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
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.
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.