Hormones | Research & Encyclopedia Articles

Hormones

Hormones are chemicals that naturally occur in the body. These chemicals direct many biochemical events, including energy storage and utilization, maintenance, and growth. Hormones are released mainly by the endocrine glands. Specific endocrine glands include the pituitary, thyroid, adrenals, parathyroids, gonads (e.g., testes and ovaries), and the islets of Langerhans which are located in the pancreas. The endocrine glands release very small quantities of hormones into the blood, but these small quantities have a large impact throughout the body.

From a chemical standpoint, hormones can be divided into three categories. The first category is made up of hormones that are derived from tyrosine, a type of amino acid. Specific hormones that belong to this category are epinephrine and norepinephrine. The second category of hormones are those that are derived from steroids, a class of chemicals based on cholesterol. Steroidal hormones include testosterone, estradiol, progesterone, cortisol, and aldosterone. The third category comprises hormones that are constructed from peptides and proteins. This category is by far the largest one and includes such hormones as insulin, glucagon, adrenocorticotropic hormone, and human growth hormone.

Most hormones are stored in the endocrine gland responsible for its construction. Steroidal hormones are the main exception to this pattern. These hormones are manufactured and released as needed. Regardless of whether a hormone is stored or not, endocrine glands rely on chemical signals from the body to start and stop hormone release, otherwise called secretion. Hormone are secreted in extremely small amounts. Their concentration in the blood is in the billionths or even the trillionths. Such small quantities are effective, however, because the hormones' targets are extremely specific. These targets are called hormone receptors. Each hormone in the body has a set of specific hormone receptors. Only these receptors will react to the presence of the hormone. The hormone receptors are located on the surface of target cells. If the hormone is present, it latches onto its receptor and triggers a biological response in the target cell.

The biological response depends on the hormone's identity and the target cell's purpose. As an example, glucagon, which is released by the islets of Langerhans, triggers a biochemical reaction in the liver. This reaction releases glucose, one of the main sugars serving the body's energy needs. The specific biochemical reaction to glucagon involves activating a particular enzyme. An enzyme is a type of protein that triggers speedy chemical reactions. The enzyme activated by glucagon is glycogen phosphorylase. In its active form, the phosphorylase breaks down glycogen, a substance composed of chains of glucose molecules.

In order for hormones to be effective, their activity must be tightly regulated. The body can only function when its chemistry is finely balanced. One of the main functions of hormones is to maintain this balance, often referred to as homeostasis. Once a hormone has done its job, there needs to be a system to stop the endocrine gland from secreting any more hormone. One such system is negative feedback. In the case of glucagon, a negative feedback system is based on the blood glucose level. Rising blood glucose levels indicate that glucagon is doing its job. When blood glucose levels have reached a certain point, the body does not need any more glucagon. The glucose itself signals the islets of Langerhans to stop the secretion of the hormone.

Since blood glucose levels move in both directions, however, there needs to be some means of lowering blood glucose when it exceeds the proper balance. Blood glucose levels normally rise following a meal. The need is met by insulin, which forms a closed negative feedback loop in partnership with glucagon. One of the actions of insulin, which is also secreted by the islets of Langerhans, is to activate a specific enzyme in the liver. This enzyme is glycogen synthase, which helps remove glucose from the blood and store it as glycogen. When blood glucose levels fall to the proper level, insulin secretion is switched off. Between glucagon and insulin, blood glucose levels remain stable, allowing the body to use its energy as efficiently as possible.

Certain hormones are not controlled by negative feedback, but through an opposite system called positive feedback. Positive feedback is based on hormones themselves triggering further secretion. This system is unusual and only comes into play with certain hormones. One such hormone is the steroidal hormone, oxytocin. Oxytocin has several uses in the body, one of which is to stimulate contractions of the uterus during childbirth. When the cervix, or opening of the uterus, begins to dilate (widen) in preparation for delivery, the brain signals the pituitary gland to secrete oxytocin. As the oxytocin circulates, the uterus contracts and cervix dilates further. The dilation triggers additional brain signals to the pituitary and even more oxytocin is released. This cycle continues until the contractions of the uterus are strong enough to force the baby through the birth canal to delivery.

Hormones remain in the blood for varying lengths of time. Their duration depends on their activity and how long it needs to last. Some hormones, such as epinephrine--the so-called fight-or-flight hormone--last only seconds in the bloodstream before breaking down. Other hormones, such as hormones secreted by the thyroid, endure for several days. The time delay between hormone secretion and biological response also varies.

Hormone secretion is a delicately balanced activity. Some systems require the coordination of more than one hormone to generate particular biological responses. The adrenal hormones are a notable example of this type of coordination. The adrenal glands, located near the kidneys, secrete many different hormones that fall into two categories: mineralcorticoids and glucocorticoids. Mineralcorticoids are important in maintaining a proper mineral balance in the body, while glucocorticoids are vital for managing carbohydrates.

Cortisol, also called hydrocortisone, is one of the most potent of the glucocorticoids. It is released in response to stress. Stress can arise from any situation in which a person feels nervous or afraid. One of the main functions of cortisol is to mobilize the body's energy sources to deal with the stressful situation. If cortisol levels remain high due to continuous stress, a person may suffer health effects such as lowered resistance to disease and high blood pressure. The secretion of cortisol is preceded by several events. First, there is a nerve impulse to the hypothalamus, an endocrine gland located near the brain. This nerve impulse causes the hypothalamus to secrete a ACTH-releasing hormone. This hormone stimulates the pituitary, another endocrine gland near the brain, to secrete adrenocorticotropic hormone (ACTH). The final step in the process occurs when the ACTH prompts the adrenal glands to manufacture and release cortisol.

ACTH is not the only hormone produced by the pituitary. Like other endocrine glands, the pituitary gland is able to produce several hormones, each of which has its own purpose. However, the amounts of each hormone are not equal. More than a third of the pituitary gland's cells are dedicated to manufacturing and secreting human growth hormone. Human growth hormone production increases sharply as a person enters adolescence. The hormone causes bones to become longer and organs to grow larger. It also alters the body composition, allowing for an increase in muscle and a relative decrease in fat. Once full growth is achieved, secretion of growth hormone drops to a lower level.

In the 1990s, scientists began studying a possible exception to the idea that certain hormone receptors only react to specific hormones. The exception is based on the molecular similarity between environmental contaminants and certain hormones. These environmental contaminants are referred to as endocrine disruptors, because they may have the potential to interfere with regular endocrine activity. The interaction of hormones and their associated receptors is highly specific. However, due to their resemblance to hormones, suspected endocrine disruptors may be able to interact with the receptors to some degree. The fit is not perfect, but the disruptors might be able to cause a partial response or possibly block the receptor from the proper hormone.

Whether there is a link between endocrine disruptors and health effects is controversial idea. Some studies suggest that endocrine disruptors in the environment may pose a serious health threat to both animals and people. In other studies, animals that have been exposed to endocrine disruptors have suffered reproductive effects such as lowered fertility or sterility. By the late 1990s, researchers were continuing to investigate suspected endocrine disruptors in the environment and the effects that these chemicals might have on human and animal health.