Hormone Classification and Structure

In addition to the nervous system, the human body also has the endocrine system to help control and regulate the activities of different organs. The main way the endocrine system controls these structures is through the production and distribution of chemicals—each called a hormone—that are secreted by cells and travel through the extracellular fluid to particular organs. When the hormone reaches its designated area, it binds with the receptors found on the surface of the cells. The binding of a hormone with a receptor on the cell membrane initiates a response. This response can begin almost instantly or can occur after longer periods of time, such as several days. Once the response begins, the outcome of a hormonal response is typically of longer duration than responses to the signals produced by the nervous system. In addition to the length of time the response lasts, there are several other differences between the endocrine and nervous systems.

• Endocrine signals work across long distances, such as the pancreas releasing insulin, which regulates blood sugar levels throughout the body. Signals from the nervous system work over short distances, such as a knee jerk response that goes from the knee to the spinal cord rather than to the brain.
• Endocrine signals can reach anywhere the blood flows. Nervous signals can only go to specific locations within the proximity of nerve cells.
• Endocrine signals cause slower responses, such as the male body's response to testosterone during puberty. Nervous signals cause more rapid responses, such as the reaction to painful stimuli.

Hormones provide and regulate many activities within the body. These include growth, development, reproduction, cellular metabolism, immune response, and nutrient balance in the blood. The organs involved with the production of hormones are generally small in size and are widely scattered across the body. The main glands that are part of the endocrine system and release hormones include:

• Pineal—located in the diencephalon of the brain and controls the body's response to time and also has some reproductive function
• Pituitary—located beneath the brain and regulates the reproductive organs, thyroid, milk production, growth, and water regulation
• Thyroid—located in the neck and regulates growth, development, and metabolism
• Parathyroid—located in the neck and regulates calcium levels
• Thymus--located behind the sternum and plays a major role in the immune system by stimulating T cells to mature
• Adrenal—located on top of the kidneys and produces adrenaline, the hormone that initiates the fight-or-flight response
• Pancreas—located to the posterior of the stomach and releases insulin to regulate blood sugar in the body
• Gonads (ovaries and testes)—located in the lower abdomen. Ovaries produce estrogen and progesterone to encourage healthy growth of female sexual characteristics, such as increased breast tissue and pubic hair. Testes produce testosterone that initiates the production of sperm and male sexual characteristics, such as facial and pubic hair.

The pituitary gland is arguably the most important of the endocrine glands because it produces so many hormones. Located in the brain, the pituitary gland and its associated structures form a much-needed connection between the brain and the rest of the endocrine system. This hypophyseal portal system is the connection of capillaries that exists between the hypothalamus and the anterior pituitary gland that serves to deliver anterior pituitary hormones. Due to its location, almost all the blood coming to the pituitary has to pass through the hypothalamus. The capillaries feeding this area are very thin and allow for direct diffusion of hormones and other materials. This benefits the pituitary because signaling hormones that turn it on or off can be easily absorbed.

Hormones can either be soluble in water (hydrophilic) or in fats (lipophilic) and are also classified by how they are formed. Hydrophilic hormones are easily dissolved in water, but have a hard time dissolving in fats. Amino acid and peptide hormones are unable to cross the cell membranes of their target cells. Most hormones of this type are made up of amino acid chains of varying lengths, with the long chains being fully formed proteins. Adrenaline (epinephrine) and norepinephrine are hydrophilic amino acid hormones (called catecholamines). These chemicals are released into the blood to increase strength and oxygen supply when danger or excitement occurs. Melatonin is another example of an amino acid hormone. It is a chemical released by the pineal gland in the brain to help the body adapt to changes in light and darkness. Some hydrophilic hormones are said to be peptide derived. An example of a peptide-derived hormone is insulin, the glucose-regulating chemical produced by the pancreas.

Lipophilic hormones are those that can easily be dissolved in fats (lipids) and not in water. Examples of these hormones include those produced by the thyroid gland (such as thyroxine, or T4) and steroids, such as cortisol, which are derived from modifications to cholesterol molecules. Steroids include the male and female sex hormones testosterone, progesterone, and estrogen, which induce the formation of secondary sex characteristics and allow for the production of gametes. These hormones are unique in that they can cross the cell membrane to enter the target cell.

A group of related signaling compounds, called eicosanoids, are also lipid soluble. These compounds are produced by just about all cell membranes and are used by the body to raise the alert of inflammation and allergic reactions. An example of an eicosanoid would be a prostaglandin, which is responsible for raising blood pressure and causing the uterus to contract during childbirth. Because eicosanoids are typically localized in their effects, meaning they are not widespread throughout the body, they are not true hormones.