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What is homeostasis?

Homeostasis is the maintenance of steady state in living organisms by control of their internal environment even though out changes in the external environment. It involves lots of organs, which coordinate together.

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What does homeostasis control?

Homeostasis controls many different aspects of the body:


water content

·     ion content

·     glucose concentration

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What controls what?


blood glucose


water levels





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Temperature Control

Body temperature is monitored and controlled by the thermoregulatory centre, the hypothalamus. It is sensitive to the temperature of the blood flowing through the brain.

 Mammals and birds can control their own body temperature. They are called endotherms, animals that cannot control their own body temperature are called ectotherms.

 Advantages of being endothermic:

·      You can live in cold or hot environments

·      Your body can work at its optimum temperature constantly

·      You can be active at night when it is cold

·      You aren’t susceptible to temperature change

Disadvantages of being endothermic

·      Requires energy

·      You can be easily spotted on infrared cameras

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When the body gets too hot one way of controlling the temperature is vasodilation:

·      Blood carries heat around the body

·      When the body gets hot the muscles lining the walls of arterioles in the skin relax

·      This causes them to dilate allowing more blood to travel to the surface of the skin and release heat through radiation.

·      This increases the rate of heat loss from the body

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When the body gets too cold one way of controlling the temperature is vasoconstriction:

·      Blood carries heat around the body

·      When the body gets cold the muscles lining the walls of arterioles in the skin contract

·      This causes them to constrict allowing less blood to travel to the surface if the skin and be lost through radiation

·      The blood is redirected through the shunt vessel which is lower in the skin

This decreases the rate of heat loss from the body

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Pili Erector

·      Each hair on our body has a muscle attached to called the pili erector muscle

·      When this muscle contracts the hair is pulled upwards

·      When the body gets cold these muscles contract

·      A layer of air is then trapped between the hairs and the skin

·      This then heats to the body temperature and keeps the body warm

·      This reduces the rate of heat loss

·      This is what causes goosebumps. It is most more effective in animals as they have more body hair

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·      When the body gets hot, sweat glands secrete sweat

·      Water in the sweat evaporates off the skin. In order to do this, it must absorb heat from the body.

      Takes away heat from the body when it evaporates

·      This increases rate at which heat is lost, cooling us

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·      Muscles require energy  to contract. This energy is supplied by respiration

·     During respiration some energy is lost as heat energy

·      When we shiver, the muscles contract repeatedly to increase the rate of respiration, causing more heat to be generated.

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Control of Blood Sugar Levels

Blood sugar levels are usually at 100mg per 100cm3 of blood. The pancreas and the liver control it. The pancreas produces two hormones to control blood glucose levels:

  • insulin
  • glucagon

Control of blood glucose concentration

Eat a meal


Blood glucose levels rise

Blood glucose levels decrease

Pancreas notices change in glucose levels

Pancreas notices change in glucose levels

Pancreas secretes insulin into the blood stream

Pancreas secretes glucagon into the blood stream

Insulin travels in the blood to the liver

Glucagon travels in the blood to the liver

Liver takes up glucose and stores it as glycogen and insulin allows glucose to pass into cells from blood

Glucagon makes liver break down glycogen, converting it back into glucose

Blood glucose level declines to set point. Stimulus for insulin release diminishes

Glucose released back into blood

Insulin levels return to normal

Glucagon levels return to normal

Blood glucose levels return to normal

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·      Diabetes is a disease of the pancreas

     Cannot produce enough insulin or cannot react to insulin, therefore they cannot control their glucose levels

·      If glucose levels get to high the kidneys will not be able to control it and glucose will appear in the urine

·      Sufferers will produce lots of urine and feel thirsty often

·      If there is no insulin then the body cannot build up stores of glycogen, therefore when the blood glucose levels drop there will be no reserve

·      If these levels drop too low it could induce a coma or even death

·      Most diabetics control it by watching their diet, exercising and taking daily injections of insulin

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Mild diabetes can be controlled through diet. Avoiding carb-rich food that is high in glucose. If carbs are eaten they should be starchy so as to be absorbed slowly avoiding a spike in blood glucose levels that cannot be controlled.

 Regular injections of insulin stop levels rising too high (hyperglycaemia) and convert glucose into glycogen for the liver. Natural glucagon can prevent the levels from falling too low. The amount of insulin injected must be balanced to the amount of carbohydrate eaten.

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