THE ENDOCRINE SYSTEM
Homeostasis, the self balancing of the body’s internal environment, is achieved and maintained by the endocrine system and the nervous systems. These systems work in concert to perform similar functions in the body: communication, integration, and control. Their communication capabilities provide the means for controlling and integrating the many different functions performed by organs, tissues, and cells. The endocrine system performs these functions by different mechanisms than the nervous system.
The endocrine system sends messages by way of chemical messengers called hormones. Minute amounts of these hormones are secreted from endocrine gland cells into the blood and distributed by the circulatory system.
Table 6-7.—Types of Receptors, Their Locations, and Affected Senses
Cells that are affected by the hormone are referred to as target organ cells. Endocrine glands secrete hormones directly into the blood, because they have no duct system. The glands of this system are often called ductless glands unlike exocrine glands that secrete their products into ducts. Many hormones can be extracted from the glands of animals or produced synthetically. Medical Officers may prescribe these naturally derived or synthetic hormones for patients deficient in them or might otherwise benefit from their use. Oxytocin (the hormone which stimulates uterine contractions during pregnancy) has been synthesized and is used during the delivery process for women who are deficient in this hormone.
The hormone-producing glands include the hypothalamus (Fig. 6-61), pituitary, pineal, thyroid, parathyroids, adrenals, pancreas, gonads (the testes and ovaries), placenta, and thymus (Fig. 6-62).
The hypothalamus, a structure in the brain, synthesizes chemicals that are secreted to the pituitary gland to release hormones and to help regulate body temperature (Fig. 6-61).
The pituitary is a small, pea-sized gland located at the base of the brain in the sella turcica, the Turkish saddle-shape depression of the sphenoid bone. The pituitary is connnected to the hypothalamus by the stalk called infundibulum. (Fig. 6-61). It is often called the master gland of the body as it influences many other endocrine glands. Although the pituitary looks like just one gland, it actually consists of two separate glands, the anterior pituitary gland and the posterior pituitary gland (Fig. 6-62).
Anterior Pituitary Gland
The anterior pituitary gland plays the more important role in influencing body functions. The five main secretions produced by the anterior pituitary gland have a broad and significant range of effects.
SOMATOTROPHS.—Somatotropin, the growth hormone, influences body growth and development. During the growth years, an overproduction of somatotropin causes Giantism, while the lack of it causes Dwarfism. An overproduction after the growth years causes acromegaly, which is characterized by the development of abnormally large hands, feet, and jaw.
THYROTROPHS.—Thyrotropin, or the thyroid-stimulating hormone (TSH), influences the growth, development, and secreting activities of the thyroid gland.
GONADOTROPHS.—Gonadotropin influences the gonads and is essential for the normal development and functioning of both male and female reproductive systems.
CORTICOTROPHS.—The adrenocorticotropin hormone (ACTH) acts primarily on the adrenal cortex (the outer portion of the adrenal glands), stimulating its growth and its secretion of corticosteroids. Corticosteroid hormones affect every cell in the body.
LACTOTROPHS.—Prolactin, (PRL): “during pregnancy, a high level of PRL promotes the development of the breasts in anticipation of milk secretion. At the birth of an infant, PRL in the mother stimulates the mammary glands to begin milk secretion2.”
Posterior Pituitary Gland
The posterior pituitary gland stores two hormones, antidiuretic hormone (ADH) and oxytocin.
ANTIDIURETIC.—The ADH hormone, promotes the conservation of water by the kidney. ADH stimulates contraction of muscles in the wall of small arteries thus increasing blood pressure by retaining fluids in the vasculature. When ADH is not produced in adequate amounts, the daily urine volume increases to 10 and 15 liters instead of the normal 1.5 liters. This condition is known as diabetes insipidus.
OXYTOCIN.—Oxytocin stimulates contraction of the muscles of the uterus, particularly during delivery of a baby. It also plays an important role in the secretion of milk in the mammary glands of nursing mothers.
The pineal gland, or pineal body, is a tiny structure resembling a pine nut located on the dorsal aspect of the brain’s diencephalons region. It produces small amounts of different hormones with melatonin being the main one. It is known as the biological clock; melantonin levels rise when sunlight is absent triggering sleepiness.
The thyroid gland, shaped like a butterfly, lies in the anterior part of the neck, below the larynx (Figs. 6-63 and 6-64). It consists of two lobes, one on each side of the upper trachea, connected by a strip of tissue called the isthmus. The thyroid secretes the iodine containing hormone thyroxin (TSH), which controls the rate of cell metabolism. Excessive secretion of thyroxin raises the metabolic rate and causes hyperthyroidism. This condition is characterized by a fast pulse rate, dizziness, increased basal metabolism, profuse sweating, tremors, nervousness, and a tremendous appetite coupled with weight loss.
Iodine is essential for the formation of thyroxin. Simple goiter, a diffuse and painless enlargement of the thyroid gland, was common in areas of the United States where the iodine content of the soil and water was inadequate. In simple goiter, the gland enlarges to compensate for the lack of iodine. To prevent formation of a simple goiter, iodine-containing foods, such as vegetables, iodized salt, and seafood, should be eaten.
A condition known as hypothyroidism is caused by an insufficient secretion of thyroxin. The patient exhibits a decrease in basal metabolism, and sweating is almost absent. There may be a weight gain and constant fatigue. The heart rate may be slow, and a simple goiter may form. There may also be personality changes characterized by slow, lethargic mental functioning. Hypothyroidism during childhood can lead to the development of cretinism. Cretinism is a condition characterized by retarded mental and physical development.
Parathyroid glands are four small round bodies located just posterior to the thyroid gland (Fig. 6-64). Their hormone, parathormone (PTH), regulates the calcium and phosphorus content of the blood and bones. The amount of calcium is important in certain tissue activities, such as bone formation, coagulation of blood, maintenance of normal muscular excitability, and milk production in the nursing mother. Diminished function or removal of the parathyroid glands results in a low calcium level in the blood. In extreme cases death may occur, preceded by strong contraction of the muscles (tetany) and convulsions.
Hyperparathyroidism, an excess of parathyroid hormone in the blood, causes calcium levels in the blood to become elevated by the withdrawal of calcium from the bones, leaving the skeleton demineralized and subject to spontaneous fractures. The excess calcium may be deposited as stones in the kidneys.
The adrenal glands are located on the superior surface of each kidney, fitting like a cap (Fig. 6-65). They consist of an outer portion, the cortex, and an inner portion, the medulla.
Specialized cells in the outer layer of the adrenal cortex produce three types of steroid hormones that are of vital importance.
MINERALOCORTICOIDS.— Mineralocorticoids are regulators of fluid and electrolyte balance. Sometimes called salt and water hormones because they regulate the excretion and absorption of sodium, chlorine, potassium, and water. In humans, aldosterone is the only physiologically important mineralcorticoid. Its primary function is the maintenance of sodium homeostasis in the blood. Aldosterone accomplishes this by increasing sodium reabsorption in the kidneys.
GLUCOCORTICOIDS.—Glucocorticoids are essential to metabolism. They increase certain liver functions and have an antiinflammatory effect. Clinically, they are used to suppress inflammatory reactions, to promote healing, to treat rheumatoid arthritis, and maintain normal blood pressure. One of the main glucocorticoids secreted in significant amounts is cortisol.
GONADOCORTICOIDS.—The adrenal cortex also produces sex hormones, some with male characteristics (androgens), others with female characteristics (estrogens). These hormones appear in different concentrations in both men and women.
The adrenal medulla secretes epinephrine (adrenalin) in the presence of emotional crises, hypoglycemia (low blood sugar), or low blood pressure. Epinephrine causes powerful contractions of many arterioles (especially in the skin, mucous membranes, and kidneys), but it dilates other arterioles (such as those of the coronary system, skeletal muscles, and lungs). Heart rate, respiration rate and depth, blood pressure, blood sugar levels, and metabolism are all increased by epinephrine. It stimulates the production of other adrenal cortical hormones.
Norepinephrine is produced in the adrenal medulla and a chemical precursor to epinephrine. Its effects are similar to those of epinephrine, but its action differs.
Despite these marked influences, the medullary tissue of the adrenal gland is not essential to life, because its various functions can be assumed by other regulatory mechanisms.
The pancreas contains exocrine and endocrine tissues. The exocrine tissue secretes digestive juice through a duct to the small intestine, while the endocrine tissue releases hormones into body fluids. The endocrine portion of the pancreas consists of cells arranged in groups, called islands (islets) of Langerhans. The islands (islets) of Langerhans contain three types of endocrine cells: alpha, beta, and delta. The alpha cells secrete the hormone glucagon. Glucagon causes a temporary rise in blood sugar levels. The beta cells secrete insulin, which is essential for carbohydrate metabolism. Insulin lowers blood sugar levels by increasing tissue utilization of glucose and stimulating the formation and storage of glycogen in the liver. Together, glucagon and insulin act to regulate sugar metabolism in the body. Delta cells produce the hormone somatostatin. Somatostatin helps regulate carbohydrates by inhibiting the secretion of glucagon.
When the islet cells are destroyed or stop functioning, the sugar absorbed from the intestine remains in the blood and excess sugar is excreted by the kidneys into the urine. This condition is called diabetes mellitus, or sugar diabetes. Insulin, a synthetic hormone, is given to patients having this disease as part of their ongoing treatment.
GONADS (TESTES AND OVARIES)
The term gonad refers to the primary sex organs of the reproductive system (male and female).
The male gonad is the testis (pl. testes), and the existence of the testes is the primary male sex characteristic (Fig. 6-66). The testes produce and secrete the male hormone testosterone, which influences the development and maintenance of the male accessory sex organs and the secondary sex characteristics.
Male Secondary Sex Characteristics
Enlargement of the larynx (Adam's apple) and thickening of the vocal cords, which produces a lower-pitched voice Thickening of the skin Increased muscle growth, broadening of the shoulder and narrowing of the waist Thickening and strengthening of the bones The male accessory sex organs include two groups of organs: the internal sex organs and the external sex organs. See section titled "Male Reproductive System” for more information on the male accessory sex organs.
The female gonads, the ovaries, produce the hormones estrogen and progesterone (Fig. 6-66). Estrogen influences the development and maintenance of the female accessory sex organs and the secondary sex characteristics, and promotes changes in the mucous lining of the uterus (endometrium) during the menstrual cycle. Progesterone prepares the uterus for the reception and development of the fertilized ovum and maintains the lining during pregnancy.
Estrogen and progesterone hormones (naturally derived) are incorporated into oral contraceptives or birth control pills. The combination of hormones released through a monthly series of pills fools the body into not preparing (building-up of uterine lining) for implantation of an embryo. As the uterus has not prepared for implantation, pregnancy cannot occur.
Figure 6-66.—Locations of some major endocrine glands.
Female Secondary Sex Characteristics: Estrogen Influenced
Development of the breasts and the ductile system of the mammary glands within the breasts Increased quantities of fatty (or adipose) tissue in the subcutaneous layer, especially in the breasts, thighs, and buttocks Increased vascularization of the skin Female accessory sex organs are also divided into internal and external accessory sex organs. See section titled "Female Reproductive System” for more information on the female accessory sex organs.
“The placenta, the tissue that forms on the lining of the uterus as an interface between the circulatory systems of the mother and developing child, serves as a temporary endocrine gland. The placenta produces the human chorionic gonadotropin (HCG).” This hormone is high during the first 3 months of pregnancy to tell the female’s gonads to maintain the uterine lining instead of falling away as in menstruation. HCG is the hormone used for early pregnancy tests.
The thymus is a gland located in the mediastinum just beneath the sternum. It is large in children and atrophies as they become adults, once they reach old age it becomes a vestige of fat and fibrous tissue. It has a critical role in the immune system, thought to stimulate the production of T cells.
The hormone Ghrelin produced by the endocrine cells in the gastric mucosa stimulates the hypothalamus to boost appetite, slow metabolism and fat burning, and may play an important role in obesity.
|David L. Heiserman, Editor||
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Revised: June 06, 2015