Regulation of Body Processes

Core Concepts

Blood glucose: the insulin–glucagon pair

After a meal, blood glucose rises; beta cells of the pancreatic islets release insulin, which tells muscle, liver, and fat cells to take up glucose and store it (as glycogen in the liver and muscle, as fat in adipose tissue), lowering blood glucose back toward the set point. Between meals, glucose falls; alpha cells release glucagon, which tells the liver to break down glycogen and make new glucose (gluconeogenesis), raising blood glucose. Somatostatin from delta cells modulates both neighbors. Fasting blood glucose has a commonly taught reference range (roughly 70–99 mg/dL in many texts — verify against current sources) that illustrates the set-point idea; when this regulation fails, the result is diabetes mellitus.

Blood calcium: PTH versus calcitonin

Calcium is needed for bone structure, muscle contraction, nerve signaling, and blood clotting, so its blood level is tightly defended. Parathyroid hormone (PTH), released when blood calcium falls, raises it three ways: it stimulates bone breakdown (resorption) to release calcium, increases calcium reabsorption in the kidneys, and activates calcitriol (active vitamin D), which boosts calcium absorption from the gut. Calcitonin, released by the thyroid when blood calcium is high, lowers it mainly by reducing bone resorption; its role in adult humans is commonly taught as minor compared with PTH's. Total blood calcium has a commonly taught reference range (roughly 9–11 mg/dL in many texts — verify against current sources).

Water and electrolytes: ADH and aldosterone

Antidiuretic hormone (ADH, also called vasopressin) is released by the posterior pituitary when blood is concentrated or blood pressure falls. It makes the kidneys reabsorb more water, concentrating the urine and restoring blood volume. Aldosterone, from the adrenal cortex, makes the kidneys reabsorb sodium (and excrete potassium); water follows sodium, so aldosterone also supports blood volume and pressure. The two work with the renin–angiotensin–aldosterone system (RAAS), a cascade triggered when the kidneys sense low blood pressure, that ultimately produces aldosterone. When ADH is missing or ineffective, the result is diabetes insipidus — large volumes of dilute urine — which is unrelated to blood sugar despite the similar name.

Growth, development, and metabolic rate

Growth hormone (GH) from the anterior pituitary stimulates growth of bone and soft tissue, largely by prompting the liver to produce insulin-like growth factors. Thyroid hormones (T3, T4) set the basal metabolic rate, are required for normal growth and for brain development in early life, and make the body more sensitive to catecholamines. Sex steroids (testosterone, estrogen) drive the growth spurt and the changes of puberty and are produced in response to the pituitary gonadotropins FSH and LH.

The stress response: fast and slow tracks

Short-term stress ("fight or flight") is handled by the sympathetic nervous system and the adrenal medulla, which release epinephrine and norepinephrine: heart rate and blood pressure rise, airways open, blood flow shifts to muscles, and stored glucose is mobilized — all within seconds. Long-term stress is handled by the hypothalamus–pituitary–adrenal (HPA) axis: the hypothalamus releases CRH, the pituitary releases ACTH, and the adrenal cortex releases cortisol, a glucocorticoid that keeps blood glucose available, supports blood pressure, and modulates inflammation and immunity. Cortisol's effects build slowly but persist — the hormonal signature of chronic stress.

Circadian rhythms

The pineal gland releases melatonin in a light-dark pattern — more at night, suppressed by daylight — helping to synchronize the sleep–wake cycle. Melatonin's exact role in human sleep is still under active research.

Common Confusions