Have you ever noticed how your skin turns red after a heavy workout, but looks pale on a freezing winter day? This visible shift is your body actively working to keep your internal temperature perfectly balanced. The human body maintains a constant core temperature of approximately .
This specific temperature is the optimum condition for human enzyme activity and metabolic rates. If your body becomes too hot, enzymes can denature; if it becomes too cold, metabolic reactions slow down to dangerous levels. The maintenance of this stable internal environment is called homeostasis.
Your brain acts as a sophisticated biological thermostat. The hypothalamus is a region in the brain that contains the thermoregulatory centre, which monitors and coordinates the control of body temperature.
It receives information from two different sources. First, receptors within the hypothalamus directly monitor the temperature of the blood flowing through the brain. Second, peripheral receptors in the skin (the epidermis and dermis) detect changes in the external temperature and send this information to the brain via electrical nerve impulses.
Thermoregulation relies on a continuous cycle called negative feedback. This is a regulatory mechanism where a change away from the ideal target value (the set point) triggers a response that reverses the change, restoring balance.
When a temperature challenge occurs, the body follows a specific coordination pathway:
When you get too hot, the hypothalamus triggers responses to increase heat loss to the environment. One key mechanism is vasodilation. The muscles in the walls of small blood vessels called arterioles relax, causing the vessels to widen (dilate).
This allows a greater volume of blood to flow through the capillaries near the skin surface, increasing heat transfer to the environment via radiation. Additionally, sweat glands secrete sweat onto the skin. As the water evaporation occurs, it transfers heat energy away from the body because it uses the latent heat of evaporation from the skin.
Finally, the microscopic hair muscles in the skin relax. The hairs lie flat, preventing an insulating layer of air from being trapped, which allows heat to be lost by convection.
When you are too cold, the body must conserve heat and generate more. The hypothalamus triggers vasoconstriction, where the muscles in the arteriole walls contract. This narrows the lumen of the blood vessels, reducing blood flow to the skin surface and minimising heat loss via radiation.
To generate heat, your skeletal muscles undergo rapid, involuntary contractions known as shivering. This requires a very high rate of respiration. Because respiration is an exothermic reaction, it releases heat energy as a byproduct, which warms the surrounding blood.
Additionally, small erector muscles in the skin contract, pulling your hairs upright to create "goosebumps". This traps a thin layer of still air against the skin. Air is a poor conductor of heat, so it acts as an insulator and reduces heat loss.
Scientists and students often model the body's cooling mechanisms in the lab. For example, sweating can be modelled by wrapping one test tube in wet cotton wool and another in dry cotton wool.
The tube with wet cotton wool cools much faster. Energy is transferred from the warm water inside the tube to the cotton wool, providing the necessary latent heat for the water in the wool to turn into a vapour.
Students often state that "capillaries dilate or constrict". Capillaries do NOT have muscular walls; you must state that the arterioles dilate or constrict.
In 6-mark questions on temperature regulation, examiners expect you to explicitly link shivering to an increased rate of respiration, specifying that respiration is an exothermic reaction.
When describing how the body monitors temperature, make sure to distinguish between the skin receptors (which send electrical impulses) and the hypothalamus (which monitors blood temperature directly).
To secure maximum marks when describing sweating, always use the keyword "evaporate" and mention the transfer of "latent heat" away from the body.
Optimum
The specific temperature (e.g., 37°C) at which human enzyme activity and metabolic rates function best.
Homeostasis
The maintenance of a stable internal environment within narrow limits, despite internal or external changes.
Hypothalamus
A region of the brain that acts as the body's thermostat by monitoring and coordinating the control of body temperature.
Thermoregulatory centre
The specific region of the hypothalamus that monitors blood temperature and coordinates homeostatic temperature responses.
Peripheral receptors
Sensory receptors located in the skin that detect changes in the external environmental temperature.
Nerve impulses
Electrical signals that travel along sensory and motor neurons, carrying information to and from the central nervous system.
Negative feedback
A regulatory mechanism where a change in a variable triggers a response that reverses the direction of that change to restore the system to its set point.
Set point
The specific target value (e.g., 37°C for body temperature) that a homeostatic system aims to maintain.
Effectors
Muscles or glands that carry out a response to a stimulus to restore the body's set point.
Vasodilation
The widening of the lumen of arterioles near the skin surface to increase blood flow and maximise heat loss.
Arterioles
Small blood vessels that supply blood to the capillaries; they have muscular walls that can contract or relax.
Radiation
The transfer of heat energy from the body to the environment without the need for physical contact.
Evaporation
The process of a liquid turning into a gas, which transfers latent heat away from the skin during sweating.
Vasoconstriction
The narrowing of the diameter of arterioles leading to skin capillaries to conserve core body heat.
Exothermic reaction
A chemical reaction (such as respiration) that releases energy in the form of heat to its surroundings.
Erector muscle
A microscopic muscle in the dermis that contracts to pull a hair follicle upright.
Insulator
A material (such as trapped still air) that is a poor conductor of heat and therefore reduces heat loss.
Put your knowledge into practice — try past paper questions for Biology B
Optimum
The specific temperature (e.g., 37°C) at which human enzyme activity and metabolic rates function best.
Homeostasis
The maintenance of a stable internal environment within narrow limits, despite internal or external changes.
Hypothalamus
A region of the brain that acts as the body's thermostat by monitoring and coordinating the control of body temperature.
Thermoregulatory centre
The specific region of the hypothalamus that monitors blood temperature and coordinates homeostatic temperature responses.
Peripheral receptors
Sensory receptors located in the skin that detect changes in the external environmental temperature.
Nerve impulses
Electrical signals that travel along sensory and motor neurons, carrying information to and from the central nervous system.
Negative feedback
A regulatory mechanism where a change in a variable triggers a response that reverses the direction of that change to restore the system to its set point.
Set point
The specific target value (e.g., 37°C for body temperature) that a homeostatic system aims to maintain.
Effectors
Muscles or glands that carry out a response to a stimulus to restore the body's set point.
Vasodilation
The widening of the lumen of arterioles near the skin surface to increase blood flow and maximise heat loss.
Arterioles
Small blood vessels that supply blood to the capillaries; they have muscular walls that can contract or relax.
Radiation
The transfer of heat energy from the body to the environment without the need for physical contact.
Evaporation
The process of a liquid turning into a gas, which transfers latent heat away from the skin during sweating.
Vasoconstriction
The narrowing of the diameter of arterioles leading to skin capillaries to conserve core body heat.
Exothermic reaction
A chemical reaction (such as respiration) that releases energy in the form of heat to its surroundings.
Erector muscle
A microscopic muscle in the dermis that contracts to pull a hair follicle upright.
Insulator
A material (such as trapped still air) that is a poor conductor of heat and therefore reduces heat loss.