5.1.1 Homeostasis

• Homeostasis is the maintenance of a constant internal environment by physiological control systems.
• Negative feedback is when a deviation from the norm is detected in the body by receptors and the body puts mechanisms in place to restore the change and bring it back to normal.

  • Involves the nervous systems and hormones.
• Positive feedback is when a deviation from the norm occurs and it triggers a response to increase this change further. This is usually very rare and doesn't happen often.

  • Example - During childbirth, when the baby's head presses on the cervix it causes the hormone oxytocin to be release, causing contractions. This results in the release of even more oxytocin, encouraging the birth of the baby.
• Thermoregulation:

  • Body temperature is too low = Insufficient kinetic energy for enzyme-controlled reactions.
  • Body temperature too high = Enzymes denature.
• Ectotherms are unable to regulate their internal temperature and can only control it by changing their behaviour and relying on external factors (eg weather).

  • Many aquatic organisms do not need to regulate their body temperature as much because water has a high specific heat capacity (remains relatively constant).
  • Cold-blooded animals (ectotherms) on land have a bigger challenge as the temperature of the air fluctuates. They tend to lay on hot rocks to absorb heat radiating from the sun.
• Endotherms are able to regulate their body temperature through nervous responses.

  Peripheral temperature receptors in the skin detect a change in the external temperature and sends an impulse along a sensory neurone to the hypothalamus, triggering a response by glands in the skin and muscles.

  • Sweat - Water has a high latent heat of vaporisation so sweat evaporates causing a cooling effect.
  • Vasodilation - Widening of blood vessels due to the relaxation of the blood vessels in the muscular walls. This enhances blood flow to areas of the body lacking oxygen / nutrients. (To cool off)
• Ectotherms are able to regulate their internal body temperature through nervous responses:

  • Vasoconstriction - Narrowing of arterioles near skin surface to restrict blood flow so more blood can travel through capillaries further away from the skin surface. This means less heat is radiating from the skin. (To stay warm)
  • Shivering - Contracting and relaxing of muscles which increases the rate of respiration and releasing more heat energy.
  • Animals have fur to insulate heat when raised, or prevent insulation when lowered. This is controlled by erector pili muscles in the skin.