ENDOCRINE GLANDS

The pituitary gland, or hypophysis, is known to secrete some major hormones (e. g. antidiuretic hormone, growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, lipotropins, prolactin) that directly regulate numerous body functions and the secretory activity of several other endocrine glands. The hypothalamus of the brain regulates the secretory activity of the pituitary gland, and, in turn, the activity of the hypothalamus is influenced by the central nervous system, and by the emotional state of the individual. The hypothalamus and pituitary gland are the major sites in which the two regulatory systems of the body (the nervous and endocrine systems) interact. Indeed, a major portion of the pituitary gland (the posterior pituitary) is an extension of the hypothalamus. As for pituitary gland, it is roughly 1 cm in diameter and weighs 0.5 to 1 g. The pituitary gland is located inferior to the hypothalamus. It is divided functionally into two parts (posterior pituitary gland and anterior pituitary gland).

The thyroid gland is composed of two lobes connected by a narrow band of thyroid tissue. The lobes are lateral to the upper portion of the trachea just inferior to the larynx. The thyroid gland is one of the largest endocrine glands with a weight of approximately 20 g. It is highly vascular and appears redder than its surrounding tissues. The thyroid hormones include both triiodothyronine (T3) and tetraiodothyronine (T4); T4 is also called thyroxine. These substances constitute the major secretory products of the thyroid gland, with 10% T3 and 90% T4. Thyroid hormones bind with intracellular receptor molecules and initiate new protein synthesis.

The adrenal glands are near the top of each kidney. Like kidneys, they lie posterior to the parietal peritoneum and are surrounded by adipose tissue. They are enclosed by a connective tissue capsule and receive a well-developed blood supply. The adrenal glands are composed of an inner medulla and an outer cortex, which are derived from two separate embryonic tissues. Unlike most glands of the body, which develop from epithelial tissue, the adrenal cortex is derived from mesoderm. The adrenal medulla is a component of the autonomic nervous system and secretes two types of hormones: epinephrine (adrenaline), 80%, and norepinephrine (noradrenaline), 20%. The adrenal cortex secretes three hormone types: mineralocorticosteroids, glucocorticoids, and sex hormones. All are similar in structure in that they are steroids, highly specialized lipids that are derived from cholesterol.

The pancreas lies behind the peritoneum between stomach and the duodenum. It is elongated structure approximately 15 cm long. It weighs 85 to 100 g. The head of the pancreas lies near the duodenum, and its body and tail extend toward the spleen. The pancreas plays a key part in the digestive process, producing enzymes essential to the digestion of food. The pancreas is both an exocrine gland and endocrine gland. The endocrine portion, consisting of pancreatic islets (islets of Langerhans), produces hormones that enter the circulatory system. Each islet is composed of alpha cells (20%), which secrete glucagons, beta cells (75%), which secrete insulin, and other cell types (5%). The remaining cells are either immature cells of questionable function or delta cells, which secrete somatostatin.