Chemical Coordination and IntegrationMind Map

Endocrine glands are ductless glands that secrete hormones directly into the blood for transport to distant or nearby target organs. They form the endocrine system, which works with the nervous system to maintain homeostasis. Exocrine glands, in contrast, have ducts and release secretions such as saliva, sweat or digestive enzymes onto epithelial surfaces or into cavities. Major endocrine glands include pituitary, pineal, thyroid, parathyroid, adrenal, pancreas, thymus, testes and ovaries. Some organs like kidney, heart, gastrointestinal tract, liver and placenta also secrete hormones. Neuroendocrine integration is best shown by the hypothalamus, which receives neural inputs and controls pituitary hormone secretion.

Hormones are non-nutrient chemical messengers produced in trace amounts by endocrine glands or endocrine tissues. They regulate physiological processes by binding to specific receptors on target cells. Based on chemical nature, hormones are commonly grouped into peptide/protein/glycoprotein hormones, steroid hormones and amino acid-derived hormones. Peptide hormones are usually water-soluble and include insulin, glucagon, growth hormone, prolactin, TSH, FSH and LH. Steroid hormones are lipid-soluble and include cortisol, aldosterone, testosterone, estrogen and progesterone. Amino acid-derived hormones include thyroid hormones, adrenaline, noradrenaline and melatonin. NEET frequently asks classification, source, target and function of hormones.

Hormones produce effects only after binding to specific receptors present on target cells. Water-soluble hormones such as peptide hormones and catecholamines usually cannot cross the plasma membrane, so they bind to cell surface receptors and activate intracellular second messengers such as cAMP, IP3, DAG or calcium ions. These second messengers change enzyme activity and produce rapid responses. Lipid-soluble hormones such as steroid hormones and thyroid hormones cross the membrane and bind intracellular receptors. The hormone-receptor complex then regulates gene expression, producing slower but longer-lasting responses. Hormonal secretion is controlled mainly by feedback regulation, especially negative feedback, which maintains homeostasis.

The human endocrine system consists of hypothalamus, pituitary, pineal, thyroid, parathyroid, adrenal glands, pancreas, gonads and thymus. The hypothalamus controls pituitary using releasing and inhibiting hormones. The pituitary secretes growth hormone, prolactin, TSH, ACTH, FSH, LH and also releases ADH and oxytocin from posterior pituitary. Pineal secretes melatonin, thyroid secretes T3, T4 and calcitonin, parathyroid secretes PTH, adrenal cortex secretes corticoids and adrenal medulla secretes catecholamines. Pancreatic islets secrete insulin and glucagon. Gonads secrete sex hormones, and thymus secretes thymosins for immune maturation. NEET heavily tests gland, hormone, target and function charts.

Endocrine disorders occur when hormones are secreted in deficient or excessive amounts, when receptors fail to respond, or when feedback regulation is disturbed. Diabetes mellitus is caused by insulin deficiency or insulin resistance and produces hyperglycemia, glycosuria and polyuria. Diabetes insipidus results from ADH deficiency or poor renal response, causing excessive dilute urine. Thyroid disorders include hypothyroidism, hyperthyroidism, goitre and cretinism. Growth hormone imbalance causes dwarfism, gigantism or acromegaly depending on age and secretion level. Adrenal cortex disorders include Addison's disease due to hyposecretion and Cushing's syndrome due to excess cortisol. NEET questions commonly ask cause, hormone involved and symptoms.