LEARNING OBJECTIVE: Identify endocrine glands and the hormone(s) they produce, and determine the effect each hormone has on the body.
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 alone and together 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, however, 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. Endocrine glands secrete hormones directly into the blood, because they have no duct system. The glands of this system are often called ductless glands. Cells that are affected by the hormone are referred to as target organ cells.
Today, 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 who are deficient in them or who might otherwise benefit from their use. For example, 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, pituitary, thyroid, parathyroids, adrenals, pancreas, and gonads (the testes and ovaries) (fig. 1-47).
The hypothalamus, a structure in the brain, synthesizes chemicals that are secreted to the pituitary
Figure 1-47.—Major endocrine glands.
gland to stimulate the release of its hormones and to help regulate body temperature (fig. 1-47).
The pituitary is a small, pea-sized gland located at the base of the brain in the sella turcica, the saddle-shape depression of the sphenoid bone (fig. 1-47). It is often called the master gland of the body because 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.
Anterior Pituitary Gland
The anterior pituitary gland plays the more important role in influencing body functions. The hormones produced by the anterior pituitary gland have a broad and significant range of effects.
SOMATOTROPIN.—Somatotropin, the growth hormone, influences body growth and development. During the growth years, an overproduction ofsomatotropin 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.
THYROTROPIN.—Thyrotropin, or the thyroid-stimulating hormone (TSH), influences the growth, development, and secreting activities of the thyroid gland.
GONADOTROPIN.—Gonadotropin influences the gonads and is essential for the normal development and functioning of both male and female reproductive systems.
ADRENOCORTICOTROPIN.—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 and are discussed in more detail later in this section.
Posterior Pituitary Gland
The posterior pituitary gland produces two hormones, antidiuretic hormone (ADH) and oxytocin.
ANTIDIURETIC.—The ADH hormone, promotes the conservation of water by the kidney. When ADH is not produced in adequate amounts, the daily urine volume is between 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 pregnancy. It also plays an important role in the production of milk in the mammary glands of nursing mothers.
The thyroid gland, shaped like a butterfly, lies in the anterior part of the neck, below the larynx (fig. 1-47). 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, 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 a loss of weight.
Iodine is essential for the formation of thyroxin. Simple goiter, a diffuse and painless enlargement of the thyroid gland, was once 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. 1-47). 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 ofblood, maintenance of normal muscular excitability, and milk production in the nursing mother. Diminished function or removal ofthe 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. 1-47). 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. They are sometimes called salt and water hormones because they regulate the excretion and absorption of sodium, chlorine, potassium, and water.
GLUCOCORTICOIDS.—Glucocorticoids are essential to metabolism. They increase certain liver functions and have an anti-inflammatory effect. Clinically, they are used to suppress inflammatory reactions, to promote healing, and to treat rheumatoid arthritis.
ANDROGENS AND ESTROGENS.—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 also stimulates the production of other adrenal cortical hormones.
Norepinephrine is also produced in the adrenal medulla. It is 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 two types of secretory tissues. The first secretory tissue secretes digestive juice through a duct to the small intestine, while the other 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 gonads 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. 1-47). 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 of the male. 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.
Male Secondary Sex Characteristics
Male secondary sex characteristics influenced by the hormone testosterone are as follows:
The female gonads, the ovaries, produce the hormones estrogen and progesterone (fig. 1-47). Estrogen influences the development and maintenance of the female accessory sex organs and the secondary sex characteristics, and promotes changes in the mucous lining ofthe 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.
Today, progesterone and estrogen hormones (naturally derived) are incorporated into oral contraceptives or birth control pills. The combination of hormones released through this monthly series of pills fools the body into not preparing (building-up of uterine lining) for implantation of an embryo. Because the uterus has not prepared for implantation, pregnancy cannot occur.
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.
Female Secondary Sex Characteristics
Female secondary sex characteristics influenced by the hormone estrogen are listed below.