7.9–7.10BHomeostasis
Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
That definition is worth learning exactly. Two parts of it carry marks: it is about the internal conditions, and it responds to changes from inside the body as well as outside it.
Homeostasis maintains optimal conditions for enzyme action and all cell functions — which is the reason it matters. Enzymes work within narrow limits of temperature and pH, so keeping those steady is what keeps every other process running.
In the human body, the conditions controlled include:
• blood glucose concentration • body temperature • water levels.
These automatic control systems may involve nervous responses or chemical responses.
Every control system, nervous or chemical, is built from the same three parts:
• Receptors — cells that detect stimuli, meaning changes in the environment. • Coordination centres — such as the brain, spinal cord and pancreas — which receive and process the information from receptors. • Effectors — muscles or glands — which bring about responses that restore optimum levels.
Name all three in order and you have the structure of almost every answer on homeostasis and response.
It is worth seeing why the internal conditions cannot simply be left alone. Enzymes have an optimum temperature and pH, and outside a narrow band around it the active site changes shape and the reaction slows or stops. Cells also need a steady water concentration: too dilute and water enters by osmosis until the cell swells, too concentrated and water leaves until it shrinks. Either way the reactions inside stop working properly.
So homeostasis is not housekeeping. It is the condition for everything else in the course happening at all — respiration, photosynthesis, digestion and the immune response are all enzyme controlled.
- Homeostasis
- The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
- Receptor
- A cell that detects a stimulus — a change in the environment.
- Coordination centre
- A part of the body, such as the brain, spinal cord or pancreas, that receives and processes information from receptors.
- Effector
- A muscle or gland that brings about a response to restore optimum levels.
The two kinds of control system differ in speed, and knowing which is which tells you what an answer should mention. A nervous response uses electrical impulses along neurones and is very fast but short lived — pulling away from something hot. A chemical response uses hormones carried in the blood and is slower but lasts longer — the control of blood glucose after a meal. Body temperature uses the nervous system; blood glucose and water levels are hormonal.
Common mistake
An effector is the muscle or gland that acts, not the action itself. "Sweating" is the response; the sweat gland is the effector. Questions asking you to name the effector want the structure.
7.11B–7.12BControl of body temperature
Biology only — not on the Combined Science specification
Body temperature is monitored and controlled by the thermoregulatory centre in the brain. The centre contains receptors sensitive to the temperature of the blood, and the skin contains temperature receptors that send nervous impulses to it. So the body is sampling temperature in two places: the blood inside and the surface outside.
Higher tier only: you should be able to explain how these mechanisms raise or lower temperature in a given context. Vasodilation widens the blood vessels near the skin surface, so more blood flows there and more energy is transferred to the surroundings. Sweat takes energy from the skin as it evaporates. Vasoconstriction does the reverse, keeping warm blood away from the surface, and shivering generates energy because muscle contraction requires respiration, which is exothermic.
Common mistake
Blood vessels do not "move closer to the surface" — they cannot move. They dilate, so more blood flows through the vessels that are already there. And sweat cools you as it evaporates, not as it is produced: sweat sitting on the skin on a humid day does very little.
- Thermoregulatory centre
- the region of the brain that monitors and controls body temperature, containing receptors sensitive to the temperature of the blood
- Vasodilation
- the widening of blood vessels near the skin surface, so more blood flows there and more energy is transferred to the surroundings
- Vasoconstriction
- the narrowing of blood vessels near the skin surface, keeping warm blood away from the surface to reduce energy transfer
Before you practise
- Homeostasis keeps conditions constant so that enzymes work and cells function.
- Every control system has receptors, a coordination centre and effectors — name all three, in order.
- The thermoregulatory centre in the brain monitors the temperature of the blood, and receptors in the skin monitor the surface.
- Too hot: sweat evaporates and takes energy with it, and vasodilation brings more blood to the surface so more energy is lost.
- Too cold: shivering releases energy from respiration, and vasoconstriction reduces the blood flow near the surface.
- Blood vessels do not move — vasodilation and vasoconstriction change the diameter of the vessel.