What Is A Hormone?
A hormone is a natural substance found in both plant and animal species. Functions of hormones include both regulation of physiological activities and maintenance of homeostasis. Hormones carry messages between cells, causing a reaction at its target. They travel either by secretion into the blood or diffusion. Hormones that travel by diffusion are known as ectohormones.
In humans, hormones are produced and excreted by different glands, organs, and tissues. The rate of production is generally determined by the body’s internal stabilization system — homeostasis. When certain factors force a body to react in order to maintain its consistency, the body produces hormones to counteract the effects of those factors. For example, when a person turns out the lights to go to sleep, the pineal gland produces melatonin — the hormone responsible for controlling the circadian rhythm.
Hormone secretion can be both stimulated and inhibited by a number of factors. First, levels of hormone secretion can be caused by other hormones — certain hormones work by carrying messages that cause the secretion or inhabitation of other hormones. Second, levels of secretion can be caused by environmental factors; for example, when a person eats a large meal, insulin is secreted by the pancreas. Third, levels of hormone secretion can be caused by neurological processes — the transfer of impulses across nerve cells. Finally, levels of secretion can be caused by both plasma concentrations of either ions or nutrients and binding globulins.
Hormone secretion has many effects on the human body with each different hormone causing a different reaction. The different types of hormones as well as their effects are discussed in detail in the section “Human Hormones.”