3.7 - Homeostasis

What homeostasis is, and the control-system model

• Homeostasis is the maintenance of a constant internal environment - regulating conditions inside the body and its cells to keep them steady in response to internal and external changes.
• It's important because cells only work properly within a narrow range of conditions.
• In particular, the enzymes that drive the body's reactions need the right temperature and pH - if conditions drift too far from normal it can be dangerous to health.
• Keeping conditions steady means responding to internal and external change while balancing what enters the body against what leaves it.

Conditions that must be kept steady

• Body temperature
• The concentration of glucose in the blood
• The body's water content

Each is kept near its normal level by an automatic control system, using both nervous and hormonal communication.

The three components of a control system

1. Receptors - Cells that detect a change (a stimulus), e.g. a change in temperature or blood glucose. They can be on their own or grouped inside a larger sense organ.
2. Coordination centre - Receives the information from the receptors, processes it, and decides on the response. Examples: the brain, the spinal cord, and the pancreas.
3. Effectors - Carry out the response that counteracts the change. Effectors are muscles (which respond by contracting) or glands (which respond by secreting a substance such as a hormone).

Every coordinated response needs all three

1. A stimulus is detected by a receptor.
2. The coordination centre processes the information.
3. The effector acts to counteract the change.

This same three-box pattern - receptor → coordination centre → effector - is shown below, and it lies behind every example of homeostasis, whatever condition is being controlled.