Chemical Coordination and Integration — Notes

A hormone is a chemical messenger made by an endocrine (ductless) gland, carried in the blood and acting in very small amounts on target cells that have the right receptor. Nervous coordination is fast and short; endocrine coordination is slower and longer lasting.

Endocrine glands of the human body: hypothalamus, pituitary, pineal, thyroid, 4 parathyroids (on the back of the thyroid), thymus, adrenals (above the kidneys), pancreas (islets of Langerhans), and the gonads (testes and ovaries). The gut also makes hormones.

Hypothalamus: controls the pituitary. It makes releasing hormones (for example TRH, CRH, GnRH, GHRH) and inhibiting hormones (for example somatostatin), which reach the anterior pituitary through the hypophysial portal vessels. It also makes oxytocin and vasopressin (ADH), which pass down nerve fibres and are stored and released from the posterior pituitary.

Pituitary (hypophysis)

Feedback control of tropic hormones: a hormone of the target gland usually inhibits the hypothalamus and pituitary: negative feedback. Thyroxine: hypothalamus (TRH) → anterior pituitary (TSH) → thyroid (thyroxine); thyroxine in the blood reduces TRH and TSH, and when thyroxine falls the inhibition is removed and secretion increases. In a simple model with thyroxine at 40 percent of normal, TSH is 160 percent and thyroxine returns to about 100 percent within 8 steps (illustrative values).

Thyroid: a butterfly-shaped gland on the front of the windpipe, with follicles that make thyroxine (T4, tetraiodothyronine) and triiodothyronine (T3), for which iodine is needed, and calcitonin (thyrocalcitonin) from separate cells.

• Thyroxine and triiodothyronine: raise the basal metabolic rate; support normal growth, development of the brain, and formation of red blood cells; help to control body temperature.
• Calcitonin: lowers blood calcium by increasing its deposition in the bones.

Parathyroids: 4 small glands on the back of the thyroid. Parathormone (PTH) raises blood calcium: it increases bone breakdown, reabsorption of calcium by the kidney tubules, and absorption of calcium in the gut (with vitamin D). PTH and calcitonin act in opposite directions to keep calcium within limits (the model range used here is 9 to 11 mg per dL; books vary).

Pineal gland: makes melatonin, which regulates the sleep-wake (diurnal) rhythm, body temperature, skin pigmentation and the menstrual cycle.

Thymus: makes thymosin, which helps the differentiation of T lymphocytes and so cell-mediated immunity; the thymus is large in children and shrinks with age, so immunity weakens in old age.

Adrenal glands: on the kidneys; the cortex has 3 layers and the medulla inside.

• Zona glomerulosa (outer): mineralocorticoids (aldosterone): reabsorption of sodium and loss of potassium and so water and blood pressure.
• Zona fasciculata (middle): glucocorticoids (cortisol): raise blood glucose (gluconeogenesis), break down proteins and fats, anti-inflammatory, reduce immunity.
• Zona reticularis (inner): small amounts of sex corticoids (androgens).
• Medulla: adrenaline (epinephrine) and noradrenaline: emergency hormones (fight or flight): faster, stronger heartbeat, wider pupils, more alertness, sweating, breakdown of glycogen, fats and proteins to raise blood glucose.

Pancreas (islets of Langerhans): alpha cells make glucagon (raises blood glucose: glycogenolysis and gluconeogenesis in the liver); beta cells make insulin (lowers blood glucose: uptake by cells, glycogenesis); delta cells make somatostatin (inhibits both). The fasting blood glucose is taken as 70 to 110 mg per dL in the chapter (book values differ); a diabetic is suspected with fasting glucose of 126 or two-hour glucose of 200 mg per dL or more in a tolerance test.

Gonads: testis (Leydig cells): androgens (testosterone): male characters, spermatogenesis; ovary: oestrogen (female characters, cycle) and progesterone (maintains pregnancy).

Gastrointestinal tract: gastrin (stomach: secretion of HCl and pepsinogen), secretin (duodenum: water and bicarbonate from the pancreas), cholecystokinin-pancreozymin, CCK-PZ (duodenum: contraction of the gall bladder and pancreatic enzymes), gastric inhibitory peptide, GIP (inhibits gastric secretion and movement).

A hormone acts only on target cells that have its receptor.

Protein, peptide and amino acid hormones (insulin, glucagon, adrenaline, TSH): water soluble, cannot cross the lipid membrane. They bind a receptor on the cell membrane, which activates adenylyl cyclase through a G protein; it converts ATP to cyclic AMP (cAMP), the second messenger, which activates protein kinases and enzymes and produces the response. (The hormone is the first messenger.) The response is fast.

Steroid hormones (cortisol, testosterone, oestrogen) and thyroid hormones: lipid soluble, they diffuse through the membrane and bind an intracellular receptor (cytoplasm or nucleus). The hormone-receptor complex binds DNA, switches genes on or off, and changes the synthesis of mRNA and proteins. The response is slower and longer lasting.

Note on books: the thyroid hormones are lipid soluble and act on nuclear receptors like steroids, although they are amino acid derivatives; adrenaline acts through cAMP. Some books use the term "chemical nature" (peptide, steroid, amino acid derivative) as the basis of the two mechanisms.

Diabetes mellitus and diabetes insipidus both give frequent urination, but the first is due to lack of insulin with high blood glucose and sugary urine; the second is due to lack of ADH, with normal blood glucose and sugar-free urine.