Endocrine Adrenal Pancreas Gonads

The body has one pair of adrenal glands, one above each kidney. Each gland has two tissue types: a centrally located adrenal medulla and an outer adrenal cortex. The adrenal medulla secretes two hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine), commonly called catecholamines. Adrenaline and noradrenaline are rapidly secreted in response to stress of any kind and during emergencies, and are called emergency hormones or hormones of Fight or Flight. They increase alertness, pupillary dilation, piloerection (raising of hairs), and sweating, increase heart beat, strength of heart contraction, and rate of respiration, and stimulate breakdown of glycogen, lipids and proteins. Underproduction of hormones by the adrenal cortex alters carbohydrate metabolism, causing acute weakness and fatigue leading to a disease called Addison's disease.

The adrenal cortex has three layers: zona reticularis (inner), zona fasciculata (middle) and zona glomerulosa (outer). It secretes hormones called corticoids. Corticoids involved in carbohydrate metabolism are glucocorticoids, of which cortisol is the main one in the body. Corticoids regulating water and electrolyte balance are mineralocorticoids; aldosterone is the main mineralocorticoid. Glucocorticoids stimulate gluconeogenesis, lipolysis and proteolysis, and inhibit cellular uptake and utilisation of amino acids. Cortisol also helps maintain the cardiovascular system and kidney function, produces anti-inflammatory reactions, suppresses immune response, and stimulates RBC production. Aldosterone acts mainly at renal tubules, stimulating reabsorption of Na+ and water and excretion of K+ and phosphate ions, helping maintain electrolytes, body fluid volume, osmotic pressure and blood pressure. Small amounts of androgenic steroids from the adrenal cortex aid growth of axial, pubic and facial hair during puberty.

The pancreas is a composite gland acting as both exocrine and endocrine gland. The endocrine pancreas consists of Islets of Langerhans, about 1 to 2 million in a normal human pancreas, representing only 1 to 2 per cent of pancreatic tissue. The two main cell types in the Islets of Langerhans are alpha-cells, which secrete glucagon, and beta-cells, which secrete insulin. Glucagon is a peptide hormone that maintains normal blood glucose levels: it acts mainly on liver cells (hepatocytes), stimulating glycogenolysis (increasing blood sugar, hyperglycemia) and gluconeogenesis, and reduces cellular glucose uptake and utilisation, making it a hyperglycemic hormone. Insulin is a peptide hormone that plays a major role in glucose homeostasis, acting mainly on hepatocytes and adipocytes to enhance cellular glucose uptake and utilisation.

As a result of insulin action, glucose moves rapidly from blood into hepatocytes and adipocytes, decreasing blood glucose levels (hypoglycemia); insulin also stimulates conversion of glucose to glycogen (glycogenesis) in target cells. Glucose homeostasis in the blood is thus jointly maintained by insulin and glucagon. Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus, which is associated with loss of glucose through urine and formation of harmful compounds known as ketone bodies. Diabetic patients are successfully treated with insulin therapy.

A pair of testes is present in the scrotal sac of males; testis is both a primary sex organ and an endocrine gland, composed of seminiferous tubules and stromal/interstitial tissue. The Leydig cells or interstitial cells, present in the intertubular spaces, produce androgens, mainly testosterone. Androgens regulate development, maturation and function of male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra), stimulate muscular growth, facial and axillary hair growth, aggressiveness, low pitch of voice, play a major stimulatory role in spermatogenesis, influence male sexual behaviour (libido), and produce anabolic effects on protein and carbohydrate metabolism. Females have a pair of ovaries, the primary female sex organ, producing one ovum per menstrual cycle and two steroid hormones, estrogen and progesterone. Estrogen is synthesised mainly by growing ovarian follicles; after ovulation the ruptured follicle becomes the corpus luteum, which secretes mainly progesterone.

Hormones are also secreted by tissues that are not endocrine glands. The atrial wall of the heart secretes a peptide hormone called atrial natriuretic factor (ANF), which decreases blood pressure: when blood pressure increases, ANF is secreted, causing dilation of blood vessels and reducing blood pressure. The juxtaglomerular cells of the kidney produce a peptide hormone called erythropoietin, which stimulates erythropoiesis (RBC formation). Endocrine cells in different parts of the gastrointestinal tract secrete four major peptide hormones: gastrin, secretin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP). Gastrin acts on gastric glands, stimulating secretion of hydrochloric acid and pepsinogen. Secretin acts on the exocrine pancreas, stimulating secretion of water and bicarbonate ions. CCK acts on both pancreas and gall bladder, stimulating secretion of pancreatic enzymes and bile juice. GIP inhibits gastric secretion and motility.

Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors, located in the target tissues only. Receptors present on the cell membrane of target cells are called membrane-bound receptors, and those present inside the target cell are called intracellular receptors, mostly nuclear receptors (present in the nucleus). Binding of a hormone to its receptor leads to formation of a hormone-receptor complex. Each receptor is specific to one hormone only, hence receptors are specific. Hormone-receptor complex formation leads to certain biochemical changes in the target tissue, altering target tissue metabolism.