Homeostasis

Homeostasis

Homeostasis is defined as the maintenance of a constant internal

environment. This is very important for organisms because:

· Chemical reactions can take place at predictable rates.

· Mechanisms are more efficient as optimum conditions can be

maintained for enzymes, etc.

· Organisms can acquire a degree of independence from the environment.

They need not be limited to one geological location where the

conditions fit their needs, but can spread out. Their activity is also

not restricted to a certain time, season, etc.

Principles of Homeostasis:

All homeostasis mechanisms follow approximately the same steps.

[IMAGE][IMAGE][IMAGE][IMAGE][IMAGE]Input Receptor Effector Negative

Feedback

[IMAGE]

Receptor: Signals the deviation of a quantity from the normal, set

level.

Effector: Brings about the necessary change needed to return the

system to the reference point.

Negative Feedback: This monitors and controls the extent of the

correction. In other words, this makes sure that, as levels return to

normal, the corrective mechanisms are scaled down (so that a shortage

or excess of the substance does not occur)

Homeostatic Functions of the Liver:

Carbohydrate Metabolism

* The livers major role in the metabolism of carbohydrates is to

convert excess glucose absorbed from the intestines into glycogen.

This is stored mostly in liver and muscle cells.

* The stored glycogen can be later reconverted to glucose when the

blood sugar level falls.

* This interconversion is under the control of the hormones insulin

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... to calcium ions by opening

channel proteins which allow the entrance of Ca+2 ions.

· The influx of these ions causes synaptic vesicles containing

acetylcholine to move towards the presynaptic membrane.

· The synaptic vesicles discharge their contents, the transmitter

fluid acetylcholine into the synaptic cleft.

· Acetylcholine diffuses across the cleft and attaches to specific

sites on the post-synaptic nerve cell so that a positive charge

develops on that part of the cell.

· When this positive charge builds up to a critical level, an action

potential is generated in the nerve cell.

· Once acetylcholine has done its job, it is broken down by an enzyme

cholinesterase.

· Synapses ensure that impulses travel in only one direction as

synaptic vesicles are present only at the presynaptic membrane.