Chemical Coordination and IntegrationNEET Zoology · Class 11 · NCERT Chapter 19

Introduction

The nervous system gives fast, short-lived signals. The endocrine system gives slower, longer-lasting signals using chemical messengers called hormones. Together, both systems coordinate the activity of every organ in your body.

Expect 2 to 3 NEET questions every year from this chapter. The questions almost always test one of three things: which gland makes which hormone, what each hormone does, and which disorder is caused by which deficiency or excess.

What is a Hormone?

A hormone is a chemical messenger that is:

• Secreted in very small amounts by an endocrine (ductless) gland.
• Carried by the blood to distant target cells.
• Acts on the target cell by binding to a specific receptor.

Endocrine glands have no ducts. They release hormones directly into the bloodstream. Examples: pituitary, thyroid, adrenal. Exocrine glands have ducts (sweat glands, salivary glands, pancreatic acini that release digestive enzymes). Some glands are both, like the pancreas.

Hypothalamus

The hypothalamus is the central command for the endocrine system. It sits at the base of the brain, just above the pituitary, and links the nervous system to the endocrine system.

• Secretes releasing hormones (GnRH, TRH, CRH, GHRH, PRH) and inhibiting hormones (GHIH/somatostatin, PIH/dopamine) that travel to the anterior pituitary and either push or block release of pituitary hormones.
• Also makes oxytocin and ADH (vasopressin), which travel down to the posterior pituitary for storage and release.

Pituitary Gland

Sits in a bony cavity called the sella turcica at the base of the brain, attached to the hypothalamus by a stalk called the infundibulum. Has two clear lobes:

Anterior Pituitary (Adenohypophysis)

Makes and releases six major hormones plus MSH. All of them are peptide hormones.

• Growth Hormone (GH): stimulates growth of long bones in childhood, protein synthesis, and cell division. Excess in children causes gigantism; in adults causes acromegaly. Deficiency in children causes dwarfism.
• Thyroid Stimulating Hormone (TSH): stimulates the thyroid to produce T3 and T4.
• Adrenocorticotropic Hormone (ACTH): stimulates the adrenal cortex to release glucocorticoids (mainly cortisol).
• Follicle Stimulating Hormone (FSH): in females, stimulates growth of ovarian follicles; in males, supports spermatogenesis via Sertoli cells.
• Luteinizing Hormone (LH): in females, the LH surge triggers ovulation and maintains the corpus luteum; in males, stimulates Leydig cells to produce testosterone.
• Prolactin: stimulates milk production in the mammary glands during pregnancy and lactation.
• Melanocyte Stimulating Hormone (MSH): acts on melanocytes to control pigmentation.

Posterior Pituitary (Neurohypophysis)

Does NOT make any hormones. Only stores and releases two hormones produced by the hypothalamus:

• ADH (Vasopressin): acts on the distal convoluted tubule and collecting duct of the kidney. Increases water reabsorption, reducing urine volume. Deficiency causes diabetes insipidus.
• Oxytocin: causes contraction of the uterus during childbirth (the foetal ejection reflex) and milk ejection (let-down reflex) during lactation.

Pineal Gland

Small endocrine gland near the back of the brain. Secretes melatonin, which regulates the 24-hour circadian rhythm of the body, especially the sleep-wake cycle. Melatonin levels rise at night. It also influences pigmentation and the immune response.

Thyroid Gland

Butterfly-shaped gland in the front of the neck with two lobes connected by an isthmus. Made of follicles filled with colloid. The follicular cells produce thyroid hormones; the parafollicular cells (C-cells) produce calcitonin.

• Thyroxine (T4) and Triiodothyronine (T3): both contain iodine. Control basal metabolic rate (BMR), body temperature, growth, and red blood cell formation. Together they are the main "metabolic" hormones.
• Calcitonin (Thyrocalcitonin): from C-cells. Lowers blood calcium by depositing calcium into bone and reducing kidney reabsorption.

Parathyroid Gland

Four small glands embedded on the back of the thyroid. Secrete parathyroid hormone (PTH), which is the most important hormone in calcium homeostasis.

• PTH raises blood calcium (it is a "hypercalcemic" hormone).
• How: by mobilising calcium from bone, increasing intestinal absorption (via vitamin D activation), and increasing reabsorption from kidney.
• Acts in opposition to calcitonin.

Thymus

Lobed organ located in the chest between the lungs, on top of the heart. Large in young children and shrinks with age. Secretes a group of peptide hormones called thymosins, which drive the maturation and differentiation of T lymphocytes. Thymus is the primary lymphoid organ for T-cell development. It also stimulates humoral immune responses.

Adrenal Gland

Two glands, each sitting on top of a kidney. Each adrenal has two distinct parts that work differently and have different embryonic origins:

Adrenal Cortex (outer)

Secretes steroid hormones called corticoids. Three groups:

• Glucocorticoids (mainly cortisol): raise blood glucose by gluconeogenesis, break down proteins and lipids. Anti-inflammatory and immunosuppressive.
• Mineralocorticoids (mainly aldosterone): act on the kidney to reabsorb sodium (and water) and excrete potassium. Raises blood pressure.
• Small amounts of sex steroids (androgens, estrogens).

Adrenal Medulla (inner)

Secretes the catecholamines: adrenaline (epinephrine) and noradrenaline (norepinephrine). These are the emergency hormones, often called "fight or flight" hormones. They raise heart rate, blood pressure, blood glucose, and dilate the bronchioles.

Pancreas

The pancreas has both exocrine and endocrine parts. Its endocrine part is made of small clusters of cells called the islets of Langerhans. These contain at least four cell types:

• Beta cells (about 70%): secrete insulin. Insulin lowers blood glucose by promoting glucose uptake into liver, muscle and fat cells, and stimulating glycogen synthesis (glycogenesis) and fat storage (lipogenesis).
• Alpha cells (about 20%): secrete glucagon. Glucagon raises blood glucose by breaking down liver glycogen (glycogenolysis) and synthesising new glucose (gluconeogenesis).
• Delta cells: secrete somatostatin (inhibits insulin and glucagon).

Insulin and glucagon act in opposition. After a meal, insulin dominates (blood sugar high). During fasting, glucagon dominates (blood sugar low).

Gonads (Testes and Ovaries)

The gonads produce both gametes and hormones.

• Testis (Leydig cells): produce testosterone under LH stimulation. Drives spermatogenesis and male secondary sexual characters (beard, deeper voice, muscle growth).
• Ovary (theca and granulosa cells, then corpus luteum): estrogen grows the uterine lining and gives female secondary sexual characters; progesterone (from corpus luteum) maintains the uterine lining for possible pregnancy. Inhibin and relaxin are also produced.

Hormones from Heart, Kidney and GI Tract

• Atrial Natriuretic Factor (ANF): from the wall of the atrium of the heart. Released when blood pressure rises. ANF causes the kidneys to excrete more sodium and water, which lowers blood pressure.
• Erythropoietin (EPO): from the juxtaglomerular cells of the kidney. Released when blood oxygen is low. Stimulates red blood cell production in the bone marrow.
• Renin: also from the kidney. Triggers the renin-angiotensin pathway which eventually raises blood pressure.
• Gastrointestinal hormones:
  • Gastrin (from stomach): stimulates HCl and pepsinogen secretion.
  • Secretin (from duodenum): stimulates the pancreas to release bicarbonate.
  • Cholecystokinin (CCK): from duodenum. Stimulates the pancreas to release digestive enzymes and the gallbladder to release bile.
  • GIP (Gastric Inhibitory Peptide): inhibits gastric secretion and motility.

Mechanism of Hormone Action

How a hormone affects its target depends on whether the hormone is water-soluble or fat-soluble:

Peptide hormones (water-soluble)

Examples: insulin, GH, ADH, oxytocin, glucagon, all anterior pituitary hormones.

• Cannot cross the cell membrane (too big, charged).
• Bind to a receptor on the cell surface.
• Activate second messengers inside the cell, mainly cyclic AMP (cAMP), IP3, and calcium ions.
• Action is fast (seconds to minutes) but short-lived.

Steroid hormones (fat-soluble)

Examples: cortisol, aldosterone, testosterone, estrogen, progesterone. Thyroid hormones T3 and T4 also behave this way although they are not strictly steroids.

• Easily cross the cell membrane.
• Bind to receptors inside the cell (in the cytoplasm or nucleus).
• The hormone-receptor complex enters the nucleus and activates specific genes.
• Action is slow (hours) but long-lasting.

Endocrine Disorders

Disorders from too little hormone (hyposecretion)

• Diabetes insipidus (ADH deficiency): kidneys cannot reabsorb water. Symptoms: very large volume of dilute urine (polyuria), extreme thirst (polydipsia). Blood sugar is normal.
• Diabetes mellitus (insulin deficiency or resistance): blood glucose rises. Symptoms: polyuria, polydipsia, polyphagia (extreme hunger), weight loss, glucose in urine.
• Goiter (iodine deficiency): thyroid swells trying to make hormone without iodine. Most common cause of an enlarged thyroid worldwide. Fix: iodised salt.
• Cretinism (thyroid hormone deficiency from birth or early childhood): stunted growth, mental retardation, poor learning, coarse dry skin.
• Myxoedema (adult hypothyroidism): low metabolic rate, weight gain, sluggishness, cold intolerance, dry skin.
• Dwarfism (GH deficiency in childhood): short stature with normal body proportions.
• Addison disease (cortisol and aldosterone deficiency): weakness, low blood pressure, weight loss, salt craving, brown skin pigmentation.

Disorders from too much hormone (hypersecretion)

• Graves disease (exophthalmic goiter): autoimmune hyperthyroidism. Enlarged thyroid plus protruding eyeballs (exophthalmos), weight loss, heat intolerance, rapid heart rate.
• Gigantism (GH excess in childhood, before growth plates close): very tall stature.
• Acromegaly (GH excess in adulthood, after growth plates close): abnormal growth of jaw, hands and feet; coarsened features.
• Cushing syndrome (cortisol excess): moon face, buffalo hump, weight gain mainly around the trunk, thin skin, easy bruising, weakness, high blood sugar.

Worked NEET Problems

Summary Cheat Sheet

• Anterior pituitary: makes GH, TSH, ACTH, FSH, LH, prolactin, MSH.
• Posterior pituitary: only stores and releases ADH and oxytocin (made by hypothalamus).
• Thyroid: T3 + T4 (need iodine) for metabolism; calcitonin (C-cells) LOWERS calcium.
• Parathyroid: PTH RAISES calcium. PTH and calcitonin oppose each other.
• Adrenal cortex: glucocorticoids (cortisol), mineralocorticoids (aldosterone), small sex hormones.
• Adrenal medulla: adrenaline + noradrenaline (fight or flight).
• Pancreas islets: beta = insulin (lowers glucose), alpha = glucagon (raises glucose).
• Testis: Leydig cells make testosterone (under LH). Ovary: estrogen + progesterone.
• Pineal: melatonin (sleep cycle). Thymus: thymosins (T-cell maturation).
• Heart: ANF (lowers blood pressure). Kidney: erythropoietin (RBC), renin.
• GI hormones: gastrin (stomach acid), secretin (bicarbonate), CCK (enzymes + bile), GIP (inhibits).
• Peptide hormone: binds surface receptor, uses cAMP. Fast and short.
• Steroid hormone: binds nuclear receptor, activates genes. Slow and long.
• Diabetes insipidus: ADH deficiency. Diabetes mellitus: insulin deficiency.
• Goiter: iodine deficiency. Cretinism: thyroid deficiency in infancy.
• Graves disease: hyperthyroid + exophthalmos. Myxoedema: adult hypothyroid.
• Dwarfism: GH deficiency (child). Gigantism: GH excess (child). Acromegaly: GH excess (adult).
• Addison disease: cortisol deficiency. Cushing syndrome: cortisol excess.

Next: use the interactive learning widgets to explore the human endocrine gland map, drill the hormone reference table, and review every major endocrine disorder, or work through the 30+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.