Homeostasis during exercise (7.11, 7.12)

Positive feedback is a control mechanism that amplifies a change by increasing the effect of the original stimulus.

This response is usually not homeostatic, as it does not restore conditions back to the normal level.

Childbirth is an example of positive feedback.

Stretching of the cervix stimulates the hypothalamus to release more oxytocin from the pituitary gland. Oxytocin strengthens uterine contractions, causing further stretching of the cervix and the release of more oxytocin. This cycle continues until birth occurs.

Blood clotting is another example of positive feedback.

Platelets adhere to damaged vessel walls, releasing chemicals to attract more platelets. This accelerates clot formation until the wound is sealed.

Negative feedback is a control mechanism that maintains homeostasis by reversing changes in the internal environment, restoring conditions back to normal (the optimal range).

1. A change is detected (e.g., body temperature rises).
2. Receptors send signals to a coordination centre (e.g., hypothalamus).
3. The coordination centre activates effectors (e.g., sweat glands).
4. Effectors respond to reverse the change (e.g., cooling the body down).
5. Once normal conditions are restored, the system switches off.

Homeostasis is the maintenance of a stable internal environment by physiological control systems that detect and respond to changes in the internal or external environment.

It is important because enzymes and cells function best within narrow ranges of temperature, and water potential. If conditions move outside these limits, metabolic processes may be disrupted and cells may be damaged.

Examples of conditions regulated by homeostasis include:

• Body temperature
• Blood glucose concentration
• Water potential of the blood
• of blood and tissues

Endotherms (e.g., mammals, birds) control body temperature internally using metabolic heat.

Endotherms show behavioural responses such as seeking shade, sweating / panting, or bathing to cool. They may curl up, huddle, or move to sunny spots to warm up.

Physiological responses also help regulate their internal temperature:

• If too hot, vasodilation, sweating, panting or reducing metabolic rate increase heat loss.
• If too cold, vasoconstriction, shivering, raised hairs / feathers and an increased metabolic rate help conserve or generate heat, returning conditions toward the optimal range.

The hypothalamus is a key control centre in the brain that acts as the thermoregulatory centre. It contains thermoreceptors that monitor blood temperature and receives information from peripheral thermoreceptors in the skin.

The hypothalamus coordinates nervous and hormonal responses by activating appropriate effectors to maintain body temperature.

When body temperature rises due to muscle activity (e.g., during exercise), the hypothalamus coordinates responses that increase heat loss and reduce heat production:

• Vasodilation (widening) of arterioles near the skin surface increases heat loss by conduction, convection and radiation.
• Sweating increases heat loss as sweat evaporates from the skin .
• Reduced metabolic heat production reduces the heat generated by the body.

When body temperature falls, the hypothalamus coordinates the following responses to reduce heat loss and increase heat production:

• Vasoconstriction (narrowing) of arterioles in the skin decreases heat loss by conduction, convection and radiation.
• Shivering increases metabolic heat production through muscle contractions.
• Reduced sweating, so heat energy is not lost by evaporation.
• Hairs stand up, trapping an insulating layer of air and reducing heat loss.