Chapter 13 - Endocrine Glands
The endocrine system is composed of glands that synthesize and secrete products, called hormones, directly into the blood or lymph rather than through a duct. Hormones are transported throughout the body where they influence only those cells that have receptors for that hormone.
The pituitary is often called the "master gland" of the body because it produces hormones that regulate other endocrine glands, as well as, have direct effects on target tissues.
Different types of cells can be identified by their morphology and of their secretion granules.
The interaction between the pituitary and its target tissues is demonstrated by the effects of castration on gonadotrophs.
The hypophyseal portal system is a system of blood vessels that connect the hypothalamus with the anterior pituitary. Its main function is to quickly transport releasing hormones to the target endocrine cells.
The thyroid gland produces hormones, triiodothyronine (T3) and thyroxine (T4), that primarily influence the metabolic rate and protein synthesis in cells throughout the body.
The thyroid is unusual in that stores large amounts of hormone outside of cells in large follicles.
The unique structure of thyroid follicles is reflected in its blood supply.
Adrenal glands produce a variety of hormones that help regulate metabolism, blood pressure, the response to stress, and other essential functions. The adrenal cortex is composed of three layers: zona glomerulosa, zona fasciculata, and zona reticularis.
The zona fasciculata is the middle and largest layer. These cells produces glucocorticoids (such as cortisol) that affect carbohydrate and protein metabolism. They stimulate gluconeogenesis in many cells and glycogen synthesis in the liver.
The zona reticularis is the innermost layer. These cells primarily produce androgens (steroid sex hormones) that are converted into more active forms by the gonads.
The adrenal medulla forms the inner core of the adrenal glands and secretes catecholamines (epinephrine and norepinephrine).
Pancreatic islets (or islets of Langerhans) are 'islands' of endocrine cells located within the exocrine pancreas. They secrete hormones (insulin and glucagon) important in the regulation of glucose in blood.
Beta cells produce inulin that decreases blood glucose levels. Its largest effects are on the liver, skeletal muscle, and adipose tissue.
Alpha Cells produce glucagon that increases blood glucose levels. Its largest effects are on the liver to stimulate the breakdown of glycogen (glycogenolysis) and synthesis of glucose from metabolites of amino acids (gluconeogensis).
Pancreatic islets are highly vascularized and receive 5-10 times higher blood flow than exocrine tissue.