CIE A Level Biology复习笔记14.1.1 Homeostasis


  • 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 (eg. 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

Exam Tip

Learn the following definition for homeostasis:Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

Principles of Homeostasis

  • The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance (ie. 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


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

Exam Tip

Although the nervous and endocrine systems are both important in homeostasis and the regulation of certain physiological factors, there are some fundamental differences between them.Information is transmitted through these two systems in different ways (electrical impulses vs. hormones). Also, the nervous system is usually required for fast, but short-lived responses, whereas the endocrine system is involved in slower, but longer-lasting responses (although this is not always the case and some hormones can act very quickly)!