Erm I'll just give you a couple of things to think about for homeostais.
I can't remember how much detail you do at higher, so I'll ramble lots and you can pick and choose.
The concept of homeostasis: the processes concerned with maintaining a stable internal environment. I.e. the physical and chemical stability of the internal environment in which cells live is the essential condition that makes possible the free, independent existence of the whole animal.
Under normal conditions and state of health, homeostasis depends on adaptive mechanisms in the body which range from control centres in the brain to hormones in the blood. Crucial homeostatic control is exerted over virtually all the properties of the blood. The most rapidly controlled of these are its pH and CO2 level.
The internal environment. This is not to be confused with the intracellular fluid, or cytoplasm. It is the fluid which bathes most cells in a complex animal, essentially an ultrafiltrate of blood.
As life evolved, cells stayed together to form multicellular organisms. Animals evolved a “skin” to allow control of the fluid their cells were bathed in. This fluid is the “internal environment".
Body Water: Note that in mammals typically 60% of body weight is water – and about 35% of body weight is intracellular water.
N.B. The steady state is not a fixed state.
The processes of homeostatic control almost all include the older (in evolution) parts of the brain; the brainstem and hypothalamus. Centres for control of respiration for example are found in the medulla, while osmoregulatory neurones are found in the supraoptic nucleus of the hypothalamus. The autonomic nervous system and endocrine system are both key parts of the homeostatic systems.
Control systems in both engineering and physiology usually involve a sensor, which measures the factor being controlled, an amplifier, which makes the signal larger, and a negative feedback system which applies the amplified error signal back to reduce the error. So the error signal is inverted, and fed-back the other way up. For example, suppose you are dehydrated. Your high blood osmolarity is then detected by osmoreceptors in the brain. The signal they produce is processed to produce (a) thirst (which drives you to drink, thereby diluting your blood and hence internal environment) and (b) the hormone ADH which minimises water loss in urine so you lose more salt than water – also diluting the blood.
So basically in an essay you want to explain all that and then mention some specific examples, eg glucose homeostasis, blood pressure, renin-angiotensin system, baroreceptors, chemoreceptors, temperature control, sympathetic/parasympathetic nervous system and their involvment in vasoconstriction/dilatation...
I honestly can't remember how much you cover of this in higher... A good reference book I use for physiology is Guyton and Hall:
http://www.amazon.co.uk/exec/obidos/ASIN/0...8499573-0603832
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hi
- i want to do well!!! 
thanx - a lot of info there to read !!
