Endocrine glands, (endo = within) on the other hand, constitute their own body system. They are "ductless" and secrete chemicals, called hormones, directly into the blood. These hormones (chemical messengers) are then transported through the blood to specific organs in the body. When they arrive at the target organ, the hormone causes a stimulant or suppressive action on a specific type of cell or tissue.
The glands that comprise the endocrine system are the pituitary, thyroid, parathyroids, pancreas, adrenals, gonads, pineal, and thymus. Additionally, mucosal cells within the duodenum and placenta are considered part of the endocrine system.
The thyroid gland is positioned just below the larynx. It is a flat structure that is roughly the shape of a shield, consisting of two lobes that lie on either side of the trachea and are connected by a broad isthmus. This is the largest of the endocrine glands, weighing between 20 and 25 gm, and has both a rich blood and lymph supply. It is comprised of many round hollow sacs, called follicles, which synthesize the thyroid hormones and store them. In between the follicles are epithelial cells that also produce a hormone.
The two main hormones produced by the thyroid gland are triiodothyronine (T3) and thyroxine (T4). These are stored in the follicles and are utilized to regulate basal metabolic rate (the rate at which the body uses oxygen to transform nutrients into energy). They also regulate normal growth and development, including mental processes and sexual maturity. Together they are referred to as thyroid hormone (TH). The third hormone, calcitonin (also called thyrocalcitonin), regulates the calcium balance of the blood. All three hormones are made up primarily of iodine, which is essential for normal thyroid function.
Embedded on the posterior surface of the lateral lobes of the thyroid gland, but structurally distinct from it, are four parathyroid glands, two superior and two inferior.
The hormone secreted by the parathyroids is called parathormone (PTH). This hormone regulates the ratio of calcium and phosphorus in the blood and bone tissues.
The pancreas is a flat, tongue-shaped fleshy organ lying against the abdominal wall in the left upper quadrant. It is 12-15 cm in length, 5-6 cm in greatest width, and functions as both an exocrine and endocrine gland.
The endocrine portion of the pancreas consists of scattered clusters of cells called the islets of Langerhans. Two types of cells make up these clusters: alpha cells and beta cells. The hormones produced by them are taken up in the blood and do not enter the duct system of the pancreas.
The alpha cells produce a hormone called glucagon. This converts glycogen into glucose and regulates the level of glucose in the blood. The beta cells produce a hormone called insulin that increases the cellular utilization of glucose. Together these hormones are essential for the metabolism of carbohydrates. There are two rare cell types as well: Delta cells that secrete somatostatin, and F (PP) cells that secrete pancreatic polypeptide (a hormone that may inhibit pancreatic exocrine activity).
Triangular-shaped adrenal glands sit atop each kidney. Each one has an outer cortex and an inner medulla that individually function as separate glands. The inner medulla produces two hormones, epinephrine and norepinephrine. These two hormones, often called adrenalin, increase cardiac output and heart rate, dilate coronary vessels, increase mental alertness, increase respiratory rate, and elevate the overall metabolic rate. The outer cortex produces a variety of hormones, including cortisol to regulate carbohydrate and protein metabolism, aldosterone to control fluid and electrolyte balance, and androgen (male hormones) that affect the growth of body and facial hair.
In all, approximately 30 hormones are produced by the adrenal glands, and modern medicine does not fully understand the interplay and effects of all of them.
prolactin - Hormone that works with other hormones to initiate the secretion of milk by the mammary glands.
vasopressin - Constricts the blood vessels causing increased blood pressure, and permit the kidneys to reabsorb water.
isthmus - Connects the two lobes of the thyroid gland.
thyroxine - Regulates normal growth and development, including mental processes and sexual maturity.
calcitonin - Regulates the calcium balance of the blood.
islets of Langerhans - Endocrine portion of the pancreas that consists of scattered clusters of cells.
glucagon - Converts glycogen into glucose and regulates the level of glucose in the blood.
insulin - Increases the cellular utilization of glucose.
epinephrine - Increases cardiac output and heart rate, dilates coronary vessels, increase mental alertness, increases respiratory rate, and elevates the overall metabolic rate.
norepinephrine - Increases cardiac output and heart rate, dilates coronary vessels, increase mental alertness, increases respiratory rate, and elevates the overall metabolic rate.
adrenaline - Another name for epinephrine and norepinephrine.
aldosterone - Controls fluid and electrolyte balance.
androgen - Affects the growth of body and facial hair.
melatonin - Plays a role in sexual maturation and in maintaining daily body rhythms.
Although there are symptoms that these pathologies produce, they are similar to the symptoms discussed in earlier chapters, including, among others, polydipsia, polyuria, hypertension, syncope, weight gain or loss, confusion, and seizures.
Improper nutrition can either overstimulate or understimulate hormone function and create somatic imbalance. Some abnormalities can even be controlled partially or completely eradicated by a good diet.
acromegaly - Disease characterized by large, coarse features, particularly of the face and hands. This results from overproduction of the growth hormone.
Addison's disease - A chronic type of adrenocortical insufficiency, characterized by hypotension, weight loss, anorexia, weakness, and a bronze-like hyperpigmentation of the skin.
cachexia - A profound and marked state of constitutional disorder; general ill health and malnutrition.
Cushing syndrome - A condition resulting from an excess of the adrenocorticotropin hormone.
cystic fibrosis - Widespread dysfunction of the exocrine glands occurring in infants, children, and young adults.
de Quervain thyroiditis - A condition characterized by fever and painful enlargement of the thyroid gland, often following a viral infection.
diabetes insipidus - A temporary or chronic disorder of the posterior lobe of the pituitary gland involving a deficiency of the vasopressin hormone.
diabetes mellitus - A syndrome characterized by hyperglycemia resulting from impaired insulin secretion or effectiveness. It is classified into two main categories, noninsulin dependent and insulin dependent.
Diabetes Mellitus Type 1 - also called insulin-dependent diabetes mellitus (IDDM). Type 1 diabetes mellitus is also referred to as juvenile diabetes. It is characterized by an abrupt onset of symptoms, usually in early adolescence, in persons who suffer from an insufficient amount of insulin being produced by the pancreas.
Diabetes Mellitus Type 2 - may be either non-insulin-dependent diabetes mellitus (NIDDM) or insulin-dependent diabetes mellitus (IDDM). This is also referred to as adult-onset diabetes, because it is usually found in adults over 45 who have a family history of diabetes, are overweight, do not exercise, and/or have high cholesterol levels.
(Note on formatting: The latest word from the American Diabetes Association is that roman numerals are no longer to be used to designate diabetes types. The preferred usage is now type 1 and type 2. Not all codebooks and dictionaries reflect this change, but some medical records may.)
dwarfism - The congenital underdevelopment of the body.
endemic - Present or prevalent in a population or geographic area at all times; said of a disease or agent.