The pituitary gland is only about the size of a pea, but it controls other glands and regulates growth and fluid balance.
The gland is attached to the undersurface of the hypothalamus by a short slender stalk called the infundibulum.
The pituitary gland contains endocrine cells surrounded by an extensive capillary network. It is also part of the hypophyseal portal system and provides entry into the circulatory system.
Anterior pituitary gland is also known by other names:
- hypophysis
- adenohypophysis
It can be subdivided into the large pars distalis (the distal part) and the slender pars intermedia (the intermediate part). Most of the endocrine cells are found in the pars distalis.
The anterior pituitary secretes nine (9) major hormones:
- ACTH (adrenocorticotropic hormone)
- stimulates the release of steroid hormones by the adrenal glands, specifically cortisol by the adrenal corte
- specifically targets cells producing hormones called glucocorticosteroids, which affect glucose metabolism;
- involved in stress response
- FSH(follicle-stimulating hormone)
- promotes egg development in women
- stimulates the secretion of estrogens and steroid hormones produced by the ovarian cells
- supports sperm production in the testes in men
- GH (growth hormone – also called somatotropin)
- stimulates cell growth and replication by accelerating the rate of protein synthesis. GH is released in spurts only during deep sleep. GHRH (growth hormone releasing hormone) increases GH in the bloodstream while another hormone called somatostatin stops the production. It causes the liver to produce somatomedin which stimulates growth.
- stimulates skeletal muscle cells and cartilage cells. These cells are more sensitive to levels of GH than other tissues. GH stimulation involves two different mechanisms: indirect and direct. For example, the indirect mechanism causes liver cells to respond by synthesizing and releasing somatomedins or IGF (insulin growth factors), which increases the rate of amino acid uptake and their incorporation into new proteins. This is particularly important after a meal when the blood contains high concentrations of glucose and amino acids. Direct actions do not appear until after blood glucose and amino acid concentrations have returned to normal levels.
- stimulates epithelia and connective tissues in cell division and the differentiation of daughter cells.
- stimulates the breakdown of glycogen reserves in adipose tissue (fat) and the release of glucose into circulation and thus playing a role in mobilizing energy reserves.
- LH (luteinizing hormone)
- induces ovulation in women
- promotes the ovarian secretion of estrogens and progestins (e.g., progesterone) which prepares the body for possible pregnancy
- stimulates production of androgens in men (e.g., testosterone). LH was once called ICSH (interstitial cell-stimulating hormone) in men because it stimulated the interstitial cells of the testes to produce sex hormones. They are now simply called androgens, with the most important one being testosterone. FSH and LH are collectively called gonadotropins because they regulate the activities of the male and female sex organs or gonads.
- LPH (lipotropins)
- may relieve pain
- MSH (melanocyte-stimulating hormone)
- released by the intermediate portion of the pituitary gland
- stimulates melanocytes of the skin
- important in the control of skin and hair pigmentation
- secreted only during fetal development, in very young children, in pregnant women, and in some disease states. MSH is virtually nonfunctioning in the human adult and circulating blood usually does not contain MSH. Synthetic forms administered to darken the skin are called the “sunless tan.”
- PRL (prolactin)
- stimulates the development of mammary glands and milk production
- has no effect in males. Although PRL has a dominant effect on the glandular cells, normal development of the mammary glands is regulated by the interaction of a number of hormones.
- Prolactin
- stimulates breasts to grow and produce milk
- stimulates the manufacture of body muscle
- TSH (thyroid-stimulating hormone)
- targets the thyroid gland to trigger the release of thyroid hormones. TSH contains thousands of amino acids and causes the epithelial cells to take hormones from the follicles and release them into circulation. However, almost all of these released hormones are unavailable because they become attached to carrier proteins in the bloodstream. Only a small number are free to diffuse into target cells of body tissues. The bound thyroid hormones are kept in reserve to be released when needed. Under normal circumstances, the bloodstream contains about a week’s supply of thyroid hormones.
Posterior pituitary gland is an extension of the hypothalamus and contains axons from two different neurons located in the hypothalamus. One group manufactures ADH, and the other, oxytocin.
- ADH (antidiuretic hormone or vasopressin) is released in response to such stimuli as a rise in the concentrations of electrolytes in the blood or a fall in blood volume or blood pressure. Production is inhibited by the use of alcohol or any drug that increases fluid excretion following consumption. ADH:
- regulates concentrations of water and electrolytes
- acts on the kidneys to decrease urine production
- causes constriction of peripheral blood vessels, thereby increasing blood pressure
Diabetes insipidus develops when the posterior pituitary no longer releases adequate amounts of ADH. Water conservation at the kidneys becomes impaired, causing excessive amounts to be lost in the urine. This causes a common symptom, constant thirst (polydipsia), because fluids consumed are not retained by the body. In severe cases, the fluid loss can reach 10 liters (10.6 US quarts) a day, causing fatal dehydration if not treated.
- Oxytocin
- In females, oxytocin stimulates smooth muscle cells in the uterus and special cells in the mammary glands. Only uterine muscles become sensitive to the hormone in the last stages of pregnancy. After delivery, oxytocin functions in milk production.
- In males, oxytocin stimulates smooth muscle contraction of the prostate gland important in the emission, ejection, and the release of secretions and sperm just before ejaculation occurs.
