THE PITUITARY GLAND
The pituitary gland (or hypophysis) hangs by a short stalk (infundibulum) from the hypothalamus and is enclosed by the sella turcica of the sphenoid bone.
Despite its small size, the pituitary gland regulates many body functions. Its two major portions are the posterior pituitary gland (neurohypophysis), which is an extension of the nerve tissue of the hypothalamus, and the anterior pituitary gland (adenohypophysis), which is separate glandular tissue. All of the hormones of the pituitary gland and their target organs are shown in Fig. 10–2. It may be helpful for you to look at this summary picture before you begin reading the following sections.
The two hormones of the posterior pituitary gland are actually produced by the hypothalamus and simply stored in the posterior pituitary until needed. Their release is stimulated by nerve impulses from the hypo-thalamus (Fig. 10–3).
Figure 10–3. Structural relationships of hypothalamus and pituitary gland. (A) Posterior pituitary stores hormones produced in the hypothalamus. (B) Releasing hormones of the hypothalamus circulate directly to the anterior pituitary and influence its secretions. Notice the two networks of capillaries.
QUESTION: In part A, name the hormones of the posterior pituitary. In part B, what stim-ulates secretion of anterior pituitary hormones?
Antidiuretic hormone (ADH, also called vaso-pressin) increases the reabsorption of water by kidney tubules, which decreases the amount of urine formed. The water is reabsorbed into the blood, so as urinary output is decreased, blood volume is increased, which helps maintain normal blood pressure. ADH also decreases sweating, but the amount of water con-served is much less than that conserved by the kidneys.
The stimulus for secretion of ADH is decreased water content of the body. If too much water is lost through sweating or diarrhea, for example, osmore-ceptors in the hypothalamus detect the increased “saltiness” of body fluids. The hypothalamus then transmits impulses to the posterior pituitary to increase the secretion of ADH and decrease the loss of more water in urine.
Any type of dehydration stimulates the secretion of ADH to conserve body water. In the case of severe hemorrhage, ADH is released in large amounts and will also cause vasoconstriction, especially in arteri-oles, which will help to raise or at least maintain blood pressure. This function gives ADH its other name, vasopressin.
Ingestion of alcohol inhibits the secretion of ADH and increases urinary output. If alcohol intake is exces-sive and fluid is not replaced, a person will feel thirsty and dizzy the next morning. The thirst is due to the loss of body water, and the dizziness is the result of low blood pressure.
Oxytocin stimulates contraction of the uterus at the end of pregnancy and stimulates release of milk from the mammary glands.
As labor begins, the cervix of the uterus is stretched, which generates sensory impulses to the hypothalamus, which in turn stimulates the posterior pituitary to release oxytocin. Oxytocin then causes strong contractions of the smooth muscle (myo-metrium) of the uterus to bring about delivery of the baby and the placenta. The secretion of oxytocin is one of the few positive feedback mechanisms within the body, and the external brake or shutoff of the feedback cycle is delivery of the baby and the placenta.
It has been discovered that the placenta itself secretes oxytocin at the end of gestation and in an amount far higher than that from the posterior pitu-itary gland. Research is continuing to determine the exact mechanism and precise role of the placenta in labor.
When a baby is breast-fed, the sucking of the baby stimulates sensory impulses from the mother’s nipple to the hypothalamus. Nerve impulses from the hypo-thalamus to the posterior pituitary cause the release of oxytocin, which stimulates contraction of the smooth muscle cells around the mammary ducts. This release of milk is sometimes called the “milk let-down” reflex. The hormones of the posterior pituitary are summa-rized in Table 10–1.
Both ADH and oxytocin are peptide hormones with similar structure, having nine amino acids each. And both have been found to influence aspects of behavior such as nurturing and trustfulness. Certain brain cells have receptors for vasopressin, and they seem to be involved in creating the bonds that sustain family life. Trust is part of many social encounters such as friendship, school, sports and games, and buy-ing and selling, as well as family life. These two small hormones seem to have some influence on us mentally as well as physically.
The hormones of the anterior pituitary gland regu-late many body functions. They are in turn regulated by releasing hormonesfrom the hypothalamus. These releasing hormones are secreted into capillaries in the hypothalamus and pass through the hypophy-seal portal veins to another capillary network in the anterior pituitary gland. Here, the releasing hormones are absorbed and stimulate secretion of the anterior pituitary hormones. This small but specialized path-way of circulation is shown in Fig. 10–3. This pathway permits the releasing hormones to rapidly stimulate the anterior pituitary, without having to pass through general circulation.
Growth hormone (GH) is also called somatotropin, and it does indeed promote growth (see Fig. 10–4). GH stimulates cells to produce insulin-like growth fac-tors (IGFs), intermediary molecules that bring about the functions of GH. Growth hormone increases the transport of amino acids into cells, and increases the rate of protein synthesis. Amino acids cannot be stored in the body, so when they are available, they must be
Figure 10–4. Functions of growth hormone.
used in protein synthesis. Excess amino acids are changed to carbohydrates or fat, for energy storage. Growth hormone ensures that amino acids will be used for whatever protein synthesis is necessary, before the amino acids can be changed to carbohydrates. Growth hormone also stimulates cell division in those tissues capable of mitosis. These functions contribute to the growth of the body during childhood, especially growth of bones and muscles.
You may now be wondering if GH is secreted in adults, and the answer is yes. The use of amino acids for the synthesis of proteins is still necessary. Even if the body is not growing in height, some tissues will require new proteins for repair or replacement. GH also stimulates the release of fat from adipose tissue and the use of fats for energy production. This is important any time we go for extended periods with-out eating, no matter what our ages.
The secretion of GH is regulated by two releasing hormones from the hypothalamus. Growth hormone– releasing hormone (GHRH), which increases the secretion of GH, is produced during hypoglycemia and during exercise. Another stimulus for GHRH is a high blood level of amino acids; the GH then secreted will ensure the conversion of these amino acids into protein. Somatostatin may also be called growth hor-mone inhibiting hormone (GHIH), and, as its name tells us, it decreases the secretion of GH. Somatostatin is produced during hyperglycemia.
Thyroid-stimulating hormone (TSH) is also called thyrotropin, and its target organ is the thyroid gland. TSH stimulates the normal growth of the thyroid and the secretion of thyroxine (T4) and triiodothyronine (T3).
The secretion of TSH is stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. When metabolic rate (energy production) decreases, TRH is produced.
Adrenocorticotropic hormone (ACTH) stimulates the secretion of cortisol and other hormones by the adrenal cortex. Secretion of ACTH is increased by corticotropin-releasing hormone (CRH) from the hypothalamus. CRH is produced in any type of phys-iological stress situation such as injury, disease, exer-cise, or hypoglycemia (being hungry is stressful).
Prolactin, as its name suggests, is responsible for lac-tation. More precisely, prolactin initiates and main-tains milk production by the mammary glands. The regulation of secretion of prolactin is complex, involv-ing both prolactin-releasing hormone (PRH) and prolactin-inhibiting hormone (PIH) from the hypo-thalamus. The mammary glands must first be acted upon by other hormones such as estrogen and proges- terone, which are secreted in large amounts by the placenta during pregnancy. Then, after delivery of the baby, prolactin secretion increases and milk is pro-duced. If the mother continues to breast-feed, pro-lactin levels remain high.
Follicle-stimulating hormone (FSH) is one of the gonadotropic hormones; that is, it has its effects on the gonads: the ovaries or testes. FSH is named for one of its functions in women. Within the ovaries are ovarian follicles that contain potential ova (egg cells). FSH stimulates the growth of ovarian follicles; that is, it initiates egg development in cycles of approximately 28 days. FSH also stimulates secretion of estrogen by the follicle cells. In men, FSH initiates sperm produc-tion within the testes.
The secretion of FSH is stimulated by the hypo-thalamus, which produces gonadotropin-releasing hormone (GnRH). FSH secretion is decreased by inhibin, a hormone produced by the ovaries or testes.
Luteinizing hormone (LH) is another gonadotropic hormone. In women, LH is responsible for ovulation, the release of a mature ovum from an ovarian follicle. LH then stimulates that follicle to develop into the corpus luteum, which secretes progesterone, also under the influence of LH. In men, LH stimulates the interstitial cells of the testes to secrete testosterone.
(LH is also called ICSH, interstitial cell stimulating hormone.)
Secretion of LH is also regulated by GnRH from the hypothalamus. We will return to FSH and LH.
The hormones of the anterior pituitary are summarized in Table 10–2.