Chemical Coordination and Integration

Besides the fast neural system, the body uses a slower, longer-lasting chemical control: the endocrine system. Its glands are ductless (endocrine) glands that pour their secretions, the hormones, straight into the blood, which carries them to distant target organs. Hormones are non-nutrient chemical messengers secreted in very small (trace) amounts and act on specific receptors. This contrasts with exocrine glands (salivary, sweat), which use ducts; the pancreas is special because it is both exocrine and endocrine.

The master link between the nervous and endocrine systems is the hypothalamus. It produces two kinds of hormones — releasing hormones (which stimulate the pituitary, e.g. GnRH) and inhibiting hormones (which stop pituitary secretion) — and so controls the pituitary gland below it.

The pituitary (hypophysis) sits in a bony cavity, the sella turcica. It has an anterior lobe (adenohypophysis, with pars distalis and pars intermedia) and a posterior lobe (neurohypophysis, the pars nervosa). The pars distalis secretes six key hormones: growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinising hormone (LH) and follicle-stimulating hormone (FSH). The pars intermedia secretes MSH.

The posterior pituitary does not synthesise hormones — it only stores and releases two hormones made by the hypothalamus: oxytocin (stimulates uterine contraction during birth and milk ejection) and vasopressin (ADH) (promotes water reabsorption in the kidney). Two GH disorders are classic NEET facts: over-secretion causes gigantism (in children) or acromegaly (in adults), while under-secretion causes pituitary dwarfism; a lack of ADH causes diabetes insipidus. Fix the gland definitions, the hypothalamic control, and the anterior-vs-posterior hormone list with these disorders.

Beyond the pituitary, several glands secrete hormones with very specific NEET-relevant roles. The pineal gland, located in the brain, secretes melatonin, which regulates the 24-hour (circadian) rhythm of the body, including the sleep–wake cycle, body temperature and pigmentation.

The thyroid gland lies in the neck and consists of two lobes. It makes the iodine-containing hormones thyroxine (T4) and tri-iodothyronine (T3), which control the basal metabolic rate (BMR), growth and development, and red blood cell formation. Because making these hormones requires iodine, a dietary deficiency causes the thyroid to enlarge into a swelling called goitre. Low thyroid activity (hypothyroidism) during pregnancy/childhood causes cretinism (stunted growth, mental retardation, deaf-mutism); in adults it can cause myxoedema. The thyroid also secretes a third hormone, thyrocalcitonin (TCT), which lowers blood calcium levels.

Embedded in the back of the thyroid are four tiny parathyroid glands. They secrete parathyroid hormone (PTH, parathormone), which raises blood calcium by stimulating the release of Ca²⁺ from bone and its reabsorption in the kidney — so PTH is a hypercalcaemic hormone.

The key NEET point is the antagonistic pair that controls blood calcium: calcitonin (TCT) lowers it while parathormone (PTH) raises it. Also worth remembering is the small thymus gland, which secretes thymosins that promote the differentiation of T-lymphocytes for immunity; the thymus is large in children and degenerates with age. For NEET, lock in melatonin (circadian rhythm), thyroxine (needs iodine; deficiency → goitre/cretinism), the calcium-regulating antagonism (calcitonin vs PTH) and thymosin's role in immunity.

One adrenal gland sits atop each kidney, and each has two distinct parts that secrete different hormones. The inner adrenal medulla secretes adrenaline (epinephrine) and noradrenaline, together called catecholamines. These are emergency or 'fight-or-flight' hormones: they increase heart rate, the force of heartbeat, breathing rate, alertness and blood glucose, preparing the body for stress. The outer adrenal cortex secretes corticoids of two main types: glucocorticoids (mainly cortisol), which regulate carbohydrate metabolism and suppress inflammation, and mineralocorticoids (mainly aldosterone), which regulate Na⁺ and water balance in the kidney. A deficiency of adrenal cortical hormones causes Addison's disease.

The pancreas is the classic example of a dual gland — both exocrine (digestive enzymes) and endocrine. Its endocrine part, the islets of Langerhans, has two important cell types. The alpha (α) cells secrete glucagon, which raises blood glucose (it is hyperglycaemic) by breaking down liver glycogen. The beta (β) cells secrete insulin, which lowers blood glucose (it is hypoglycaemic) by promoting uptake of glucose into cells and its storage as glycogen. The two are antagonistic and keep blood sugar steady; a lack of insulin causes diabetes mellitus, marked by high blood glucose and glucose in the urine.

The gonads are also endocrine glands. The testes contain the interstitial Leydig cells that secrete androgens (mainly testosterone), controlling the development of male accessory organs and secondary sexual characters and sperm production. The ovaries secrete oestrogen (development of female characters, regulation of the menstrual cycle) and, after ovulation, progesterone (supports pregnancy).

A few other tissues also act as endocrine sources: the heart secretes atrial natriuretic factor (ANF) (lowers blood pressure), the kidney secretes erythropoietin (stimulates red blood cell formation) and renin, and the gastrointestinal tract secretes gastrin, secretin and CCK. For NEET, fix the adrenal medulla (emergency hormones) vs cortex (corticoids), the insulin/glucagon antagonism with diabetes mellitus, and the gonadal hormones.

Hormones travel in the blood to almost every cell, yet each acts only on its target cells — those that carry the matching receptor. Receptors on the cell membrane are membrane-bound receptors; receptors inside the cell are intracellular receptors. The hormone–receptor complex is what produces the response, and the location of the receptor depends on the chemical nature of the hormone, which gives two distinct mechanisms — a core NEET comparison.

The first mechanism is for peptide, protein and amine hormones (such as insulin, glucagon and adrenaline). These hormones are water-soluble and cannot pass through the lipid cell membrane, so they bind membrane-bound receptors on the outside of the cell. This binding activates the production of a second messenger inside the cell — most commonly cyclic AMP (cAMP) — which then sets off a chain of biochemical changes that bring about the hormone's effect. Because no new protein needs to be made first, these effects are usually rapid.

The second mechanism is for steroid hormones and thyroid hormones (such as cortisol, testosterone, oestrogen and thyroxine). These hormones are lipid-soluble, so they pass easily through the cell membrane and bind intracellular receptors in the cytoplasm or nucleus. The hormone–receptor complex then acts on the DNA, regulating gene expression — switching specific genes on or off — which changes the kinds and amounts of proteins the cell makes. Because they work through gene expression, these effects are usually slower but longer-lasting.

So the key contrast is: water-soluble (peptide) hormones → membrane receptor → second messenger (cAMP) → fast response, whereas lipid-soluble (steroid/thyroid) hormones → intracellular receptor → gene regulation → slower, lasting response. For NEET, remember which hormones fall into each class, where the receptor is, and the role of the second messenger versus gene regulation.