Hormones

Hormones elicit a response at their target tissue, target organ, or target cell type through receptors. Receptors are molecular complexes which specifically recognize another molecule-in this case, a particular hormone. When the hormone is bound by its receptor, the receptor is usually altered in some way that it sends a secondary message through the cell to do something in response. Hormones that are proteins, or peptides (smaller strings of amino acids), usually bind to a receptor in the cell's outer surface and use a second messenger to relay their action. Steroid hormones such as cortisol, testosterone, and estrogen bind to receptors inside cells. Steroids are small enough to and chemically capable of passing through the cell's outer membrane. Inside the cell, these hormones bind their receptors and often enter the nucleus to elicit a response. These receptors bind DNA to regulate cellular events by controlling gene activity.

Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. Plasma concentrations of all hormones are assessed at some site which has receptors binding that hormone. The site keeps track of when the hormone level is low or high. The major area which records this information is the hypothalamus. A number of hormones are secreted by the hypothalamus which stimulate or inhibit additional secretion of other hormones at other sites. The hormones are part of a negative or positive feedback loop.

Most hormones work through a negative feedback loop. As an example, when the hypothalamus detects high levels of a hormone, it reacts to inhibit further production. And when low levels of a hormone are detected, the hypothalamus reacts to stimulate hormone production or secretion. Estrogen, however, is part of a positive feedback loop. Each month, the Graafian follicle in the ovary releases estrogen into the bloodstream as the egg develops in ever increasing amounts. When estrogen levels rise to a certain point, the pituitary secretes luteinizing hormone (LH) which triggers the egg's release of the egg into the oviduct.

Not all hormones are readily soluble in blood (their main transport medium) and require a transport molecule that will increase their solubility and shuttle them around until they get to their destination. Steroid hormones, in particular, tend to be less soluble. In addition, some very small peptides require a carrier protein to deliver them safely to their destination, because these small peptides could be swept into the wrong location where they would not elicit the desired response. Carrier proteins in the blood include albumin and prealbumin. There are also specific carrier proteins for cortisol, thyroxin, and the steroid sex hormones.