Homeostasis is the maintenance of a constant internal environment within the body.


It includes

·      Control of the water balance of the blood

·      Control of blood sugar level

·      Control of body temperature


·      Control of blood urea level


Each of these internal ‘Controls’ is maintained by a separate ‘Mechanism’


All ‘mechanisms’ for homeostasis share common features:

·      A specific sensor is able to detect the value of the factor which is being monitored

·      Any deviation from the desired value (norm) is corrected so that the norm is more-or-less maintained

·      The corrective mechanism involves negative feedback


Negative Feedback

Control mechanism in which a change from the norm triggers off a response, or responses, which allow the norm to be reset






The sugar carried in the blood is glucose. The level of glucose in the blood is closely monitored and has a norm of approximately 80mg per 100cm3 blood


The regulation of glucose involves the pancreas and the liver

·      In the pancreas there are groups of special cells known as ‘Islets of Langerhans’. These cells secrete 2 hormones Insulin and Glucagon.

·      If the blood sugar level rises, e.g. after a heavy meal, these cells detect this and release more insulin and less glucagon.

·      The insulin travels to the liver and ‘tells’ it to do a number of things (1) convert glucose to glycogen(stored in the liver and muscles), (2) Convert glucose to fat

·      As a result the blood sugar level falls





Significant variation in the internal temperature could have damaging effects on the body’s enzymes

Humans are ‘Endotherms(Warm blooded- keep constant body temperature in hot or cold climate)’ as  opposed to ‘Ectotherms(Reptiles and Fish, their body temperatures vary according to outside temperature)


Heat can be gained or lost from the body in four ways:

·      Radiation-Transfer of heat to or from the body to other objects (via the air)

·      Conduction-Transfer of heat to or from the body by direct contact with another object e.g. radiator, window

·      Convection- Transfer of heat to or from the body via moving air

·      Evaporation- The loss of heat used to change water from a liquid to a vapour, e.g. when water leaves the skin surface or gas exchange system




·      Sweating

·      Flattening of hairs on the skin

·      Vasodilation and dilation of shunt vessels

·      More blood in the circulation-blood which is stored in the liver and spleen is released into the circulation. More blood therefore reaches the skin surface, therefore more heat is lost

·      Metabolic rate falls. This is a long-term effect under the control of the hormone thyroxine

·      Lack of shivering- this spasmodic contraction of voluntary muscle to produce heat does not occur


The response to heat LOSSES is the reverse of these changes.



The hypothalamus in the brain acts as a thermostat and is sensitive to changes in blood temperature. If the temperature of the blood entering the hypothalamus falls, it sends impulses to organs causing then to reduce heat loss. The reverse happens if the temperature of the blood entering the hypothalamus rises.




These sense changes in the external temperature and are useful in:

·      Informing the brain of these changes so that the brain can alter behaviour e.g. if the person feels a cold draught he/she may close a window

·      Protection, e.g. if a person picks up an object which is very hot then he/she drops it very quick


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