Homeostasis Principles
Homeostasis is a fundamental concept in biology — the maintenance of a stable internal environment despite fluctuations in external conditions. Effective homeostasis is essential for enzyme function, cell survival, and the normal operation of all body systems. This lesson covers the principles of feedback, thermoregulation, and the roles of key hormones.
Key Definition: Homeostasis is the maintenance of a constant internal environment within narrow limits, achieved through physiological processes involving receptors, coordinators, and effectors operating via feedback mechanisms.
The Homeostatic Control System
All homeostatic mechanisms share a common pattern:
- Stimulus — a change in the internal or external environment (deviation from the set point).
- Receptor (sensor) — detects the change and sends information to the coordinator.
- Coordinator (control centre) — processes the information and determines the appropriate response (e.g., the hypothalamus, medulla, pancreas).
- Effector — carries out the corrective response (muscles or glands).
- Response — the action that returns the variable to the set point.
Negative Feedback
Negative feedback is the primary homeostatic mechanism. The response produced by the effector opposes (reverses) the original change, bringing the variable back towards the set point.
Characteristics of Negative Feedback
- The response is always in the opposite direction to the deviation.
- It is a continuous process — the system constantly monitors and adjusts.
- It results in oscillation around the set point, not a perfectly constant value.
- It involves multiple pathways, so there is a response for deviations above and below the set point.
Examples
- Blood glucose too high → insulin secreted → glucose taken up by cells → blood glucose falls.
- Core body temperature too high → vasodilation and sweating → heat lost → temperature falls.
- Blood water potential too low → ADH released → more water reabsorbed in kidneys → water potential rises.
Positive Feedback
In positive feedback, the response amplifies the original change, moving the variable further from the set point. Positive feedback is relatively rare and usually occurs in situations where a rapid, decisive response is needed.
Examples
- Oxytocin during labour: Uterine contractions stimulate the release of oxytocin from the posterior pituitary. Oxytocin stimulates stronger contractions, which in turn stimulate more oxytocin release. The cycle continues with increasing intensity until the baby is born, at which point the stimulus (pressure on the cervix) is removed.
- Blood clotting: Damage to a blood vessel exposes collagen, which activates clotting factors. Each activated factor activates the next in a cascade, rapidly amplifying the response until a fibrin clot seals the wound.
- Depolarisation during an action potential: Opening of Na⁺ channels leads to depolarisation, which opens more Na⁺ channels, causing rapid and complete depolarisation.
Why is Positive Feedback Less Common?
- Positive feedback is inherently destabilising — it moves the system away from equilibrium.
- It requires an external event to stop the cycle (e.g., birth of the baby, wound sealed).
- In most physiological situations, stability (negative feedback) is preferred.
Thermoregulation
Humans are endotherms (they generate their own body heat) and are homeothermic (they maintain a relatively constant core body temperature of approximately 37 °C).
Maintaining a constant body temperature is vital because:
- Enzymes have an optimum temperature; deviations reduce the rate of metabolic reactions.
- At high temperatures, enzymes may denature (lose their tertiary structure), with potentially fatal consequences.
- At low temperatures, metabolic reactions slow excessively.
The Role of the Hypothalamus
The hypothalamus is the thermoregulatory centre in the brain:
- It contains central thermoreceptors that monitor the temperature of the blood passing through it.
- It receives nerve impulses from peripheral thermoreceptors in the skin, which detect changes in external temperature.
- It has a set point of approximately 37 °C and initiates responses when the core temperature deviates from this.
Responses When Core Temperature Rises Above 37 °C