plasma that surround and bathe the tissues.

Homeostatic regulation

The adjustment of physiological systems within the body, involving a receptor, a control centre and an effector.

Receptor

The part of a homeostatic mechanism that receives information that something in the environment is changing, for example thermoreceptors and mechanoreceptors.

Control centre

The integration centre that receives and processes information from the receptor and commands the effector.

Effector

The part of a homeostatic mechanism that responds to the commands of the control centre by opposing or enhancing the stimulus.

Set point

The specific value at or near which a controlled condition is kept by continuous adjustments.

Renin-angiotensin system (RAS)

A hormonal system (renin, angiotensin II and aldosterone) that regulates blood pressure and fluid balance by increasing sodium and water reabsorption and vascular tone.

Feedback mechanism

A physiological regulation system in a living body, in the shape of a loop, that works to return the body to its normal internal state.

Negative feedback

A loop in which a change in a given direction causes a change in the opposite direction; the predominant mechanism of homeostasis.

Positive feedback

A loop in which a change in a given direction causes additional change in the same direction, pushing the organism further out of homeostasis.

Insulin

Hormone released by the pancreas when blood glucose is high; it lowers blood glucose and regulates glycogen storage.

Glucagon

Hormone released by the pancreas when blood glucose is low; it raises blood glucose.

Parathyroid hormone (PTH)

Hormone released by the parathyroid gland when blood calcium falls; it raises blood calcium by increasing absorption through the intestines and kidneys.

Oxytocin

Hormone that stimulates contraction of the uterus; in childbirth more oxytocin is produced until the contractions are powerful enough to produce birth.

Osmoregulation

The maintenance of constant osmotic conditions (water and solute concentration) in the body.

Osmoconformers

Animals that change the osmotic concentration of their body fluids according to that of the surrounding medium; they are isotonic to their environment.

Osmoregulators

Animals that maintain an internal osmotic concentration different from that of the surrounding medium (hypotonic or hypertonic to it).

Osmolytes

Organic substances such as urea and trimethylamine oxide (TMAO) that raise the osmotic (solute) concentration of body fluids.

Trimethylamine oxide (TMAO)

An osmolyte that, with urea, keeps the blood of sharks and rays at a high osmotic concentration and stabilises proteins against the effect of urea.

Teleosts

Bony fishes.

Ionocytes

Special salt cells that actively transport salts from the external dilute medium into the body, found in amphibian skin and fish gills.

Rectal glands

Salt-secreting glands in the wall of the rectum of some marine fishes that remove salts into the digestive tract.

Anhydrobiosis

The ability of desert mammals to tolerate a strong degree of dehydration through special metabolic and behavioural adaptations.

Metabolic water

Water produced as a by-product of the breakdown of carbohydrates and derived from cellular oxidation; 90 percent of the water used by the kangaroo rat.

FBISE 2026 questions

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

Exam focus

1. Describe the negative feedback mechanism with an example (blood glucose, calcium or body temperature) and a flow diagram. (long)
   The 7-mark Section C question of Set D (Q.3 OR). Book long question 5.
   Go to answer
2. Explain the physical responses of the body to heat (vasodilation and sweating) and draw a sketch. (long)
   Set B Q.6 OR (6 marks) and Set D Q.2 (xiv) (two ways of losing heat). Book long question 2, Figure 8.1.
   Go to answer
3. Explain the role of the receptor, control centre and effector in a homeostatic mechanism. (short)
   Set B Q.2 (xiv); book long question 4 and short question 6.
   Go to answer
4. Compare ectotherms and endotherms, and poikilotherms and homeotherms, with examples. (short)
   Set D Q.2 (ix) OR; book long question 1 and MCQ 1.
   Go to answer
5. State the problems faced by osmoregulators, especially marine bony fishes. (short)
   Set B Q.2 (ix) OR asks the challenges of marine osmoregulators; book short question 2.
   Go to answer
6. Name the responses of the body to cold (shivering, vasoconstriction, piloerection, higher metabolic rate) and say how each helps. (short)
   Set B MCQ 3 (shivering) and Set D MCQ 4 (vasoconstriction) test the cold responses.
   Go to answer
7. Compare negative and positive feedback loops with two examples of each. (long)
   Not asked in 2026 but a book long question (6 and 7) and the natural companion of the Set D Q.3 OR question.
   Go to answer
8. Describe the osmoregulatory adaptations of freshwater, marine and terrestrial animals, with labelled figures. (short)
   Related to Set B Q.2 (ix) OR; book short question 3 and Figures 8.9 and 8.10.
   Go to answer