Chemical Coordination and Integration — Class 11 Biology

This chapter describes the endocrine system, which provides chemical coordination and integration through hormones. Endocrine glands are ductless and secrete hormones, which are non-nutrient chemicals acting as intercellular messengers in trace amounts. The human endocrine system includes the hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas and gonads, while organs such as the heart, kidney and gastrointestinal tract also secrete hormones. The hypothalamus produces releasing and inhibiting hormones that control the pituitary. The pituitary has the anterior part (secreting GH, prolactin, TSH, ACTH, LH and FSH) and the posterior part (releasing oxytocin and vasopressin); disorders include gigantism, dwarfism, acromegaly and diabetes insipidus. The pineal gland secretes melatonin, which regulates diurnal rhythms. The thyroid secretes thyroxine, important for basal metabolic rate, and its iodine deficiency causes goitre. The parathyroid raises blood calcium, the thymus produces thymosins for immunity, and the adrenal medulla secretes the fight-or-flight hormones adrenaline and noradrenaline while the cortex secretes glucocorticoids and mineralocorticoids. The pancreatic islets secrete insulin and glucagon to regulate blood glucose, and their imbalance causes diabetes mellitus. Gonads secrete androgens, estrogen and progesterone. Hormones act by binding to membrane-bound or intracellular receptors, generating second messengers or regulating gene expression.