Unit 16: Human Endocrine System — Long Questions

The hypothalamus contains special neurosecretory cells that can both conduct nerve impulses and secrete hormones, making it the master controller that governs the pituitary (the "master gland").

Control of posterior pituitary Neurosecretory cells of the hypothalamus make two hormones, oxytocin and antidiuretic hormone (ADH). The axons of these cells transport oxytocin and ADH to the posterior pituitary, where they are stored and later released into the blood. The posterior pituitary itself does not synthesize any hormone; it only stores and releases hormones made by the hypothalamus.

Control of anterior pituitary Blood vessels connect the hypothalamus with the anterior pituitary. Neurosecretory cells secrete hormones that travel through these blood vessels to the anterior pituitary. These hypothalamic hormones are of two kinds: releasing hormones, which stimulate the anterior pituitary to make and secrete its hormones, and inhibiting hormones, which inhibit production and secretion of anterior pituitary hormones. Examples include Growth Hormone Releasing Hormone (stimulates GH secretion), Somatostatin (inhibits GH), Thyrotropin Releasing Hormone/TRH (stimulates TSH secretion), Adreno-corticotropin releasing hormone (stimulates ACTH secretion), Prolactin Inhibiting Factor/dopamine (inhibits prolactin secretion), and Gonadotropin Releasing Hormone (stimulates FSH and LH secretion). In this way, the hypothalamus directs both the storage/release of posterior pituitary hormones and the synthesis/secretion of anterior pituitary hormones.

Hormones can be grouped into four chemical categories:

1. Peptide and protein hormones: Made of chains of amino acids; water-soluble and cannot pass through cell membranes, so they bind to receptors on the cell surface. Examples: insulin and glucagon (pancreas), growth hormone (anterior pituitary), oxytocin (made in hypothalamus, released by posterior pituitary).

2. Glycoprotein hormones: Conjugated proteins consisting of a protein combined with a carbohydrate group; large, complex and polar, so they also cannot cross the cell membrane. Examples: TSH, FSH and LH (all from anterior pituitary).

3. Amino acid derivative hormones: Small molecules derived from a single amino acid (tyrosine or tryptophan). Their ability to cross membranes depends on whether they behave more like a protein or a lipid: epinephrine and norepinephrine (adrenal medulla) cannot cross membranes, while thyroxine (T4) and triiodothyronine (T3) (thyroid gland) can cross membranes.

4. Steroid hormones: Derived from cholesterol (a lipid); lipid-soluble, so they can cross the cell membrane and act directly on the nucleus. Examples: cortisol and aldosterone (adrenal cortex), testosterone (testes), estrogen and progesterone (ovaries).

The pituitary gland has three lobes: anterior, posterior and intermediate.

Anterior pituitary hormones (1) Growth Hormone (GH) stimulates growth of muscles, bones and other tissues, and regulates metabolism. (2) Adrenocorticotropic Hormone (ACTH) stimulates the adrenal cortex to secrete corticosteroids (cortisol, aldosterone). (3) Thyroid-stimulating Hormone (TSH) stimulates the thyroid gland to produce thyroxine. (4) Luteinizing Hormone (LH) stimulates ovaries to secrete progesterone and estrogen and triggers ovulation in females; in males, it stimulates testosterone production. (5) Follicle-stimulating Hormone (FSH) stimulates development of ovarian follicles in females and sperm development in males. (6) Prolactin stimulates mammary glands to produce milk.

Posterior pituitary hormones (made by hypothalamus, stored/released here): (1) Antidiuretic Hormone (ADH) stimulates water retention by kidneys. (2) Oxytocin stimulates milk ejection and uterine contractions during childbirth.

Intermediate pituitary secretes melanocyte stimulating hormone (MSH), which stimulates melanin production and release by melanocytes.

Abnormalities due to imbalance Over-secretion of GH in a growing child causes gigantism (height well above average, enlarged hands, feet, facial features); over-secretion of GH in adults causes acromegaly (thickening of bones, enlarged hands, feet, skull, nose, jawbone). Deficiency of GH causes dwarfism (slow development, short stature, but normal body proportions and normal brain/IQ). Over-secretion of TSH causes hyperthyroidism; under-secretion causes hypothyroidism. Under-secretion of ADH causes diabetes insipidus (excessive dilute urine and thirst); over-secretion of ADH may lead to kidney problems. Over-secretion of oxytocin can cause rupturing of the uterine wall; under-secretion inhibits normal labour.

The thyroid gland is a butterfly-shaped gland located on either side of the trachea. It secretes: (1) Thyroxine (T4) and triiodothyronine (T3) - the thyroid produces mainly T4, which converts into the more active T3. These hormones increase heart rate and breathing rate, stimulate cellular respiration, set the basal metabolic rate, enhance glucose catabolism and cholesterol synthesis in the liver, and in children promote skeletal growth and brain development. (2) Calcitonin - maintains proper blood calcium levels; when blood calcium rises too high, calcitonin stimulates calcium absorption into bones, lowering its level in blood.

Diseases due to imbalance Hypothyroidism (deficiency of thyroid hormones) causes growth retardation, lethargy, weight gain, constipation, high cholesterol, and low heart rate and body temperature. If hypothyroidism occurs during fetal/childhood development, it causes cretinism (stunted growth and mental retardation). Severe hypothyroidism can lead to myxoedema, a life-threatening build-up of mucopolysaccharides causing swelling and thickening of skin and tissues. Hypothyroidism caused by iodine deficiency results in goitre (swelling of the thyroid gland). Hyperthyroidism (excess thyroid hormones) causes weight loss, and high blood pressure, heart rate and body temperature; Graves' disease is an autoimmune cause of hyperthyroidism, which may also cause exophthalmia (protruding eyeballs).

One adrenal gland lies just above each kidney, composed of an outer adrenal cortex and an inner adrenal medulla.

Adrenal medulla Produces epinephrine (adrenaline) and norepinephrine (noradrenaline). Secretion is controlled by the nervous system. When a person is stressed, these hormones prepare the body for "fight or flight": increased heart rate, increased blood pressure, dilation of bronchioles, elevated blood glucose, and reduced blood flow to skin and digestive organs.

Adrenal cortex Secretion is controlled by anterior pituitary hormones (ACTH). It produces corticosteroids of two types: (1) Glucocorticoids, e.g., cortisol, maintain glucose homeostasis by stimulating breakdown of muscle protein into amino acids (carried to liver) and stimulating gluconeogenesis (conversion of amino acids to glucose) in the liver, and help the body respond to stress. (2) Mineralocorticoids, e.g., aldosterone, regulate mineral balance by stimulating the kidneys to reabsorb sodium (and water) from urine, maintaining blood volume and pressure, and stimulating secretion of excess potassium into urine.

Imbalance Over-secretion of adrenal medulla hormones may cause hypertension and aggressive behaviour; under-secretion causes failure to cope with emergency situations. Over-secretion of cortisol causes Cushing disease (excessive protein breakdown, high blood sugar, high blood pressure, thin bones, muscle loss, fragile skin, depression, excess facial hair in women), often due to excess ACTH from the pituitary. Under-secretion of cortisol causes Addison's disease (general metabolic disturbance, muscle weakness, loss of salts, developing slowly).

The pancreas is mostly an exocrine gland, producing pancreatic juice secreted into the pancreatic duct. However, it also functions as an endocrine gland through specialized cell patches called the Islets of Langerhans, distributed throughout the pancreas. Each islet has two main cell types: Beta (β) cells, which are more numerous and secrete insulin, and Alpha (α) cells, which secrete glucagon.

Insulin is secreted when blood glucose rises. It lowers blood glucose by increasing the rate of glucose uptake into body cells (e.g., skeletal muscles and fat cells), promoting glycogenesis (conversion of glucose to glycogen) in the liver, increasing glucose use in cellular respiration, promoting conversion of excess glucose to fats, and inhibiting gluconeogenesis.

Glucagon is secreted when blood glucose falls below normal. It raises blood glucose mainly by promoting hydrolysis of stored liver glycogen, releasing glucose into blood for use by cells.

Imbalance Under-secretion of insulin causes diabetes mellitus (hyperglycaemia), in which cells cannot take up glucose from blood; the resulting excess glucose inhibits water reabsorption by kidneys, causing large volumes of urine, dehydration, and kidney damage. In Type I diabetes, patients lack insulin-secreting beta cells and need daily insulin injections (produced by genetically engineered bacteria, since insulin would be digested if taken orally). In Type II diabetes, insulin levels are normal but cells have reduced sensitivity to insulin; these patients manage it through diet, exercise and other medicines rather than insulin. Over-secretion of insulin causes hypoglycaemia (below-normal blood glucose), with symptoms of lethargy, dizziness, nervousness, and in extreme cases, unconsciousness.

Thymus gland Located beneath the sternum between the lungs. It secretes thymosin, which stimulates the maturation of T cells that help defend the body from pathogens.

Pineal gland Located near the base of the brain. It secretes melatonin, which regulates sleep patterns.

Kidneys Produce renin, which controls the production of aldosterone in the adrenal glands; erythropoietin, which stimulates the bone marrow to produce red blood cells; and calcitriol, which increases blood calcium level by promoting calcium absorption from the intestines.

Walls of digestive tract Endocrine cells in the stomach wall secrete gastrin, which stimulates gastric glands to release gastric juice when food is eaten. Endocrine cells in the small intestine wall secrete secretin, which stimulates the pancreas to release pancreatic juice, and cholecystokinin, which stimulates the gallbladder to release bile.