Chemical Coordination and Integration

(a) Exocrine gland: A gland that has ducts and pours its secretion through them onto a surface or into a body cavity.

(b) Endocrine gland: A ductless gland that releases its secretion directly into the blood.

(c) Hormone: A non-nutrient chemical messenger produced in trace amounts that acts on specific target tissues to regulate body functions.

- Hypothalamus: Releasing hormones and inhibiting hormones, such as GnRH and somatostatin.
- Pituitary: GH, PRL, TSH, ACTH, LH, FSH, MSH, oxytocin, vasopressin.
- Thyroid: Thyroxine (T4), Triiodothyronine (T3), thyrocalcitonin (TCT).
- Parathyroid: Parathyroid hormone (PTH).
- Adrenal: Adrenaline, noradrenaline, cortisol, aldosterone, small amounts of androgenic steroids.
- Pancreas: Insulin and glucagon.
- Testis: Androgens, mainly testosterone.
- Ovary: Estrogen and progesterone.
- Thymus: Thymosins.
- Atrium of heart: Atrial natriuretic factor (ANF).
- Kidney: Erythropoietin.
- G-I tract: Gastrin, secretin, cholecystokinin (CCK), gastric inhibitory peptide (GIP).

- Hypothalamic hormones → Pituitary
- Thyrotrophin (TSH) → Thyroid
- Corticotrophin (ACTH) → Adrenal cortex
- Gonadotrophins (LH, FSH) → Gonads (testis and ovary)
- Melanotrophin (MSH) → Melanocytes

(a) Parathyroid hormone (PTH): Increases the **Ca$^{2+}$ level in blood. It acts on bones to cause bone resorption, on kidneys to increase reabsorption of calcium, and on the intestine to increase calcium absorption. It is therefore a hypercalcemic hormone.

(b) Thyroid hormones: T3 and T4 regulate the basal metabolic rate, support red blood cell formation, control metabolism of carbohydrates, proteins and fats, help in maintaining water and electrolyte balance, and are important for normal growth and development.

(c) Thymosins: These hormones help in the differentiation of T-lymphocytes, thus supporting cell-mediated immunity. They also promote the production of antibodies and help in humoral immunity.

(d) Androgens: Mainly testosterone; they regulate development and function of male accessory sex organs, cause male secondary sex characters like facial and axillary hair, low pitch of voice and muscular growth, and stimulate spermatogenesis and male sexual behaviour.

(e) Estrogens: They stimulate growth and activity of female secondary sex organs, development of ovarian follicles, female secondary sex characters, and mammary gland development. They also regulate female sexual behaviour.

(f) Insulin and Glucagon: Insulin lowers blood glucose by increasing cellular uptake of glucose and promoting glycogenesis, so it is a hypoglycemic hormone. Glucagon raises blood glucose by stimulating glycogenolysis and gluconeogenesis, so it is a hyperglycemic hormone**.