ENDOCRINE GLANDS

The human body has two types of glands, duct and ductless glands. Duct glands (also known as exocrine glands) are glands which have ducts (canals) which limit the area of their secretions. Examples of duct glands are tear glands, sweat glands and salivary glands. Ductless glands are also known as endocrine glands. Endocrine glands release their secretions, called hormones, into the blood stream to be carried to other locations where they take their action. Examples of endocrine glands are: the pituitary, thyroid, parathyroids, adrenals, pancreas, testes, and ovaries. The study of endocrine glands is called endocrinology.

The word hormone was derived from the Greek word hormodezein which means "to arouse." While many hormones do arouse to create action in glands and organs, their primary function is to regulate glands and organs.

The pituitary gland is called the "master gland" because its various hormones regulate all other endocrine glands. The pituitary gland is located in the brain near the top of the brain stem, and is connected to the brain by a short stalk called the infundibulum. The pituitary gland is about the size of a cherry and is divided into three regions. Of these, the anterior (front part) lobe is the largest. The posterior (back part) is only slightly smaller than the anterior lobe. Between the anterior and posterior lobes is third region, the pars intermedia. Each of these regions produces its own group of hormones.  

The anterior lobe of the pituitary produces the somatotrophic hormone, which in people is commonly called the human growth hormone (HGH). If this hormone is produced in greater than normal quantities, the bones grow very long. HGH acts to prevent the epiphyseal junction from forming, thus making continued growth possible in these bones. The result of this greater than normal production of HGH is pituitary giantism, and they may grow to be 9 feet tall. Giantism is accompanied by a corresponding increase in size of the internal organs. If there is an increased secretion of HGH after the appearance of the epiphyseal line, the long bones do not grow. However, there is growth in the bones of the face, hands and feet. This condition is called acromegaly.

Deficiency of HGH causes pituitary dwarfism. If dwarfism is recognized early in a child or adolescent, HGH produced through genetic engineering can be administered causing the child to grow.

A second hormone produced by the anterior pituitary is prolactin. In mammals, prolactin stimulates the production of milk in the mammary glands which is used to nourish the young. It may also stimulate maternal behavior. A third hormone secreted by the anterior pituitary is the thyroid stimulating hormone. This hormone causes the thyroid gland to function, and if this hormone is absent, the thyroid gland shuts down.

The anterior lobe of the pituitary also produces the adrenocorticotropic hormone (ACTH). ACTH influences the amount of various corticoids produced by the cortex of the adrenal glands. Corticoids help regulate the amount of minerals in our bodies, as well as carbohydrate metabolism.

The anterior pituitary also produces the two gonadotrophic hormones, the follicle-stimulating hormone and the luteinizing hormone. The follicle-stimulating hormone (FSH) causes both egg cells and sperm cells to mature so that reproduction can occur. The luteinizing hormone (LH) works with the follicle-stimulating hormone to prepare the female body to nourish a developing child during pregnancy.

The posterior lobe of the pituitary gland secretes two hormones, the antidiuretic hormone (ADH) and the oxytocic hormone. ADH controls the reabsorption of water from the kidneys into the blood stream. If this hormone is deficient, the blood stream does not reabsorb water from the kidneys and urine is formed in excess amounts. The resulting disease, called diabetes insipidus, may result in the excretion of as much as 10 gallons of urine per day. ADH also increases blood pressure. Once it was believed that a separate hormone, called vasopressin, acted on the walls of arteries to produce an increase in blood pressure (hypertension); but it was recently discovered that vasopressin is, in actuality, ADH. A diuretic drug stimulates urination, usually by decreasing the action of ADH. By doing this, diuretic drugs are useful in controlling high blood pressure.

The oxytocic hormone (oxytocin) causes the contractions which lead to child-birth. The oxytocic hormone also increases blood pressure and decreases the formation of urine during pregnancy.

The pars intermedia produces a hormone called intermedin. Little is known of the function of this hormone in people. In fish it functions to darken the scales.

The thyroid gland is located in the approximate area of the throat, lying along side of the larynx. It is shaped like the letter H. The thyroid gland uses iodine and the amino acid tyrosine to produce the hormones thyroxin and triiodothyronine. Both of these hormones function to regulate cellular metabolism. Metabolism refers to all of the processes that make energy available to cells. As such, these hormones regulate the conversion of glycogen (stored glucose) to glucose.

The deficiency of thyroxin results in a reduction in the metabolism rate. This condition is called hypothyroidism. If hypothyroidism occurs during infancy or childhood the result is called cretinism. Cretinism is characterized by stunted physical growth and mental retardation. If hypothyroidism occurs in an adult, it results in such symptoms as: reduced body temperature, a decrease in blood pressure and heart rate, dryness of the hair and skin, loss of energy, weight gain and depression. Hypothyroidism is treated medically by administering thyroxin.

The opposite condition, hyperthyroidism, results from an over-production of thyroxin. The symptoms of hyperthyroidism include: rapid heart rate, increase in blood pressure, increase in metabolism (resulting in weight-loss), oily skin, an increase in body temperature, excess sweating, and nervousness. People suffering from hyperthyroidism may eat very large amounts of food and still lose weight. Hyperthyroidism is treated by removing part of the thyroid gland and by the use of drug therapies.

A goiter is an enlarged thyroid gland, which results in a swelling in the neck. A goiter can result from either hyper or hypothyroidism, but it is more common in hypothyroidism. A goiter is generally caused by a lack of iodine. The thyroid stimulating hormone of the pituitary gland causes the thyroid gland to work. But without iodine, no thyroxin is produced. The pituitary gland responds to this by making the thyroid gland work harder and harder. Therefore, like a muscle that you work with a barbell, the thyroid gland gets bigger. We get most of our iodine from seafood and from vegetables grown in soil which contains iodine. Iodine is often supplemented in the diet by including it in salt (iodized salt).

 

On the back side of the thyroid gland are four small bodies known as the parathyroid glands. These are the smallest endocrine glands. The parathyroid glands produce the parathyroid hormone which plays an important role in regulating calcium and phosphate ions in blood. These two ions are important in nerve and muscle function, and in maintaining bone structure. Vitamin D is important to the parathyroid glands because it promotes retention of calcium and phosphate. Without vitamin D bones cannot develop and they become soft and bend. This is the disease called rickets.

The adrenal glands look like caps sitting on top of each kidney. Each adrenal is composed of two distinct regions: an outer area called the adrenal cortex, and an inner part called the adrenal medulla.

The adrenal cortex produces hormones that belong to a group of chemical compounds known as corticosteroids, corticoids, or just steroids. The adrenal cortex makes many different steroids. The exact number is not known, but it is believed to be over 30. The adrenal cortex has three layers, and each layer produces different corticosteroids. The steroids corresponding to the three layers are: the mineralocorticoid (outer layer), glucocorticoids (middle layer) and the androgenic steroids (inner layer).

Aldosterone is the main mineralocorticoid. It functions to cause the kidneys to reabsorption sodium and excrete potassium. This helps to keep these two ions in balance. Hydrocortisone is the principle glucocorticoid. Hydrocortisone help regulate the metabolism of carbohydrates, proteins and fats. It also helps us cope with stress. Glucocorticoids cause a rapid destruction of certain white blood cells, thereby lowering our resistance to disease. Because they depress the immune system, glucocorticoids are used to treat many autoimmune diseases. The production of glucocorticoids is under the control of the adrenocorticotropic hormone (ACTH). ACTH is produced by action of the pituitary gland.

The third group of steroids is the androgenic steroids. These hormones are called androgens and are male sex hormones. Testosterone is the main androgen. While the adrenals of males and females produce near equal amounts of androgens, males have an additional supply of androgens produced by the testes.

If the adrenal cortex produces insufficient amounts of glucocorticoids and mineralocorticoids the result is called Addison's Disease. The symptoms of Addison 's include: electrolyte imbalance, fatigue, muscular weakness, vomiting, diarrhea, loss of weight, low blood pressure, and excess pigmentation of the skin (bronzing). Addison's Disease can often be controlled with hormone replacement therapy. Excess production of ACTH or a tumor within the adrenal cortex results in Cushing's Disease. The symptoms of Cushing's include: obesity, high blood pressure, a round "moon" face, muscular weakness, a tendency to bruise easily, and poor healing of skin lesions.

The adrenal medulla produces epinephrine and norepinephrine, often called adrenalin and noradrenaline. Both of these hormones are involved in the stress reaction called the "fight or flight response." When confronted with danger, these two hormones prepare our body to fight or to run. These hormones constrict the blood vessels of the kidneys and digestive system, dilate blood vessels in the heart and brain, raise blood pressure, and increase respiration. They constrict the blood vessels of the skin so that we will lose less blood should we be bitten, cut or shot. They rapidly convert glycogen to glucose, so that we might have extra energy. And these hormones cause more neurotransmitters to be available to our nervous system, so that our nerves can activate more muscle cells making us faster and stronger. All of these changes prepare us to respond to danger.

The pancreas lies between the kidneys. Cells known as the islets of Langerhans are scattered throughout the pancreas. The islets of Langerhans have two types of cells: alpha cells which produce a hormone called glucagon, and the beta cells which produce a hormone called insulin. Insulin is the hormone which escorts glucose across the cell membrane. Without insulin, glucose cannot enter the cell and, therefore, cannot be used to produce energy. Glucagon functions to cause the liver to convert more glycogen to glucose, thereby, raising the blood sugar level.

If the Islets of Langerhans stop or lessen their production of insulin, the result is diabetes mellitus. The symptoms of diabetes include: loss of weight, excessive thirst, increase in urination, itching in the skin, and fatigue. Because glucose cannot enter cells, the blood sugar level is high. This high blood sugar level causes sugar to be excreted in urine. High volumes of water are needed to remove the sugar from the kidneys. Since water is used for this purpose, thirst occurs to replace this lost water.

Since glucose is not available for cells to produce energy, protein is used instead. This produces weight-loss, as the protein is no longer available to form protoplasm. When cells attempt to use fat to produce energy, the fat is incompletely burned and forms ketone bodies. Ketone bodies are highly toxic and damage many parts of the body. They may also be at least partially responsible for diabetic coma (unconsciousness).

There are many causes for diabetes. Two common types are juvenile diabetes and adult-onset diabetes. In juvenile diabetes, because of genetics, it is suspected that when the child contracts a specific virus, the thymus gland makes a white blood cell to eat the virus. However, this particular white blood cell thinks that beta cells do not belong to the body and eats them, too. The result is no insulin producing cells at all. Such diabetics require an injection of insulin before each meal.

In adult-onset diabetes there are beta cells which still produce insulin. However, because of factors, such as being over-weight, for example, there is not enough insulin to serve these large fat cells. Adult-onset diabetes may be controlled by diet, by drug treatments or insulin injections. Insulin must be injected as digestion destroys insulin.

For many years diabetics injected insulin acquired from animals. However, sometimes insulin from a certain animal would cease to control their disease, and they would have to switch to insulin obtained from another animal. Today human insulin can be made by genetic engineering, so that such sensitization does not occur.

The opposite of diabetes is hypoglycemia. Hypoglycemia occurs when the pancreas produces too much insulin. Since insulin escorts glucose across the cell membrane, there is much sugar inside of cells and a low blood sugar level. This low blood sugar is read by the hypothalamus which causes a perception of hunger. Therefore, hypoglycemics appear to be hungry almost all of the time. For this reason hypoglycemics often eat excessively and gain weight. Hypoglycemics often feel faint, weak and develop the "shakes." Most hypoglycemia can be controlled by diet--eating six small protein meals each day.

The gonads are sex glands and consist of the ovaries in females and the testes in males. Testes produce sperm and male sex hormones, and ovaries produce female sex hormones.

The effect of sex hormones usually becomes noticeable between the ages of 12--15 years of age when most boys and girls reach puberty. Puberty is the biological event which brings the child into adolescence. It occurs about one to two years earlier in girls than in boys. Puberty begins when the pituitary gland releases the gonadotropic hormone. The gonadotrophic hormone acts on the gonads causing them to become functionally mature. When this occurs, the person will soon become reproductively mature, and the gonads will greatly increase their production of sex hormones.

These sex hormones produce the secondary sex characteristics for the sexes. These are things such as facial hair in males, the appearance of breasts in females, pubic and axillary hair (hair in the arm pits), lower voice for both sexes, and the other characteristics which describe men and women.

Three different groups of hormones are produced by the various organs of the female reproductive system. These are called the estrogenic hormones (commonly called estrogen). The first group, composed primarily of the hormone beta-estradiol is responsible for maintaining the uterine environment, and it also effects behavior. The second group, composed primarily of progesterone, makes the uterus favorable to the reception of a developing embryo. The third, chorionic gonadotrophin, prevents ovulation during pregnancy.

The male sex hormones are called androgens. The main androgen is the steroid hormone known as testosterone. Testosterone causes males to have larger muscle mass, and to be more aggressive in behavior.

The pineal body (gland), attached to the lower surface of the brain, is sometimes considered an endocrine gland. The pineal gland secretes two hormones: serotonin and melatonin. Serotonin functions as a neurotransmitter, and has been proven to be involved in some depression. The pineal gland is stimulated to produce serotonin by sunlight. The lack of sunlight in winter can result in a type depression called Seasonal Affective Disorder or SAD. SAD can usually be treated with fluorescent lights, or with antidepressant drugs such as Prozac. Melatonin may be a hormone which triggers sleep. Melatonin may also be useful to correct "jet lag," as it can be used to alter our "biological clocks."