⚡ Nervous responses
The information is carried as signals by the nervous system.
Homeostasis » Homeostasis and Control Systems
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.5.1
- What homeostasis is and why your body needs it
- Which three things the human body keeps under control
- The difference between nervous and chemical responses
- The three parts of every control system: receptor, coordination centre and effector
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Unlock This CourseWhat is homeostasis?
Your body is always being pushed around. You walk out into the cold. You run for a bus. You skip lunch. Each of these changes the conditions inside your body. Yet your cells need those conditions to stay in a narrow range, or they stop working properly.
The job of keeping conditions steady is called homeostasis. It happens in single cells and in whole organisms.
Key terms:
- 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.
- Optimum conditions: the conditions in which cells and enzymes work best.
Two ideas in that definition matter a lot. Regulation means the body does not just let things drift. It checks and corrects. Internal and external changes means the trigger can come from inside you (for example, using up glucose as you exercise) or from outside you (for example, a hot day).
Why do cells need steady conditions?
Remember, enzymes speed up the reactions in your cells, and they only work well in the right conditions. If the temperature or pH moves too far from the optimum, an enzyme can lose its shape and stop working.
That is why homeostasis matters. It maintains the optimal conditions for enzyme action and for all cell functions. If the control fails, enzymes work too slowly or stop altogether, and the cells cannot do their jobs.
Remember
Homeostasis does not freeze conditions at one exact number. It keeps them close to the optimum, so the cells can carry on working.
What does the human body control?
The spec names three conditions you must know. Each has its own control system, and each gets its own lesson later in this theme.
🍬 Blood glucose concentration
The amount of glucose in your blood. Cells need a steady supply of glucose.
🌡 Body temperature
Kept close to the temperature at which your enzymes work best.
💧 Water levels
The amount of water in your body, so that your cells keep working properly.
You will see how each one is controlled in Controlling Blood Glucose, Controlling Body Temperature and Water Balance and the Kidneys. In this lesson you only need the big idea they all share.
Two kinds of response
These control systems are automatic. You do not have to think about them. They may involve nervous responses or chemical responses.
⚗ Chemical responses
The information is carried by chemicals instead.
You will study how each works in The Nervous System and Reflexes and The Endocrine System. For now, just know that a control system can use either kind of response.
The three parts of every control system
Every control system in the body, whatever it controls, is built from the same three kinds of part. This is the most important idea in the lesson.
Key terms:
- Stimulus: a change in the environment.
- Receptor: a cell that detects stimuli.
- Coordination centre: a part of the body that receives and processes information from receptors. Examples are the brain, the spinal cord and the pancreas.
- Effector: a muscle or a gland that brings about a response, which restores optimum levels.
👁 1. Receptors
Cells that detect a stimulus, such as a change in temperature or glucose level.
🧠 2. Coordination centre
Receives the information from the receptors and processes it. The brain, spinal cord and pancreas are examples.
💪 3. Effectors
Muscles or glands. They bring about the response that restores optimum levels.
So the order is always: stimulus, then receptor, then coordination centre, then effector, then response.
Worked example: a thermostat
A central heating system works in a similar way. The room gets cold (the change). The thermostat detects it (like a receptor). The control box processes the signal (like a coordination centre). The boiler switches on (like an effector). The room warms back to the set temperature. Your body does the same job, with cells instead of electronics.
Putting it together
Imagine one of your internal conditions drifts away from the optimum. Receptors detect the change. A coordination centre processes the information. An effector brings about a response that restores the level. When the level is back at the optimum, the job is done.
Notice what the effector does. It does not just respond. It responds in a way that restores the optimum level. That is the whole point of homeostasis.
Common mistakes
1. Mixing up the coordination centre and the effector. The coordination centre processes information. The effector (a muscle or gland) brings about the response.
2. Forgetting that receptors are cells which detect stimuli. They are not the response itself.
3. Saying homeostasis stops conditions changing outside the body. It keeps the internal conditions steady, even when the outside changes.
Exam-style question
(a) What is meant by homeostasis? [2 marks]
(b) Name the three types of part found in every control system and state the job of each. [3 marks]
(c) Explain why homeostasis is important for enzymes. [2 marks]
Model answer
(a) The regulation of the internal conditions of a cell or organism (1) to maintain optimum conditions for function, in response to internal and external changes (1).
(b) Receptors detect stimuli (changes in the environment) (1). Coordination centres receive and process information from the receptors (1). Effectors, which are muscles or glands, bring about responses that restore optimum levels (1).
(c) Homeostasis maintains optimal conditions for enzyme action (1), so that enzymes and cells can carry on working properly (1).
Exam tip
In a question about a control system, name the receptor, the coordination centre and the effector in that order, and use the word 'restore' when you describe what the effector does.