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The Human Endocrine System

Endocrinology is the study of chemical communication systems that provide the means to control a huge number of physiologic processes. Like other communication networks, endocrine systems contain transmitters, signals and receivers that are called, respectively hormone producing cells, hormones and receptors.

The first step in understanding endocrinology is to explore the meaning of such terms as hormone, receptor and target cell, and to obtain an understanding of how chemical communication is controlled.

How is it that humans and animals maintain quite constant blood concentrations of glucose throughout their lives despite wildly varying frequencies of meals? If your blood glucose concentration drops much below 1 mg per ml, your neurons will begin to misbehave, leading ultimately to coma and death. Yet skipping breakfast is rarely life-threatening.

The answer is that a battery of chemical messengers - hormones - are secreted into blood in response to rises and falls in blood glucose concentration and stimulate metabolic pathways that pull glucose concentrations back into the normal range.

Definitions

Pheromones - chemical messengers that travel between organisms and which cause physiological changes in the organism detecting the phermone.

Prostaglandins. - modified lipids that are not produced by specialized glands; they are released into the interstitial fluids and cause local physiological effects.

Important Concepts

1. The endocrine system of vertebrates comprises the pineal gland, hypothalamus, pituitary glands, thyroid, parathyroid, thymus, adrenal glands, pancreas, placenta (when present) and the gonads (ovaries in females and testes in males).

2. Insulin and glucagon are peptide hormones produced in the pancreas. Insulin allows the uptake of glucose into cells and stimulates the conversion of glucose into glycogen in the liver and skeletal muscles. Glucagon stimulates the breakdown of glycogen in the liver. Insulin and glucagon are examples of a pair of hormones that act antagonistically and whose production is regulated by the same thing - in this case, the concentration of blood glucose. Many hormones exist in these kinds of pairs.

3. The hypothalamus secretes releasing hormones, which stimulate the release of hormones from the anterior pituitary. The anterior pituitary secretes several tropic hormones, hormones that have glands as their targets. This is another common way for hormone production to be regulated.

4. The endocrine and nervous system interact at several points. The hypothalamus is brain tissue and thus receives information from the peripheral nervous system. The adrenal medulla evolved from nervous tissue and is stimulated by nerve impulses. Some of the hormones are similar or the same as neurotransmitters.

 

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