Control of internal environment of living organisms.

Most of the body's homeostatic systems rely on negative feedback systems to resist change. In these systems there needs to be a detector which can measure the value that needs to be controlled. When the detector finds that the value is higher then should be it sends information to an effector which will then correct the level. This happens till the detector finds the value is too low and the effector then changes what it is doing so the value then rises. Information is therefore feed back to the detector from the effector, the feedback is called negative due to it stopping the effector doing one thing and stimulating it to do the opposite. As there is some delay for the information to pass from the detector to the effector levels are not normally kept absolutely constant. This means that there are usually some fluctuations to values which are around the ideal value or set point.

In humans there are specialised cells that act as detectors. These cells are in organs and act as detectors for particular substances and many of the organs act as effectors (kidneys are effectors in the regulation of water). The information travels along nerves (temperature regulation) or in the blood as chemicals called hormones (regulation of water content).

Hormones are secreted by endocrine glands also know as ductless glands as they do not have a duct for carrying the secretion to a particular point. The hormones pass into the blood which then flows through the capillaries and to the glands.

There are three different main groups of animal hormones –

• Amino acids (and catecholamines): All water-soluble molecules (thyroxine and adrenaline)
• Proteins: Water soluble (insulin and glucagon)
• Steroids: Lipids. Fat soluble molecules, made from cholesterol (oestrogen and testosterone)