The pituitary gland, located at the base of the brain in proximity to the nasal cavity, is a small endocrine gland about the size of a pea weighing approximately 600 mg. The pituitary gland is referred to as the "master gland" because it is responsible for the regulation of many other endocrine glands and body systems. Growth, development, metabolism, reproduction, and stress homeostasis are among the functions influenced by the pituitary. Functionally, the gland consists of two distinct sections: the anterior pituitary lobe (adenohypophysis) and the posterior pituitary lobe (neurohypophysis). The pituitary receives neural and hormonal input from the inferior hypothalamus via blood vessels and neurons contained in the pituitary stalk (infundibulum).
FIGURE 46-1. Hypothalamic-pituitary-target-organ axis. The hypothalamic hormones regulate the biosynthesis and release of eight pituitary hormones. Stimulation of each of these pituitary hormones produces and releases trophic hormones from their associated target organs to exert their principal effects. These trophic hormones regulate the activity of endocrine glands. Subsequently, increased serum concentration of the trophic hormones released from the target organs can inhibit both the hypothalamus and the anterior pituitary gland to maintain homeostasis (negative feedback). Inhibin is produced by the testes in the male and ovaries in the female during pregnancy. In-hibin directly inhibits pituitary production of follicle-stimulating hormone (FSH) through a negative feedback mechanism. Melanocyte-stimulating hormone (MSH) produced by the anterior pituitary is not illustrated in the figure [(-), inhibit; (+), stimulate; ACTH, adrenocorticotropic hormone (corticotropin); ADH, antidiuretic hormone (vasopressin); CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GABA, y-aminobutyric acid; GH, growth hormone (somatotropin); GHIH, growth hormone-inhibiting hormone (somatostatin); GHRH, growth hormone-releasing hormone; GnRH, gonadotropin-releasing hormone; IGF-I, insulin-like growth factor I; LH, luteinizing hormone; LHRH, luteinizing hormone-releasing hormone; PRH, prolactin-releasing hormone; T3, triiodothyronine; T4, thyroxine; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone (thyrotropin).]
The posterior pituitary is innervated by nervous stimulation from the hypothalamus, resulting in the release of specific hormones to exert direct tissue effects. The hypothalamus synthesizes two hormones, oxytocin and vasopressin. These hormones are stored in and released from the posterior pituitary lobe. Oxytocin exerts two actions, it: (a) promotes uterine contractions during labor and (b) contracts the smooth muscles in the breast to stimulate the release of milk from the mammary gland during lactation. Vasopressin is an antidiuretic hormone essential for proper fluid and electrolyte balance in the body. Specifically, vasopressin increases the permeability of the distal convoluted tubules and collecting ducts of the nephrons to water. This causes the kidney to excrete less water in the urine. Consequently, the urine becomes more concentrated as water is conserved.
In contrast to the posterior pituitary lobe, the anterior pituitary lobe is under the control of several releasing and inhibiting hormones secreted from the hypothalamus via a portal vein system. The anterior pituitary, in turn, synthesizes and secretes six major hormones. Figure 46-1 summarizes the physiologic mediators and effects of each of these hormones.
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