Homeostasis (Cambridge (CIE) A Level Biology)
Revision Note
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 (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
Examiner Tips and Tricks
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 (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
Examiner Tips and Tricks
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)!
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