The chemical messengers released by endocrine glands are called hormones.Hormones are leased into the bloodstream and travel all over the body.Some hormones can affect millions of cells simultaneously.Their effects last for minutes or even hours or days.Many hormones are secreted constantly;and the amount secreted changes as needed.

Like neurotransmitters, hormones work by binding to receptors on target cells.One special class of hormones, steroids, is particularly powerful because steroids can bind to sites inside the cells.Steroids are lipid molecules that can pass easily through the target cell membrane.These hormones, then, can interact directly with the cell's DNA, the genetic material, to change cell activity.These hormones are carefully regulated by the body because of their ability, even in very small amounts, to control target cells.

Thyroid hyperplasia Source: BioMed Central

Hormonal Control

There are three basic ways in which endocrine organs function to maintain hormone levels and body function.Some hormones are directly controlled by the nervous system.Other hormones are part of a hierarchy of hormonal control in which one gland is contolled by the release of hormones from another gland higher in the chain, which is controlled by another gland's release of hormones yet higher in the chain.Orders are sent from one organ to another.This is very similar to a relay race at a track meet where the baton is smoothly handed from one runner to the next.For example, the hypothalamus has control over the pituitary, which has control over the adrenal gland, which secretes the hormone cortisol.Increased cortisol secretion is one way that the body copes with stress, and as cortisol levels rise in the blood, further release of hormones at the hypothalamus is depressed.

Peptide Hormones

Hormones come in two classes: peptide hormones and steroid hormones. Peptide hormones are made from amino acids. They are essentially protein molecules, but some are very small and are referred to as peptides. Because they cannot cross cell membranes, peptide hormones must bind to receptors outside the cell on its surface. Peptide hormones generally cause their effects rapidly. They do this by turning existing enzymes in the cell on or off. Some examples of peptide hormones are insulin, prolactin, and glucagon.

Steroid Hormones

Steroid hormones are made from cholesterol. They are lipids and can easily cross the cell membrane, so they bind to receptors inside the cell. Steroid hormones generally cause their effects more slowly than peptide hormones. They cause their effects by binding to DNA and changing which genes get transcribed. some examples of steroid hormones are aldosterone, estrogen, and testosterone.

Most hormones are named according to where they are secreted or what they do.For example, growth hormone stimulates cells to grow.Prolactin increases milk production.Most hormones are known by their abbreviations.Adrenocorticotropic hormone, for example, is abbreviated ACTH, which is much easier to say and write.



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