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Homeostasis is the maintanance of a constant internal environment. This environment includes tissue fluid, blood and cytoplasm.

Homeostasis comes from 2 words:

  • Homeos- same
  • Stasis- static

What things need to be kept constant?

  • Temperature
  • Water content
  • Salt levels not only NaCl, but Na, K, Cl, K, etc.
  • pH
  • Glucose levels

Why should they be kept constant

Any change:

  • Affects enzyme activity
  • May affect osmostic concentration of cells

- if enzymes are affected, they may be denatured, reactions will stop, that's a disaster.


Principles of Homeostasis

Set point/norm-
Each condition has a level at which the body operates at its best, the optimum condition (e.g. 36.8°C-37°C for body temperature; 80mg Glucose per 100ml of blood).
Sensitive cells around the body which detect changes from the norm.
An organ or a gland which brings about the required change back to the Norm.
Negative feedback-
A mechanism where a rise in something results in its own decrease, for example, a rise in body temperature sets off sweating and other processes which eventually lower the body temperature.

The skin and temperature control

The skin is the main organ which is involved in temperature control (Thermoregulation).

There are 2 locations for receptor which detect changes to body temperature:

1. Thermoregulatory centre: Located in the hypothalamus in the base of the brain, has receptors which detect changes in the temperature of the blood passing through.

They mainly detect changes in the core body temperature: This is the temperature of internal vital organs like the brain, heart, liver. The core body temperature is strictly controlled and cannot vary much.

2. Skin: Has receptors which detect skin surface temperature. This can vary a bit.

Skin structure diagram comes here

Some key terms

Hypothermia (hypo=below; thermia= temperature)- when the core body temperature goes below the Norm.

Hyperthermia (hyper= above; thermia= temperature)- when the core body temperature goes above the Norm.


What happens when the core body temperature goes up (Hyperthermia)?

-Usually this happens during exercise when respiring muscles are producing excess heat energy.

  • Sweat glands produce sweat ( a salty solution)- Heat energy is extracted from the body to evaporate the sweat, reducing the tempearture.
  • Vasodilation- blood vessels (arteries) supplying skin capillaries with blood dilate (increase in diameter), this allows more blood to flow near the surface (skin looks red) of the skin, enabling heat loss through conduction and radiation.

As sweating results in loss of water and salts, more water has to be taken in on a hot day to replace it.

The kidneys also respond by reducing the amount of urine produced i.e. water is conserved.= Small volumes of dark coloured urine on a hot day.


What happens when the core body temperature goes down (Hypothermia)?

When we are cold:

  • Hairs on our skin stand up as muscles holding them contract (piloerection)- This traps a layer of insulating air (air is a good insulator) around the skin preventing heat loss.
  • Shivering- Involuntary and rapid contraction of muscles generates heat energy (through respiration and friction) to bring the temperature back to the Norm.
  • Vasoconstriction- Blood vessels (arteries) supplying skin capillaries with blood constrict (reduce diameter), this restricts blood flow to the surface of the skin (skin appears pale). Less heat is lost.

Since sweating (water loss) is reduced on a cold day, the kidneys respond by producing large volumes of watery urine.