ABSTRACT

The Endocrine System deals with all of the glands that produce hormones and the hormones themselves. The hypothalamus is considered the “Commander and Chief” while the pituitary is considered the “master gland” and is usually under the control of the hypothalamus. Hormones are classified by their mechanism of action: generally, there are two distinct mechanisms of action, one is to bind to cell surface and cause the release of a second messenger within the cell, and then cause a cascade of reactions leading to a change in metabolism. The other mechanism of action is that the hormones pass through the cell membrane and bind to a binding protein and then interact with the DNA to alter gene expression and then they alter metabolism. The posterior pituitary releases oxytocin and antidiuretic hormone directly into the circulation. The anterior pituitary releases stimulating hormones that go to a gland and cause release of the hormone of interest from the gland of interest, and the hormone then circulates in blood, binds to the target tissue, and causes changes in metabolism. The adrenal medulla releases epinephrine and norepinephrine which have strong effects on cardiovascular function and the “fight or flight” response, while the adrenal cortex releases cortisol which is also active in the “fight or flight” response and stimulates muscle protein breakdown and gluconeogenesis (the making of new glucose from a non-glucose source). It is also important in the release of aldosterone which primarily leads to Na+ and reabsorption in the kidney. The pancreatic hormones are insulin and glucagon. Insulin results in a lowering of blood glucose as a result of glucose uptake by cells while glucagon acts to breakdown liver glycogen to glucose, thus raising blood glucose concentrations. The major sex hormones are testosterone which increases muscle mass, estrogen, and progesterone, of which the latter two stimulate the development of secondary female sex characteristics. The thyroid secretes T3 and T4, the hormones that stimulate thermogenic metabolism in addition to calcitonin which lowers blood calcium. The parathyroid gland acts to secrete hormones that raise blood calcium.