Coordination and Regulation

• Neural and endocrine systems coordinate bodily functions.
• Hormones mediate coordination, especially for functions needing continuous regulation.

Endocrine Glands and Hormones

• Endocrine glands are ductless and secrete hormones.
• Hormones are non-nutrient intercellular messengers produced in trace amounts.
• Invertebrates: simple endocrine systems with few hormones.
• Vertebrates: many chemicals act as hormones.

Human Endocrine System

• Includes pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads.
• Gastrointestinal tract, liver, kidney, and heart also produce hormones.

Hypothalamus

• The hypothalamus is the basal part of diencephalon, forebrain
• It regulates body functions and contains neurosecretory cells (nuclei) to create hormones.
• Hypothalamic hormones affect pituitary hormone synthesis/secretion.
• Releasing hormones stimulate pituitary hormone secretion.
• Inhibiting hormones inhibit pituitary hormone secretion.
• Gonadotrophin releasing hormone (GnRH) stimulates pituitary gonadotrophin release.
• Somatostatin inhibits pituitary growth hormone release.
• Hypothalamic hormones travel through axons, are released near nerve endings, and reach the anterior pituitary via a portal circulatory system.
• The posterior pituitary is under direct neural control by the hypothalamus.

Pituitary Gland

• Located in the sella tursica, attached to the hypothalamus by a stalk
• Divided into adenohypophysis and neurohypophysis.
• Adenohypophysis includes the anterior pituitary (pars distalis) and pars intermedia.
  • Anterior pituitary produces growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH), and follicle stimulating hormone (FSH).
  • Pars intermedia secretes melanocyte stimulating hormone (MSH).
• Neurohypophysis (posterior pituitary) stores and releases oxytocin and vasopressin, which are produced by the hypothalamus.
• Oversecretion of GH: gigantism; low secretion of GH: pituitary dwarfism.
• Excess GH in adults causes Acromegaly, leading to disfigurement and premature death if unchecked.
• Prolactin regulates mammary gland growth and milk formation.
• TSH stimulates thyroid hormone synthesis/secretion.
• ACTH stimulates adrenal cortex steroid hormone synthesis/secretion (glucocorticoids).
• LH and FSH stimulate gonadal activity and are called gonadotrophins.; LH stimulates androgen production in males; FSH and androgens regulate spermatogenesis.
• LH induces ovulation and maintains the corpus luteum in females.
• FSH stimulates ovarian follicle growth in females.
• MSH regulates skin pigmentation by acting on melanocytes.
• Oxytocin stimulates smooth muscle contraction, uterine contraction during childbirth, and milk ejection.
• Vasopressin (ADH) stimulates water and electrolyte resorption in the kidney, reducing water loss through urine
• Impaired ADH production: Diabetes Insipidus (kidney's diminished ability to conserve water, leading to dehydration).

Pineal Gland

• The pineal gland is located on the dorsal side of the forebrain
• It secretes melatonin.
• Melatonin regulates the 24-hour (diurnal) rhythm, including sleep-wake cycles and body temperature.
• Melatonin influences metabolism, pigmentation, menstrual cycle, and defense capability.

Thyroid Gland

• The thyroid has two lobes on either side of the trachea joined by the isthmus of connective tissue.
• Follicles and stromal tissues make up the thyroid gland and thyroid follicles makes thyroxine (T4) and triiodothyronine (T3).
• Iodine is essential for thyroid hormone synthesis.
• Iodine deficiency: hypothyroidism and goiter.
• Hypothyroidism during pregnancy causes cretinism (stunted growth, mental retardation).
• Hypothyroidism in adult women can cause irregular menstrual cycles.
• Hyperthyroidism results from increased thyroid hormone synthesis/secretion, which adversely affecting body physiology.
• Exopthalmic goitre (Graves' disease) characterized by thyroid gland enlargement, protruding eyeballs, increased metabolic rate, and weight loss, a form of hyperthyroidism.
• Thyroid hormones regulate basal metabolic rate, support red blood cell formation, and control the metabolism of carbohydrates, proteins, and fats.
• Water and electrolyte balance is influenced by thyroid hormones.
• The thyroid gland secretes thyrocalcitonin (TCT) to regulate blood calcium levels.

Parathyroid Gland

• Humans have four parathyroid glands on the back of the thyroid gland.
• Parathyroid glands secrete parathyroid hormone (PTH).
• PTH secretion is regulated by calcium ion levels.
• PTH increases blood calcium levels, stimulates bone resorption, and increases calcium reabsorption in the renal tubules and from digested food.
• PTH is a hypercalcemic hormone and plays a significant role in calcium balance with TCT.

Thymus

• Thymus: lobular structure between lungs, behind the sternum, on the ventral side of aorta.
• Thymus plays a role in immune system development.
• Thymosins are secreted and play a role in T-lymphocyte differentiation for cell-mediated immunity.
• Thymosins promote antibody production for humoral immunity.
• Thymus degenerates in old age, decreasing thymosin production and weakening immune responses.

Adrenal Gland

• Adrenal glands are located on top of each kidney.
• Gland consists of the adrenal medulla and the adrenal cortex.
• Underproduction of adrenal cortex hormones causes Addison’s disease.
• Adrenal medulla produces adrenaline (epinephrine) and noradrenaline (norepinephrine), known as catecholamines.
• Catecholamines are secreted during stress and emergencies (fight or flight response).
• Catecholamines increase alertness, pupillary dilation, piloerection, sweating, heart rate, contraction strength, and respiration rate.
• Catecholamines stimulate glycogen breakdown.
• Adrenal cortex has three layers: zona reticularis, zona fasciculata, and zona glomerulosa.
• Adrenal cortex secretes corticoids; Glucocorticoids are involved in carbohydrate metabolism (cortisol is the main one).
• Mineralocorticoids regulate water and electrolyte balance (aldosterone is the main one).
• Glucocorticoids stimulate gluconeogenesis, lipolysis, and proteolysis, while inhibiting amino acid uptake.
• Cortisol maintains the cardiovascular system and kidney functions, produces anti-inflammatory reactions, suppresses the immune response, and stimulates RBC production.
• Aldosterone promotes reabsorption of Na⁺ and water and excretion of K+ and phosphate ions in the renal tubules, maintaining electrolyte balance, fluid volume, osmotic pressure, and blood pressure.
• The adrenal cortex secretes small amounts of androgenic steroids, aiding in the growth of axial, pubic, and facial hair during puberty.

Pancreas

• Pancreas: composite gland acting as both exocrine and endocrine gland.
• Endocrine pancreas consists of Islets of Langerhans (1-2 million in a human pancreas making 1-2% of the tissue)
• Islets of Langerhans contain α-cells and β-cells.
  • α-cells secrete glucagon.
  • β-cells secrete insulin.
• Glucagon raises blood glucose levels by acting on hepatocytes to stimulate glycogenolysis and gluconeogenesis (hyperglycemia), also reduces cellular glucose uptake and utilisation.
• Insulin is a peptide hormone that regulates glucose homeostasis.
• Insulin enhances cellular glucose uptake and utilisation in hepatocytes and adipocytes, promoting glycogenesis and lowering blood glucose levels (hypoglycemia).
• Glucose homeostasis is maintained by insulin and glucagon.
• Prolonged hyperglycemia causes diabetes mellitus, which leads to glucose loss in urine and ketone body formation.

Testis

• Testes in the scrotal sac and perform functions as a primary sex organ and an endocrine gland.
• Composed of seminiferous tubules and stromal or interstitial tissue.
• Leydig cells (interstitial cells) produce androgens, mainly testosterone.
• Androgens regulate the development, maturation, and function of male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra, etc.).
• Androgens stimulate muscular growth, facial/axillary hair growth, aggressiveness, and lower voice pitch.
• Androgens stimulate spermatogenesis and act on the central neural system to influence libido.
• They have anabolic effects on protein and carbohydrate metabolism.

Ovary

• Ovaries are located in the abdomen, and produce one ovum per menstrual cycle, as well as estrogen and progesterone.
• Estrogen and progesterone produced by ovarian follicles and stromal tissues.
• Growing ovarian follicles mainly secrete estrogen.
• Corpus luteum (structure from ruptured follicle) mainly secretes progesterone after ovulation.
• Estrogens stimulate growth and activity of female secondary sex organs, secondary sex characteristics, and mammary gland development, and they regulate sexual behavior.
• Progesterone supports pregnancy, acts on mammary glands to stimulate alveoli formation for milk storage, and supports milk secretion.

Hormones of Heart, Kidney, and Gastrointestinal Tract

• Hormones secreted by tissues other than endocrine glands.
• The heart’s atrial wall secretes atrial natriuretic factor (ANF), which lowers blood pressure by dilating blood vessels.
• Kidney juxtaglomerular cells produce erythropoietin, stimulating erythropoiesis (RBC formation).
• Gastrointestinal tract secretes gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP); gastrin stimulates gastric glands to secrete hydrochloric acid and pepsinogen.
• Secretin acts on the exocrine pancreas to stimulate secretion of water and bicarbonate ions.
• CCK acts on the pancreas and gall bladder (secretion of pancreatic enzymes/bile juice).
• GIP inhibits gastric secretion and motility.
• Non-endocrine tissues secrete growth factors for tissue growth/repair.

Mechanism of Hormone Action

• Hormones bind to specific proteins called hormone receptors on target tissues.
• Membrane-bound receptors are on the cell membrane.
• Intracellular receptors, mostly nuclear receptors, are inside the target cell.
• Hormone binding forms a hormone-receptor complex.
• Hormone-Receptor complex formation causes biochemical changes in target tissue.
• Physiological functions are regulated by hormones.
• Hormones are divided by chemical nature: - Peptide, polypeptide, protein hormones: insulin, glucagon, pituitary hormones, hypothalamic hormones. - Steroids: cortisol, testosterone, estradiol, progesterone. - Iodothyronines: thyroid hormones. - Amino-acid derivatives: epinephrine.
• Hormones that interact with membrane-bound receptors generate second messengers (cyclic AMP, IP3, Ca++) that regulate cell metabolism.
• Hormones interacting with intracellular receptors regulate gene expression/chromosome function through the hormone-receptor complex.
• Actions result in physiological/developmental effects.