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What is homeostasis?

Homeostasis is the dynamic equilibrium that maintains health within the body, in spite of the continual changes taking place both internally and in the external environment. This is achieved by attempting to maintain a constant internal state, and all the systems of the body are involved in the effort. Whenever an imbalance occurs, regulatory systems become active to restore normal conditions, usually by meeting one change with another in an effort to regain the balance. Ill health is the consequence of failure to re-achieve this state.

For the maintenance of health in the human body, fairly constant conditions must be maintained in the fluid that feeds the cells. To achieve this, several factors must be continually monitored and regulated, and kept within narrow limits. These include temperature, concentrations of glucose, osmotic pressure, oxygen and carbon dioxide levels, and ion concentrations (such as calcium, sodium, etc). As an example, the concentration of glucose in the blood never normally drops below 70mg per 100ml, nor rises above 110mg per 100ml.

The process of maintaining these constant, or near constant, conditions is termed homeostasis. It is achieved by the actions of all the organs and tissues, which act to rebalance the system, and which are therefore called 'control systems'. They do this by a feedback process, where one change in the system is met with another, opposite change, in an effort to regain normal conditions. In order for a control system to know there is an imbalance, there are receptors (such as nerve endings) that monitor changes in the internal environment. The majority of the control systems are alerted by these receptors through either the nervous system or the endocrine system. Feedback through the endocrine system, which utilises and controls hormones in the blood stream, tends to be slower than through the nervous system, which utilises nerve impulses.

Examples :

1. The endocrine system :

An increase in the level of calcium in the blood upsets the calcium homeostasis in the body. The thyroid gland detects the change, and releases the hormone calcitonin. The calcitonin acts on the kidneys, bone and intestine to decrease the level of calcium released into the blood - and the calcium homeostasis level is restored.

2. The nervous system:

A decrease in body temperature upsets the temperature homeostasis (normally 37 degrees C). The temperature receptors (specialised nerve endings) inform the hypothalamus in the brain by way of the nervous system. The hypothalamus responds by causing constriction of skin blood vessels to remove blood from the body surface (and thus reduce heat loss), 'goose bumps' to raise the hair follicles on the skin (to create insulation), and induces shivering (to create heat). Thus the body temperature is raised, and the temperature homeostasis is restored.

This type of feedback is termed negative feedback, because it acts directly to reduce an imbalance, and it is the main type of feedback that operates to restore homeostasis in the body. Positive feedback is much more rarely seen, and its action is to strengthen or reinforce changes rather than reduce them - a good example is the action of the hormone oxytocin on the uterus during the birth process. The uterine muscle does not normally contract, but dilation of the cervix during labour causes stretch receptors to transmit nerve impulses to the hypothalamus in the brain, and this results in the release of oxytocin, a hormone that induces the uterus to contract. The contractions push the foetus through the cervix, and as the size of the foetus pushes and stretches the cervix even more, the result is that more oxytocin is released. This positive feedback will continue until the baby has cleared the birth canal and the cervix is no longer stretched.

Homeostasis is constantly being disturbed or challenged within the body, either externally, such as from overexposure to extremes in temperatures, or internally, such as from chemical imbalances or even psychological stress. While these changes are usually mild and temporary, lengthy or intense changes can lead to the failure of homeostasis, and the onset of disease. It is at this point that help is needed to regain health.

Reference: Clinically Oriented Anatomy, Moore

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