Chapter 19: The Endocrine System
Closely tied to and sometimes hard to distinguish from the CNS.
The endocrine system includes all endocrine cells and tissues of the body.
Four chemical types of hormones:
Endocrine stimuli:
Endocrine reflexes are the series of steps in an endocrine stimulus and response. May be as simple as a low blood calcium level causing the parathyroid glands to secrete parathormone (PTH) which causes increased osteoclast activity which increases circulating calcium.
Usually more complicated such as the TRH-TSH-then resulting in thyroid hormone (T3, T4) production
Endocrine organs
Hypothalamus
Exerts effects on endocrine system by
Posterior lobe is actually an extension of the brain and releases oxytocin (OT) and antidiuretic hormone (ADH). An insufficient level of ADH will result in diabetes insipidus, resulting in polyuria and polydipsea (excessive urine production and excessive drinking).
Anterior lobe:
Thyroid stimulating hormone (TSH): causes thyroid to release thyroid hormone
Adrenocorticotropic hormone (ACTH): stimulates release of glucocorticoids from the adrenal glands
Follicle stimulating hormone (FSH): promotes development of oocytes in the female and supports sperm production in the male.
Luteinizing hormone (LH): induces ovulation in women, stimulates the production of androgens (like testosterone) in men. FSH and LH are gonadotropins because they regulate the activity of the male and female sex organs.
Prolactin (PRL): stimulates the development of mammary glands and production of milk.
Growth Hormone (GH): stimulates cell growth and replication. Skeletal muscle is particularly responsive to GH and this has resulted in abuse by body builders. Pituitary Dwarfs are the result of the pituitary being unable to produce adequate GH.
Melanocyte Stimulating Hormone (MSH): Increases rate of melanin production. Secreted only during fetal development, young children, and pregnant women.
The thyroid gland lies in the anterior/inferior neck. It lies anterior/lateral to the trachea. It has right and left lobes joined by a small band of thyroid tissue called the isthmus which crosses the midline at about the middle/lower portion of the gland.
Thyroid follicles manufacture, store and release thyroid hormones. The thyroid cells (simple cuboidal epithelium) surround the follicle and secrete thyroglobulin into follicle.
The follicles actively transport Iodide ions into the follicle where it is bound in the hormone production process.
The hormones of the thyroid are T4 (thyroxin) and T3 (triiodothyronine).
Mechanism of release: TRH (thyrotropin releasing hormone) is released by the hypothalamus which causes the pituitary to release TSH (thyroid stimulating hormone) which acts at the thyroid to cause release of T4 and T3. T4 accounts for the greatest percentage of thyroid hormone. Thyroid hormone up-regulates metabolism of cells.
C cells in the thyroid gland produce calcitonin (CT). Calcitonin inhibits osteoclasts and stimulates calcium excretion in the kidneys. Calcitonin therefore lowers serum calcium levels.
4 small glands located on the posterior surface of the thyroid gland. Secrete parathyroid hormone (PTH) in response to low serum calcium levels. Effects are exactly the opposite of calcitonin, it decreases renal loss of calcium and stimulates osteoclast activity. Also stimulates production of the renal hormone calcitrol which promotes intestinal absorption of calcium. The net result is to increase serum calcium levels.
Thymus:
Gland inside the thoracic cavity in the anterior mediastinum. Largest in children (peaks in size at puberty) and gradually involutes in the adult. Produces hormones important in the development and maintenance of the immune response. The thymus will be considered further in the lymphatic system.
Located adjacent to the superior aspect of the kidneys (hence the other name: suprarenal glands). Two discrete regions: the external adrenal cortex and internal adrenal medulla.
The adrenal cortex produces several hormones collectively known as corticosteroids.
Mineralocorticoids (MC) of which aldosterone is the most important, regulates ionic composition of urine.
Glucocorticoids: (such as cortisol) alter glucose metabolism (hence the name glucocorticoids). Cortisol is active in itself but the liver converts some to cortisone which is another active glucocorticoid. These hormones speed up glucose synthesis and glycogen formation, particularly in the liver. Also plays role in blood pressure regulation, insulin release, and inflammatory response.
Androgens: stimulate some secondary sexual characteristics in men and women. Not important in adult men because testes produce testosterone. In women, adrenal androgens help develop muscle mass, increase blood cell production and help support libido.
Hormones of the adrenal medulla:
Epinephrine and norepinephrine; hormones of the fight or flight response. Increases cellular metabolism and mobilize energy reserves. Peak activity is about 30 seconds after release so the effect outlasts the initial jolt from the sympathetic nervous system.
The kidneys and heart produce hormones that for the most part regulate blood pressure and volume. In the kidneys, the most important are renin, which as part of the renin-angiotensinogen system which results in the release of aldosterone, and calcitrol which has been mentioned before and plays a role in calcium absorption. Also erythropoetin stimulates red blood cell production in the bone marrow.
Pancreas and other endocrine tissues of the digestive system:
Anatomy (fig 19.10)
Pancreas primarily produces exocrine secretions involved in digestion, but scattered throughout the pancreas are islet cells (islets of Langerhans).
Alpha cells: produce glucagon which raises blood glucose levels by increasing glycogen breakdown.
Beta cells: produce insulin which lowers blood glucose by increasing glucose uptake and glycogen synthesis
Delta cells: produce somatostatin an inhibiting hormone that acts as an antagonist to growth hormone, insulin, glucagon and others.
F cells: produce pancreatic polypeptide which inhibits gallbladder contraction (not a very interesting hormone).
Endocrine tissues of the reproductive system:
Male testes produce androgens, primarily testosterone. Testosterone promotes production of sperm, influences secondary sexual characteristics, and stimulates muscle growth.
Ovaries: oocytes develop into gametes within follicles in response to FSH from pituitary gland. Follicle cells produce estrogens which aid in the maturation of the oocytes as well as stimulates the endometrial cavity. Typically one follicle releases the oocyte (often accompanied by pain, or middlesmerch, in the middle of the menstrual cycle) and remaining follicles release estrogens and progestins (including progesterone), which help the movement of the oocytes along the fallopian tube and further helps in stimulation and maintenance of the endometrium.
Pineal gland:
Part of the epithalamus. Secretes the hormone melatonin. Melatonin inhibits release of FSH and LH. Melatonin production rises at night and contributes to our circadian rhythm. Also powerful antioxidant.
