Short Notes on Chemical Coordination and Control for NEET 2026 | NCERT key points

19.1 Endocrine Glands and Hormones

• The neural system provides rapid, point-to-point coordination, but it is short-lived.
• The endocrine system provides continuous, long-term regulation using hormones.
• Both systems work together to coordinate and regulate body functions.
• Endocrine glands are known as ductless glands because they lack ducts.
• Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
• Invertebrates have simple endocrine systems, while vertebrates have a large number of hormones.

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Nervous system transmits fast, short-lived signals via neurons (electrical + chemical synapses) for precise, point-to-point control, whereas the Endocrine system uses hormones secreted by glands, acts more slowly, and has long-lasting, widespread effects — both systems coordinate body functions and often act together (hypothalamus-pituitary link).

Feature	Nervous System	Endocrine System
Messenger / Signal	Electrical impulses & neurotransmitters	Hormones (chemical messengers)
Speed of action	Very fast (milliseconds)	Slow (seconds to hours) — but effects last longer
Duration of effect	Short-lived	Long-lasting
Mode of transmission	Through neurons (axons/synapses)	Through blood (endocrine glands → bloodstream)
Target specificity	Highly specific (innervated cells)	Broad — hormones can act on many tissues with appropriate receptors
Control examples (NEET-relevant)	Reflex actions, rapid muscle responses	Growth, metabolism, reproduction, stress response (adrenaline/cortisol)
Structural units / organs	Neurons, brain, spinal cord, peripheral nerves	Ductless glands: pituitary, thyroid, adrenal, pancreas, gonads, etc.
Integration link	Hypothalamus (neural control)	Hypothalamus → Pituitary (endocrine control)

NEET PYQ topic (highlighted) (Based on NCERT emphasis and common NEET question patterns.)

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19.2 Human Endocrine System

• The human endocrine system consists of organised endocrine glands and diffused hormone-producing cells.
• Major organised endocrine glands include: Pituitary, Pineal, Thyroid, Adrenal, Pancreas, Parathyroid, Thymus, and Gonads (Testis & Ovary).
• Other organs like the gastrointestinal tract, liver, kidney, and heart also produce hormones.

19.2.1 The Hypothalamus

• The hypothalamus is the basal part of the diencephalon (forebrain).
• It contains neurosecretory cells called nuclei that produce hormones.
• Hypothalamic hormones are of two types:
  • Releasing Hormones: Stimulate the secretion of pituitary hormones.
    • Example: Gonadotrophin-releasing hormone (GnRH) stimulates the pituitary to release gonadotrophins. (AIPMT PYQ)
  • Inhibiting Hormones: Inhibit the secretion of pituitary hormones.
    • Example: Somatostatin inhibits the release of Growth Hormone from the pituitary. (NEET UG PYQ)
• Hormones from the hypothalamus reach the anterior pituitary via a portal circulatory system.
• The posterior pituitary is under the direct neural regulation of the hypothalamus. (NEET UG PYQ)

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19.2.2 The Pituitary Gland

• Located in the bony cavity called sella tursica and is attached to the hypothalamus by a stalk.(infundibulum) (AIPMT PYQ)
• Divided into: Adenohypophysis (anterior pituitary) and Neurohypophysis (posterior pituitary).
• Adenohypophysis consists of two portions:
  • Pars Distalis (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). In humans, it is almost merged with pars distalis.
• Hormone Functions & Disorders:
  • GH: Oversecretion causes Gigantism (in children) and Acromegaly (in adults). Low secretion causes pituitary dwarfism. (NEET UG PYQ)
  • PRL: Regulates mammary gland growth and milk formation.
  • TSH: Stimulates thyroid hormone synthesis and secretion.
  • ACTH: Stimulates synthesis of glucocorticoids from adrenal cortex.
  • LH & FSH (Gonadotrophins):
    • In males, LH stimulates androgen synthesis, and FSH & androgens regulate spermatogenesis. (AIPMT PYQ)
    • In females, LH induces ovulation and maintains the corpus luteum. FSH stimulates growth of ovarian follicles. (NEET UG PYQ)
  • MSH: Acts on melanocytes to regulate skin pigmentation.
• Neurohypophysis (Posterior Pituitary or Pars Nervosa):
  • Stores and releases two hormones synthesised by the hypothalamus:
    • Oxytocin: Stimulates vigorous uterine contraction during childbirth and milk ejection.
    • Vasopressin (Anti-diuretic Hormone or ADH): Acts on the kidney tubules to stimulate water and electrolyte reabsorption, reducing water loss through urine. (NEET UG PYQ)
    • Disorder: Impairment of ADH synthesis/release leads to Diabetes Insipidus (water loss and dehydration). (NEET UG PYQ)

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19.2.3 The Pineal Gland

• Located on the dorsal side of the forebrain.
• Secretes melatonin, which plays a key role in regulating the 24-hour (diurnal) rhythm of the body. (NEET UG PYQ)
• Influences sleep-wake cycle, body temperature, metabolism, pigmentation, and menstrual cycle.

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19.2.4 Thyroid Gland

• Composed of two lobes on either side of the trachea, interconnected by the isthmus.
• Follicular cells synthesise thyroxine (T4) and triiodothyronine (T3).
• Iodine is essential for thyroid hormone synthesis.
  • Iodine Deficiency: Leads to hypothyroidism and an enlarged thyroid gland, commonly called goitre. (AIPMT PYQ)
• Thyroid Disorders:
  • Cretinism: Hypothyroidism during pregnancy leading to stunted growth, mental retardation, and abnormal skin in the baby. (NEET UG PYQ)
  • Irregular menstrual cycle: Can be caused by hypothyroidism in adult women.
  • Hyperthyroidism: Increased secretion of thyroid hormones. Caused by thyroid cancer or nodules.
  • Exopthalmic goitre (Graves' disease): A form of hyperthyroidism characterised by an enlarged thyroid, protruding eyeballs, increased basal metabolic rate, and weight loss. (NEET UG PYQ)
• Thyroid hormones also support RBC formation, metabolism of carbohydrates, proteins and fats, and maintenance of water and electrolyte balance.
• The thyroid gland also secretes Thyrocalcitonin (TCT), which regulates (decrease blood calcium level) blood calcium levels.

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19.2.5 Parathyroid Gland

• Four glands located on the back of the thyroid gland.
• Secretes Parathyroid Hormone (PTH).
• Secretion is regulated by circulating calcium levels.
• PTH is a hypercalcemic hormone; it increases blood levels. (AIPMT PYQ)
• It does this by:
  • Stimulating bone resorption (dissolution/demineralisation).
  • Stimulating reabsorption of Ca²+ by renal tubules.
  • Increasing Ca²+ absorption from digested food.
• PTH works with TCT to maintain calcium balance in the body.

19.2.6 Thymus

• The thymus is a lobular structure located between the lungs, behind the sternum on the ventral side of the aorta.
• It plays a major role in the development of the immune system.
• This gland secretes the peptide hormones called thymosins.
• Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. In addition, they also promote the production of antibodies to provide humoral immunity. (NEET UG PYQ)
• The thymus degenerates in old individuals, resulting in a decreased production of thymosins and weakened immune responses.

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19.2.7 Adrenal Gland

• The body has one pair of adrenal glands, each located above a kidney.
• Each gland is composed of a centrally located tissue called the adrenal medulla and an outer tissue called the adrenal cortex.

Adrenal Medulla

• Secretes two hormones called adrenaline (epinephrine) and noradrenaline (norepinephrine). These are commonly called catecholamines. (AIPMT PYQ)
• They are known as emergency hormones or hormones of "Fight or Flight" due to their rapid secretion in response to stress. (NEET UG PYQ)
• Their effects include increased alertness, pupilary dilation, piloerection, sweating, increased heart rate, strength of heart contraction, and rate of respiration.
• They stimulate the breakdown of glycogen into glucose (hyperglycemia), as well as the breakdown of lipids and proteins.

Adrenal Cortex

• Divided into three layers: zona reticularis (inner), zona fasciculata (middle), and zona glomerulosa (outer).
• Secretes hormones called corticoids.
  • Glucocorticoids (e.g., cortisol): Involved in carbohydrate metabolism. They stimulate gluconeogenesis, lipolysis, and proteolysis. Cortisol is also anti-inflammatory, suppresses the immune response, and stimulates RBC production.
  • Mineralocorticoids (e.g., aldosterone): Regulate water and electrolyte balance. Aldosterone acts on renal tubules to stimulate the reabsorption of Na⁺ and water and excretion of K⁺ and phosphate ions. (NEET UG PYQ)
  • Small amounts of androgenic steroids are also secreted, which play a role in the growth of axial, pubic, and facial hair during puberty.
• Addison’s disease is caused by the underproduction of hormones by the adrenal cortex, leading to altered carbohydrate metabolism, acute weakness, and fatigue. (AIPMT PYQ)

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19.2.8 Pancreas

• The pancreas is a (composite/mixed/hetro-crine/) gland that functions as both an exocrine and endocrine gland.
• The endocrine part consists of Islets of Langerhans (1-2% of pancreatic tissue).
• The two main cell types are α-cells and β-cells.
• The α-cells secrete glucagon, while the β-cells secrete insulin. (NEET UG PYQ)
• Glucagon: A peptide hormone that maintains normal blood glucose levels. It acts on hepatocytes to stimulate glycogenolysis and gluconeogenesis, causing an increase in blood sugar (hyperglycemia). It is a hyperglycemic hormone. (AIPMT PYQ)
• Insulin: A peptide hormone that regulates glucose homeostasis. It acts on hepatocytes and adipocytes to enhance glucose uptake and utilisation, causing a decrease in blood glucose levels (hypoglycemia). It also stimulates the conversion of glucose to glycogen (glycogenesis).
• Δ (Delta) cells secrete Somatostatin (PYQ)
• PP (F) cells secrete Pancreatic Polypeptide (PYQ)
• Diabetes mellitus is a complex disorder caused by prolonged hyperglycemia, associated with loss of glucose in urine and the formation of ketone bodies. (NEET UG PYQ)

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19.2.9 Testis

• A pair of testes is present in the scrotal sac of males.
• It performs dual functions as a primary sex organ and an endocrine gland.
• The Leydig cells (interstitial cells), located in the intertubular spaces, produce a group of hormones called androgens (mainly testosterone). (AIPMT PYQ)
• Androgens regulate the development and functions of male accessory sex organs, stimulate muscular growth, facial and axillary hair growth, aggressiveness, and low pitch of voice.
• They also play a major stimulatory role in the process of spermatogenesis.

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19.2.10 Ovary

• Females have a pair of ovaries located in the abdomen.
• Ovary is the primary female sex organ that produces an ovum. It also produces two groups of steroid hormones: estrogen and progesterone.
• Estrogen is synthesised and secreted mainly by the growing ovarian follicles.
• After ovulation, the ruptured follicle converts into a corpus luteum, which secretes mainly progesterone.
• Estrogen stimulates the growth of female secondary sex organs, development of ovarian follicles, appearance of female secondary sex characters (e.g., high pitch of voice), and mammary gland development.
• Progesterone supports pregnancy and stimulates the formation of alveoli and milk secretion in mammary glands. (NEET UG PYQ)

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19.3 Hormones of Heart, Kidney and Gastrointestinal Tract

• Heart: The atrial wall secretes atrial natriuretic factor (ANF), which decreases blood pressure by causing dilation of blood vessels. (AIPMT PYQ)
• Kidney: The juxtaglomerular cells produce erythropoietin, a peptide hormone that stimulates erythropoiesis (RBC formation). (NEET UG PYQ)
• Gastro-intestinal Tract: Secretes four major peptide hormones:
  • Gastrin: Stimulates gastric glands to secrete HCl and pepsinogen.
  • Secretin: Stimulates the exocrine pancreas to secrete water and bicarbonate ions.
  • Cholecystokinin (CCK): Acts on both the pancreas and gall bladder to stimulate the secretion of pancreatic enzymes and bile juice, respectively.
  • Gastric Inhibitory Peptide (GIP): Inhibits gastric secretion and motility.

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19.4 Mechanism of Hormone Action

• Hormones bind to specific proteins called hormone receptors located in the target tissues.
• Receptors can be membrane-bound (on the cell membrane) or intracellular (inside the cell).
• Hormone-receptor complex formation leads to biochemical and physiological changes.
• On the basis of chemical nature, hormones are grouped into:
  1. Peptide, Polypeptide, Protein hormones (e.g., insulin, glucagon, pituitary hormones).
  2. Steroids (e.g., cortisol, testosterone, estradiol, progesterone).
  3. Iodothyronines (thyroid hormones).
  4. Amino-acid derivatives (e.g., epinephrine).
• Mechanism for Membrane-Bound Receptors: Hormones do not enter the cell, but generate second messengers (e.g., cyclic AMP, IP₃, Ca⁺⁺) which regulate cellular metabolism. (NEET UG PYQ)
• Mechanism for Intracellular Receptors: Hormones (e.g., steroid hormones, iodothyronines) enter the cell and the hormone-receptor complex regulates gene expression or chromosome function. (AIPMT PYQ)

Gland	Origin/Location	Hormone(s) Secreted	Nature	Target Organ/Site of Action	Function (NEET/AIPMT Key Points)
Hypothalamus	Basal part of the diencephalon (forebrain)	GnRH, Somatostatin, ADH/Oxytocin (synthesizes)	Peptide	Pituitary Gland (Ant. & Post.)	Controls Pituitary. GnRH stimulates Gonadotrophin release. Somatostatin inhibits GH release.
Pituitary	Sella turcica (bony cavity)	GH, PRL, TSH, ACTH, LH, FSH, MSH, Oxytocin, Vasopressin (ADH)	Peptide/Protein	Bone, Liver, Kidney, Gonads, Uterus, Mammary Glands	GH disorders: Gigantism, Acromegaly. LH induces ovulation. ADH prevents water loss (Diabetes Insipidus).
Pineal	Dorsal side of forebrain	Melatonin	Peptide	CNS/Pigment Cells	Regulates the 24-hour (diurnal) rhythm (Sleep-wake cycle).
Thyroid	Trachea (connected by Isthmus)	T₃/T₄ (Thyroxine), Thyrocalcitonin (TCT)	Iodothyronines / Peptide	All Body Cells / Bone	Iodine deficiency → Goitre/Hypothyroidism (causes Cretinism). TCT lowers blood Ca²⁺.
Parathyroid	Back of Thyroid Gland (Four glands)	Parathyroid Hormone (PTH)	Peptide	Bone, Renal Tubules	Hypercalcemic hormone; Increases blood Ca²⁺ levels by stimulating bone resorption.
Thymus	Between lungs, behind sternum	Thymosins	Peptide	T-lymphocytes	Differentiation of T-lymphocytes for cell-mediated immunity.
Adrenal Medulla	Central part of Adrenal Gland	Adrenaline/Noradrenaline (Catecholamines)	Amino-acid derivative	Heart, Blood Vessels, Liver, Muscles	'Fight or Flight' hormones. Increase heart rate and blood glucose (glycogenolysis).
Adrenal Cortex	Outer part of Adrenal Gland	Cortisol (Glucocorticoid), Aldosterone (Mineralocorticoid)	Steroid	Liver, Kidney Tubules	Aldosterone stimulates Na⁺ reabsorption. Underproduction causes Addison's disease.
Pancreas	Composite Gland (Islets of Langerhans)	Glucagon (α-cells), Insulin (β-cells)	Peptide	Liver, Adipose Tissue	Glucagon → Hyperglycemia (glycogenolysis). Insulin → Hypoglycemia (glycogenesis). Failure causes Diabetes Mellitus.
Testis	Scrotal sac (Leydig cells)	Androgens (Testosterone)	Steroid	Male Accessory Organs	Regulates development of accessory sex organs and spermatogenesis.
Ovary	Abdomen (Follicles, Corpus Luteum)	Estrogen, Progesterone	Steroid	Uterus, Mammary Glands	Estrogen develops secondary sex characters. Progesterone supports pregnancy.
Heart	Atrial wall	Atrial Natriuretic Factor (ANF)	Peptide	Blood Vessels	Decreases blood pressure by causing dilation of blood vessels.
Kidney	Juxtaglomerular cells	Erythropoietin	Peptide	Bone Marrow	Stimulates erythropoiesis (RBC formation).
GI Tract	Endocrine cells in mucosa	Gastrin, Secretin, CCK, GIP	Peptide	Gastric Glands, Pancreas, Gall Bladder	Regulate digestive secretions (e.g., Secretin stimulates water and bicarbonate ions from pancreas).