As a result of negative feedback, hormone concentrations remain relatively stable, although they may fluctuate slightly above and below average concentrations.


As a result of negative feedback, hormone concentrations remain relatively stable, although they may fluctuate slightly above and below average concentrations.

Pituitary Gland Releasing Hormones
Hypothalamic releasing hormones stimulate cells of the anterior lobe to secrete hormones. Nerve impulses originating in the hypothalamus stimulate nerve endings in the posterior lobe of the pituitary gland to release hormones.

The brain controls most of the pituitary gland's activities (fig. 13.10). For example, the pituitary gland's posterior lobe releases hormones into the bloodstream in response to nerve impulses from the hypothalamus. However, releasing hormones from the hypothalamus primarily control secretions from the anterior lobe. These releasing hormones are carried in the blood via a capillary net associated with the hypothalamus. The vessels merge to form the hypophyseal (hi"po-fiz'e-al) portal veins that pass downward along the pituitary stalk and give rise to a capillary net in the anterior lobe. Thus, substances released into the blood from the hypothalamus are carried directly to the anterior lobe. The hypothalamus, therefore, is an endocrine gland itself, yet it also controls other endocrine glands. This is also true of the anterior pituitary.

The arrangement of two capillaries in series is quite unusual and is called a portal system. It occurs in only three places in the body: the hepatic portal vein connects intestinal capillaries to special liver capillaries called sinusoids; the efferent arteriole of kidney nephrons connects two sets of capillaries; and the hypophyseal portal vein gives rise to a capillary net in the anterior pituitary lobe.

During fetal development, a narrow region appears between the anterior and posterior lobes of the pituitary gland. Called the intermediate lobe (pars intermedia), this region produces melanocyte-stimulating hormone (MSH), which regulates the formation of melanin — the pigment found in the skin and in portions of the eyes and brain. The region atrophies during prenatal development and appears only as a vestige in adults.

Upon reaching the anterior lobe of the pituitary, each of the hy-pothalamic releasing hormones acts on a specific population of cells. Some of the resulting actions are inhibitory (prolactin release-inhibiting hormone and somatostatin), but most stimulate the anterior pituitary to release hormones that stimulate the secretions of peripheral endocrine glands. In many of these cases, important negative feedback relationships regulate hormone levels in the bloodstream. Figure 13.11 shows this general relationship.

The following section titled "Anterior Pituitary Hormones" discusses the hormones of the anterior pituitary as a group. The hormones of the peripheral endocrine glands are then discussed individually.

Shier-Butler-Lewis: I III. Integration and I 13. Endocrine System I I © The McGraw-Hill

Human Anatomy and Coordination Companies, 2001

Physiology, Ninth Edition

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