The Endocrine Glands and Their Hormones
The remainder of this chapter deals with hormones and the tissues that produce them. Refer to Figure 8-2 to locate each of the endocrine glands as you study them. Although most of the discussion centers on the endocrine glands, it is important to note that many tissues other than the endocrine glands also secrete hormones. That is, they produce substances that act on other tissues, usually at some distance from where they are produced. These tissues include the brain, digestive organs, and kidney.
The pituitary, or hypophysis, is a small gland about the size of a cherry. It is located in a saddlelike depression of the sphenoid bone just posterior to the point where the optic nerves cross. It is surrounded by bone except where it connects with the hypothalmus of the brain by a stalk called the infundibulum. The gland is divided into two parts, the anterior lobe and the posterior lobe (Fig. 8-3). The pituitary is often called the master gland because it releases hormones that affect the working of other glands, such as the thyroid, gonads (ovaries and testes), and adrenal glands. (Hormones that stimulate other glands may be recognized by the ending -tropin, as in thyrotropin, which means “acting on the thyroid gland.”) However, the pituitary itself is controlled by the hypothalamus, which sends secretions and nerve impulses to the pituitary through the infundibulum (see Fig. 8-3).
Control of the Pituitary The hormones produced in the anterior pituitary are not released until chemical messengers called releasing hormones arrive from the hypothalamus. These releasing hormones travel to the anterior pituitary by way of a special type of circulatory pathway called a portal system. By this circulatory “detour,” some of the blood that leaves the hypothalamus travels to capillaries in the anterior pituitary before returning to the heart. As the blood circulates through the capillaries, it delivers the hormones that stimulate the release of anterior pituitary secretions. Hypothalamic releasing hormones are indicated with the abbreviation RH added to an abbreviation for the name of the hormone stimulated. For example, the releasing hormone that controls growth hormone is GHRH. Two anterior pituitary hormones are also regulated by inhibiting hormones (IH) from the hypothalamus. Inhibiting hormones suppress both growth hormone, which stimulates growth and metabolism, and prolactin, which stimulates milk production in the mammary glands. These inhibiting hormones are abbreviated GHIH (growth hormone-inhibiting hormone) and PIH (prolactin-inhibiting hormone) The two hormones of the posterior pituitary (antidiuretic hormone, or ADH, and oxytocin) are actually produced in the hypothalamus and stored in the posterior pituitary. Their release is controlled by nerve impulses that travel over pathways (tracts) between the hypothalamus and the posterior pituitary.
Hormones of the Anterior Lobe
* Growth hormone (GH), or somatotropin, acts directly on most body tissues, promoting protein manufacture that is essential for growth. GH causes increase in size and height to occur in youth, before the closure of the epiphyses of long bones. A young person with a deficiency of GH will remain small, though well proportioned, unless treated with adequate hormone. GH is produced throughout life. It stimulates protein synthesis and is needed for maintenance and repair of cells. It also stimulates the liver to release fatty acids for energy in time of stress.
* Thyroid-stimulating hormone (TSH), or thyrotropin, stimulates the thyroid gland to produce thyroid hormones.
* Adrenocorticotropic hormone (ACTH) stimulates the production of hormones in the cortex of the adrenal glands.
* Prolactin (PRL) stimulates the production of milk in the breasts.
* Follicle-stimulating hormone (FSH) stimulates the development of eggs in the ovaries and sperm cells in the testes.
* Luteinizing hormone (LH) causes ovulation in females and sex hormone secretion in both males and females; in males, the hormone is sometimes called interstitial cell-stimulating hormone (ICSH). FSH and LH are classified as gonadotropins, hormones that act on the gonads to regulate growth, development, and function of the reproductive systems in both males and females.
Figure 8-2 The endocrine glands.
Figure 8-3 The hypothalamus, pituitary gland, and target tissues. Arrows indicate the hormones’ target issues and feedback pathways.
Hormones of the Posterior Lobe
* Antidiuretic hormone (ADH) promotes the reabsorption of water from the kidney tubules and thus decreases water excretion. Large amounts of this hormone cause contraction of smooth muscle in blood vessel walls and raise blood pressure. Inadequate amounts of ADH cause excessive water loss and result in a disorder called diabetes insipidus. This type of diabetes should not be confused with diabetes mellitus, which is due to inadequate amounts of insulin.
* Oxytocin causes contractions of the uterus and triggers milk ejection from the breasts. Under certain circumstances, commercial preparations of this hormone are administered during or after childbirth to cause uterine contraction.
Tumors of the Pituitary The effects of pituitary tumors depend on the cell types in the excess tissue. Some of these tumors contain an excessive number of the cells that produce growth hormone. A person who develops such a tumor in childhood will grow to an abnormally tall stature, a condition called gigantism. Although people with this condition are large, they are usually very weak.
If the GH-producing cells become overactive in the adult, a disorder known as acromegaly develops. In acromegaly, the bones of the face, hands, and feet widen. The fingers resemble a spatula, and the face takes on a coarse appearance: the nose widens, the lower jaw protrudes, and the forehead bones may bulge. Multiple body systems may be affected by acromegaly, including the cardiovascular and nervous systems. Tumors may destroy the secreting tissues of the pituitary so that signs of underactivity develop. Patients with this condition often become obese and sluggish and may exhibit signs of underactivity of other endocrine glands that are controlled by the pituitary, such as the ovaries, testes, or thyroid. Pituitary tumors also may involve the optic nerves and cause blindness. Evidence of tumor formation in the pituitary gland may be obtained by radiographic examinations of the skull. The pressure of the tumor distorts the sella turcica, the saddlelike space in the sphenoid bone that holds the pituitary. Computed tomography (CT) and magnetic resonance imaging (MRI) scans are also used to diagnose pituitary abnormalities.