The endocrine system

• The thyroid gland is located at the base of the neck.
• Parathyroid hormone regulates calcium levels in blood, bones, and other tissues.
• Thyroxin (T4) and triiodothyronine (T3) are produced by the thyroid gland.
• Calcitonin, which regulates calcium levels, is also produced by the thyroid gland.
• Adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline).
• Adrenal cortex produces mineralocorticoids, glucocorticoids, and sex steroids.
• Growth hormone promotes cell division and tissue repair.
• Mineralocorticoids regulate salt and water balance.
• Glucocorticoids control metabolism and immune response.
• Sex steroids include estrogen, progesterone, testosterone, and dihydrotestosterone.
• Prolactin stimulates milk production during pregnancy and lactation.
• Oxytocin causes uterus contractions during childbirth and breastfeeding.
• Pituitary gland releases growth hormone, prolactin, adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyrotropin (thyroid stimulating hormone or TSH), melanocyte-stimulating hormone (MSH), oxytocin, and antidiuretic hormone (ADH).
• Hormones are chemical substances secreted by endocrine cells into the extracellular fluids that regulate the metabolic activity of other cells in the body.
• Hormones can be classified chemically as either amino acid-based molecules or steroids.
• Exposure to pesticides, industrial chemicals, dioxin, and other soil and water pollutants diminishes endocrine function, which may explain the higher cancer rates among older adults in certain areas of the country.
• All older people have some decline in insulin production, and type 2 diabetes mellitus is most common in this age group.
• Steroid hormones are made from cholesterol, include the sex hormones made by the gonads (ovaries and testes) and the hormones produced by the adrenal cortex.
• A given hormone affects only certain tissue cells or organs, referred to as target cells or target organs.
• For a target cell to respond to a hormone, specific protein receptors to which that hormone can attach must be present on the cell’s plasma membrane or in its interior.
• The functions of the endocrine system include metabolism and tissue maturation, ion regulation, water balance, immune system regulation, heart rate and blood pressure regulation, control of blood glucose and other nutrients, control of reproductive functions, and uterine contractions and milk release.
• If blood calcium ion level falls too low, neurons become extremely irritable and overactive, causing tetany, which may be fatal.
• When sodium is reabsorbed, water follows, thus, the mineralocorticoids help regulate both water and electrolyte balance in body fluids.
• The thymus is located in the upper thorax, posterior to the sternum, and is large in infants and children, decreasing in size throughout adulthood.
• Severe hyperparathyroidism causes massive bone destruction, making the bones very fragile and causing spontaneous fractures.
• When the blood level of aldosterone rises, the kidney tubule cell reabsorb increasing amounts of sodium ions and secrete more potassium ions into the urine.
• The two adrenal glands curve over the top of the kidneys like triangular hats, and it is structurally and functionally two endocrine organs in one.
• The parathyroid glands are tiny masses of glandular tissue most often on the posterior surface of the thyroid gland, and parathyroid hormone (PTH) is the most important regulator of calcium ion homeostasis of the blood.
• The adrenal cortex produces three major groups of steroid hormones, collectively called corticosteroids: Mineralocorticoids (aldosterone), Glucocorticoids, and Androgens.
• Mineralocorticoids (aldosterone) are produced by the outermost adrenal cortex cell layer and are important in regulating the mineral (salt) content of the blood, particularly the concentrations of sodium and potassium ions.
• These hormones target the kidney tubules that selectively reabsorb the minerals or allow them to be flushed out of the body in urine.
• Extreme overproduction of thyroxine results in a high basal metabolic rate, intolerance of heat, rapid heartbeat, weight loss, nervous and agitated behavior, and a general inability to relax.
• The thymus produces a hormone called thymosin and others that appear to be essential for normal development of a special group of white blood cells (T lymphocytes) and the immune response.
• Graves’ disease is a form of hyperthyroidism where the thyroid gland enlarges and the eyes bulge (exophthalmos).
• Dwarfism and mental retardation can result from hypothyroidism if discovered early, and hormone replacement will prevent mental impairment.
• Hyperthyroidism generally results from a tumor of the thyroid gland.
• Calcitonin is the second important hormone product of the thyroid gland, which decreases the blood calcium ion level by causing calcium to be deposited in the bones.
• Human placental lactogen (hPL) works cooperatively with estrogen and progesterone in preparing the breasts for lactation.
• The high estrogen and progesterone blood levels maintain the lining of the uterus and prepare the breasts for producing milk.
• The female and male gonads produce sex cells and sex hormones that are identical to those produced by adrenal cortex cells.