The definition of homeostasis is the maintenance of a constant internal environment. Positive and negative feedback loops control internal conditions within set limits. 

  • For example, our optimum body temperature is approximately 37°C – This is the set value/limit
  • If the temperature goes beyond 37 then negative feedback brings it back down 
  • If the temperature falls below 37 then positive feedback brings it back up 

Maintenance of Internal Body Temperature

Mammals and birds are endothermic (can maintain a constant body temperature even if the outside temperature is different). Body temperature is maintained at around 37 degrees. The body loses heat when the environment is too hot, and retains heat when the environment is too cold. The brain has blood temperature receptors that can modulate the temperature accordingly via certain mechanisms. To maintain this temperature, the organism needs energy which comes from respiration.

Layers of the skin:


  • The cells in the epidermis are made at its base and always dividing through mitosis.
    • The new cells gradually move towards the surface of the skin.
    • When they die, they fill with keratin (a protein)
    • Top layer of skin is made of these dead cells and is called the cornified layer.
      • This layer protects the softer cells under it as it is waterproof and hard. It grows thicker on parts of the body that are more exposed to watery areas – soles of feet
    • The pigmentation of the skin is from melanin which absorbs harmful UV rays to prevent damage.
    • Epidermis layers fold inward and form a hair follicle (layer of skin surrounding hair)


  • Made of connective tissue – elastic and collagen fibres
    • As a person grows old, fibres lose elasticity making skin wrinkly
    • Contains sweat glands
    • Contains blood vessels and nerve endings – which are sensitive to changes in the environment and help keep you aware of changes in the environment
  • Adipose Tissue
    • Below Dermis
    • Layer of fat
    • Made of cells with large drops of oil
      • Help insulate body against heat loss
      • Can be used as energy reserve
  • Hypothalamus
    • Part of brain that helps in keeping constant body temperature
    • Coordinates activities to maintain set limit
    • Acts like a thermostat

Mechanisms to retain heat:

  • Basic insulation
    • The fat layer in the skin acts as an insulator
    • Skin hair follicles stand up to trap a layer of air around the skin which is also an insulator 
  • Shivering
    • Increased metabolism in muscles increase heat circulation
  • Vasoconstriction
    • Heat is carried in the blood 
    • If blood goes near the skin surface, then heat radiates out of the body
    • Constriction of the skin arterioles reduce the amount of blood flowing near the skin surface to retain heat in the blood 

Mechanisms to lose heat

  • Sweating
    • Sweat is a mixture of water, salt and urea 
    • Water evaporates from sweat which causes the skin (and body) to cool down
  • Vasodilation
    • Dilation of skin arterioles increase the amount of blood flowing near the skin surface to allow more heat to radiate out of the body 

Negative Feedback – In order to preserve homeostasis to reach the fixed point inside the body, negative feedback loops are used. Negative feedback loops are distinguished by their tendency to either raise or decrease a stimulus, thereby inhibiting the stimuli’ capacity to function as it did before receptor sensing.

NOTE: Fancy terminology aside, this means that the hypothalamus performed an action to e.g.: reduce body temperature. The effects of those actions are constantly fed back to the hypothalamus. When the temperature is optimum, the action stops.

Negative feedback means the effector decreases the action. It is called feedback because information is fed back based on the actions (hypothalamus gets knowledge of the effect of the actions that were used to initially reduce temperature).

When it gets feedback and realizes temperature doesn’t need to be dropped, the actions stop or fewer are used. This means if there were 5 actions being performed, they’ll go down in number or stop (so 0 actions). The number reduced, hence negative.

Control of Sugar Levels

Blood sugar levels are controlled by two hormones that are secreted by groups of cells called the islets of Langerhans in the pancreas.

  • Insulin – Causes liver to use blood glucose and covert it to glycogen for storage inside liver cells. Too much in the blood can cause osmosis from cells to blood, which leads to not enough water in cells to carry out normal metabolic processes.
  • Glucagon – Causes the liver to convert glycogen to glucose and release it into the blood. Too little glucose in the blood means that cells won’t be getting a constant stream of glucose which is required for respiration and will die when deprived of it.

Essentially, insulin causes blood sugar levels to fall and glucagon causes blood sugar levels to rise. 

When glucose levels are too high, negative feedback will reduce it back down

  • Pancreas secretes insulin 
  • Liver converts blood glucose to glycogen 
  • Blood sugar levels fall 

When glucose levels fall too low, positive feedback will increase it back up

  • Pancreas secretes insulin 
  • Liver converts blood glucose to glycogen 
  • Blood sugar levels fall 

Type 1 diabetes is caused by insufficient insulin production leading to a very high blood glucose level.

The symptoms of type 1 diabetes include

  • Increased hunger 
  • Frequent urination 
  • Blurred vision 
  • Tiredness & fatigue 
  • Unexplained weight loss
  • Hyperglycaemia – High glucose concentration

The management of type 1 diabetes includes:

  • Controlling sugar intakes 
  • Monitoring sugar levels 
  • Insulin injections 


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