The Endocrine System
The functions of the Endocrine System are to maintain homeostasis in the body by releasing chemicals called hormones. The major prosses controlled by hormones are reproduction; growth and developement; mobilization of body defenses against stressors; maintenance of electrolyte, water, and nutrient balance of the blood; and regulation of cellular metabolism and energy balance. The Endocrine System regultaes processes that go on for relatively long periods of time, and in some instances, continuously.
Although the endocrine organs are small compared to organs of other systems and lack the structural and anatomical continuity of other organ systems, their role in maintaining homeostasis makes them very powerful. The key to this power comes from hormones that are produced and secreted. Hormones are defined as any chemical substance, secreted by cells into the extracellular fluids, that regulate metabolic activity of other cells in the body. Almost all hormones are amino acid-based molecules or steroids. Steriod hormones include the sex hormones made by the gonads and hormones produced bythe adrenal cortex. All of the others are amino acid derivatives. Local hormones called prostiglandins are the only hormones made from highly active lipids found in the cell's plasma membrane. Although blood-borne hormones circulate to all the organs of the body, a given hormone only affects certain tissue cells, called target cells. In order for a target cell to respondto a hormone, specific protein receptors must be present on its plasma membrane or in its interior, to which that hormone can attach. Only when this binding occurs can the hormone affect the workings of a cell.
The term hormone comes from a Greek word meaning "to arouse." The body's hormones do just that. They arouse or bring about their effects on the body's cells primarily by altering cellular activity- that is by increasing or decreasing the rate of a normal, or usual metabolic process rather than by stimulating a new one. Typically one or more of the following occurs when hormones bind to a cell:
1. Change in plasma membrane permeability or electrical stste.
2. Activation or inactivation of enzymes.
3. Stimulation of the genetic materia-l itself to produce the instructions for making particular enzymes or other proteins.
Negative feedback mechanisms are the chief means of regulating blood levels of nearly all hormones. In this system, hormone secretion is triggered by some internal or external stimulus; then rising hormone levels inhibit further hormone release (even while promoting responses in their target organs). As a result, blood levels of many hormones vary within a very little range.
The stimuli that activate the endocrine organ fall into three categories: hormonal, humoral, and neural. The most common is the hormonal, in which endocrine organs are put into action by other hormones. For example, many anterior pituitary hormones stimulate other endocrine organs to release their hormones into the blood. As the hormones produced by the final target glands increase in the blood, they feed back to inhibit the release of anterior pituitary hormones and thus their own release. Changing blood levels of certain ions and nutrients are known as humoral stimuli. For example, the release of insulin by the pancreatic cells is promoted by increasing blood sugar levels. Since insulin increase the ability of cells to take up glucose, the blood sugar level soon decreases. This ends the stimulus for the insulin production. When nerve fibers stimulate hormone release and the target cells are said to respond to neural stimuli. The classic example of this is the sympathetic nervous system stimulation of the adrenal medulla to release norepinephrine and epiniphrine during periods of stress. These are the main stimuli for the endocrine glands.
The Endocrine Glands
Pituitary Gland
The pituitary gland is about the size of a grape and has two functional lobes: the anterior pituitary (glandular tissue) and the posterior pituitary (nervous tissue).
Hormones of the Anterior Pituotary
There are six anterior pituitary lobe hormones: two of them (growth hormone and prolactin) exert their major effects on nonendocrine targets. The remaining four (thyrotropic hormone, adrenocortiotropic hormone, and the two gonadotropic hormones) are all tropic hormones. Tropic hormones stimulate their endocrine glands which produce their hormones. All anterior pituitary hormones (1) are proteins or peptides, (2) act through second-messenger systems, and (3) are regulated by hormonal stimuli, and usually negative feedback.
Hormones of the Posterior Pituitary
The posterior pituitary is not really an endocrine gland in the strict sense because it does not make the peptide hormones that it releases. Instead it simply acts as a storage area for hormones made by hypothalamic neurons. Oxytocin is released in significant amounts only during childbirth an din nursing women.Antidiuretic hormone (ADH) is a chemical that inhibits or prevents urin production.
Thyroid Gland
The thyroid gland is located at the bas of the throat just inferior the Adam's apple. It consists of two lobes connected by and isthmus. The thyroid gland produces two hormones: thyroid hormone and calcitonin. Thyroid hormone is actually two iodine-containing hormones: thyroxine and triiodothyronine. Together, thyroid hormone controls the rate at which glucose is burned (or oxidized) and turned into body heat and chemical energy. Every cell in the body is a target for this hormone because they akk depend on a continuous supply of energy. Calcitonin, or thyrocalcitonin, decreases calcium levels by causing calium to be deposited in the bones.
Parathyroid Glands
The parathyroid glands are tiny masses of glandular tissue found on the posterior surface of the thyroid gland. There are two lobes on each gland, and these four lobes secrete parathyroid hormone or parathormone. This hormone acts as a counter to calcitonin. When calcium levels drop in the blood, it breaks down bone and releases calcium into the blood.
Adrenal Gland
The adrenal glands rest above the kidneys. Although it looks like one gland, like the pituitary gland, it has both glandular (cortex) and neural tissue (medulla) parts.
The adrenal cortex produces three major groups of of steroid hormones collectively called corticosteroids- the mineralocorticoids, the glucocorticoids, and the sex hormones. The the minerocorticoids are produced by the outermost layer of the adrenal cortex and regulate the mineral content of the blood. The glucocorticoids (produced in the middle layer of the cortex) promote normal cell metabolism and help the body to resist long-term stressors, primarily by increasing blood glucose levels. Both male and female sex hormones are produced by the innermost layer of the adrenal cortex throughout life in relatively small amounts.
The medulla developes from a knot of nervous tissue. When stimulated by the sympathetic nervous system, it produces epinephrine norepinephrine. Together, these help the body react to a "fight-or-flight" situation.
written by Michael Simpson
