医学英语,chapter10 Endocrine System.doc

Chapter 10 Endocrine System Anatomy and Physiology of the Endocrine System The endocrine system is a collection of glands that secrete hormones directly into the bloodstream. Hormones are chemicals that act on their target organs to either increase or decrease the target’s activity level. Hormones are released into the extracellular spaces surrounding secretory cells. Paracrine hormones travel only short distances and affect nearby cells. Other hormones are carried by the blood to all parts of the body and may produce general effects. In either case, the physiological action of a particular hormone is restricted to its target cells – those cells that possess specific receptors for the hormone molecules. In other words, a hormone’s target cells have receptors that other cells lack. Endocrine glands and their hormones help regulate metabolic processes. They control the rates of certain chemical reactions, aid in transporting substances through membranes, and help regulate water balance, electrolyte balance, and blood pressure. In this way the endocrine system is instrumental in maintaining homeostasis, that is, adjusting the activity level of most of the tissues and organs of the body to maintain a stable internal environment. The body actually has two distinct types of glands: exocrine glands and endocrine glands. Exocrine glands release their secretions into a duct that carries them to the outside of the body. For example, sweat glands release sweat into a sweat duct that travels to the surface of the body. Endocrine glands, however, release hormones directly into the bloodstream. The secretion of the thyroid gland directly enters the bloodstream. Because they have no ducts, they are referred to as ductless glands. The endocrine system consists of the following glands: the pituitary gland, the thyroid gland, parathyroid glands, the adrenal glands, the thymus gland, the pancreas, testes in the male, and ovaries in the female. The endocrine glands as a whole affect