1- homeostasis

What does homeostasis mean?

It means ‘staying the same’, it refers to the process of keeping the internal body environment in a steady state.

This is highly important as a great deal of the hormone system and autonomic nervous system are dedicated to homeostasis.

What factors are controlled by homeostasis and why?

• Body temperature- to keep enzymes working near their optimum temperature and stop them denaturing.

• Blood pH- to keep enzymes working near their optimum pH.

• Blood glucose concentration- to ensure there is enough glucose available for cellular respiration, but not enough to lower the blood water potential and dehydrate cells.

• Blood water potential- to prevent loss or gain of water from cells by osmosis.

• Blood O₂ and CO₂ concentration- to ensure there is sufficient oxygen for respiration and to prevent carbon dioxide from forming carbonic acid.

• Blood pressure- to maintain blood flow around the body without losing too much fluid in capillary beds.

• Blood ion concentration- ions, like Na⁺, K⁺ and Ca²⁺, are needed for nerve impulses and metabolic reactions, while others, like CO₃^2- and PO₄ ^3- are needed as pH buffers.

Why do the factors homeostasis maintains inevitably change?

Due to normal activities like respiring, eating, exercising etc.

So homeostasis must constantly deal with these changes by making more changes itself.

Therefore, it is described as dynamic equilibrium because the factors don’t stay absolutely constant and set in stone, but fluctuate slightly over time.

Do all homeostatic mechanisms use positive or negative feedback to maintain a constant value (set point)?

Negative feedback

What is negative feedback?

Whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal.

It also means that the bigger the change the bigger the corrective mechanism.

What is positive feedback?

When a change stimulates a further change in the same direction, away from the set point. So it cannot be used in homeostasis and in fact is potentially dangerous.

It is usually associated with a breakdown in the normal homeostasis mechanism (e.g. potentially fatal hypothermia, when the body cools down below the set point).

But it is used occasionally when fast, explosive responses are required.

Give examples of when positive feedback is used:

• In blood clotting, activated platelets release chemicals that activate more platelets, causing a rapid increase in the number of platelets and the clotting cascade.

• In childbirth the pressure of the foetus on the cervix causes stretch receptors to the hypothalamus, which release the hormone oxytocin, which stimulates the uterus to contract, which stimulates the stretch receptors again.

In both of these cases the feedback loop is stopped by other events (the sealing of the wound in blood clotting and the expulsion of the baby in childbirth).