Endocrine System PDF

Physiology Dr. Mohamed Balfas Endocrine System The endocrine system consists of glands that secrete hormones. They are commonly referred to as the ductless glands because the hormo...

Physiology Dr. Mohamed Balfas Endocrine System The endocrine system consists of glands that secrete hormones. They are commonly referred to as the ductless glands because the hormones are secreted and diffuse directly into the bloodstream. Figure 1: Positions of the endocrine glands. Hormones: Are chemical substances produced by the glands of the endocrine system and released directly into the bloodstream to be carried to the tissues they affect. Chemistry: Chemically, hormones are classified into: 1. Proteins: Most hormones are proteins or related compounds composed of amino acids. 2. Steroid hormones: Are hormones derived from lipids, and include those produced by the adrenal cortex and the sex glands.. Important note; all hormones are extremely potent, that is, they are effective in very small amounts. Regulation of hormone secretion: 1. Hormones are secreted when there is a need for their effects. Each hormone has a specific stimulus for secretion. 2. The secretion of most hormones is regulated by negative feedback mechanisms. 1 Physiology Dr. Mohamed Balfas Mechanisms of hormone action: A hormone affects cells that have receptors for it (called target cells). Receptors are proteins that may be in the cell membrane, or inside the target cell. A protein hormone binds to a membrane receptor; stimulates formation of cyclic AMP from ATP, which activates the cell’s enzymes to bring about the cell’s characteristic response to the hormone. Steroid hormones diffuse easily through plasma membrane and nuclear membrane. Only after they are inside the cell, steroid hormones bind to receptor proteins located within the cytoplasm or nucleus forming hormone-receptor complex. If this complex in the cytoplasm, it moves into the nucleus. Within the nucleus, steroid-receptor complexes activate certain genes, which initiate protein synthesis that influence cellular functions. Figure 2: Mechanisms of hormone action. (A) Mechanism of the action of protein hormones. (B) Mechanism of the action of steroid hormones. Pituitary gland: The pituitary gland lies below the hypothalamus, to which it is attached. It is called the “master gland” because it synthesizes and releases several hormones that affect other glands and organs of body. It weighs about 500 mg and consists of two parts; the anterior pituitary lobe and the posterior pituitary lobe. 2 Physiology Dr. Mohamed Balfas Figure 3: The pituitary gland. Anterior pituitary lobe hormones: - Growth hormone (GH): GH accelerates overall body growth by increasing the mass of both skeletal and soft body tissues. An abnormal increase in growth hormone before puberty leads to gigantism. An abnormal deficiency in growth hormone before puberty leads to dwarfism. Figure 4: Examples of the effect of abnormalities in growth hormone secretion on growth. - Thyroid-stimulating hormone (TSH); stimulates the growth of the thyroid gland and the secretion of thyroxine (T4) and triiodothyronine (T3). - Adrenocorticotropic hormone (ACTH); stimulates the secretion of hormones by the adrenal cortex. - Prolactin: is responsible for lactation. 3 Physiology Dr. Mohamed Balfas - Follicle-stimulating hormone (FSH); it affects the ovaries in female and testes in male. Within the ovaries FSH stimulates the growth of follicles. In men, FSH initiates sperm production within the testes. - Luteinizing hormone (LH); in women, LH is responsible for ovulation, the release of a mature ovum from the ruptured follicle in the ovary. The ruptured follicular cells then turned into a gland (corpus luteum) which secretes estrogen and progesterone. In men, LH stimulates the testes to secrete testosterone. Posterior pituitary lobe hormones: The two hormones of the posterior pituitary lobe are actually produced by the hypothalamus, and just stored in the posterior lobe until needed. - Antidiuretic hormone (ADH); increases the reabsorption of water by kidney, which decreases the amount of urine formed. - Oxytocin; stimulates mainly contraction of the uterus during labor, and release of milk from the mammary glands (breast). Figure 5: Hormones of the pituitary gland and their target organs. 4 Physiology Dr. Mohamed Balfas Thyroid gland: It is a butterfly-shaped structure lying over the anterior surface of the trachea. Figure 6: Thyroid gland. The thyroid gland is composed of numerous follicles. Each follicle consists of follicular cells enclosing a cavity containing a substance termed colloid from which thyroid hormones are produced. Thyroid gland secretes two hormones, thyroxine (tetraiodothyronine or T4) and triiodothyronine (T3). Figure 7: Cells of the thyroid gland. Physiologic effects of thyroid hormones: 1. It increases body metabolism, and hence it increases body heat production. 2. Development of the nervous system in the fetus and neonate. The parathyroid glands: The parathyroid glands, usually four in number, are embedded in the dorsal surface of the thyroid gland. The parathyroid glands secrete parathyroid hormone (PTH) which regulates serum calcium levels. 5 Physiology Dr. Mohamed Balfas Figure 8: The parathyroid glands. Pancreas: The hormone-producing cells of the pancreas are called islets of Langerhans; they contain alpha cells that produce glucagon and beta cells that produce insulin. Figure 9: (A) The pancreas. (B) Microscopic section showing islets of Langerhans. Insulin increases the transport of glucose from the blood into cells. A deficiency of insulin hormone leads to a disease called diabetes mellitus. The overall effect of glucagon, is to raise the blood glucose level. 6 Physiology Dr. Mohamed Balfas The adrenal glands: The two adrenal glands are located one on top of each kidney. Each adrenal gland consists of two parts: an inner adrenal medulla and an outer adrenal cortex. Figure 10: The adrenal glands. Adrenal medulla: It secretes epinephrine (adrenalin) and norepinephrine (noradrenalin), which prepare the body for “fight or flight.” Adrenal cortex: The adrenal cortex secretes three types of steroid hormones: aldosterone, cortisol, and sex hormones. The sex hormones are produced in very small amounts, and their importance is not known. Aldosterone: Aldosterone increases the reabsorption of sodium and the excretion of potassium by the kidneys. Cortisol: Cortisol—by enhancing gluconeogenesis “formation of glucose from amino acids and glycerol”— provide more glucose to be used by brain cells, while promoting other body tissue cells to use fatty acids as an alternative energy source instead of glucose. This process is called the glucose sparing effect, and it is important because it conserves glucose for use by the brain. Cortisol also has an anti-inflammatory effect, so preventing excessive tissue destruction. 7