Homeostasis (CIE A Level Biology)

• In order to function properly and efficiently, organisms have different control systems that ensure their internal conditions are kept relatively constant
• The process of maintaining constant internal body conditions is known as homeostasis
• Homeostasis is critically important for organisms as it ensures the maintenance of optimal conditions for enzyme action and cell function
• Sensory cells can detect information about the conditions inside and outside of the body
• Examples of physiological factors that are controlled by homeostasis in mammals include:
  • Core body temperature
  • Metabolic waste (e.g. carbon dioxide and urea)
  • Blood pH
  • Concentration of glucose in the blood
  • Water potential of the blood
  • Concentration of the respiratory gases (carbon dioxide and oxygen) in the blood

• The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance (i.e. to keep certain physiological factors, such as blood glucose concentration, within certain limits)
• Negative feedback control loops involve:
  • A receptor (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
  • A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
  • An effector (muscles and glands) – to carry out a response
• Outcome of a negative feedback loop:
  • The factor / stimulus is continuously monitored
  • If there is an increase in the factor, the body responds to make the factor decrease
  • If there is a decrease in the factor, the body responds to make the factor increase

Negative feedback diagram

A flow diagram of a negative feedback control loop

• Homeostasis in mammals relies on two different coordination systems to transfer information between different parts of the body:
  • Nervous system – information is transmitted as electrical impulses that travel along neurones
  • Endocrine system – information is transmitted as chemical messengers called hormones that travel in the blood