Chemical Coordination and Integration: NEET notes

1. Endocrine glands and hormones

NCERT §19 (opening text) and §19.1

• The neural system gives point-to-point coordination that is quick but does not last. Nerve fibres do not reach every cell, and cell functions need regulating all the time, so a second kind of coordination is needed; hormones provide it.
• The neural and endocrine systems work together to coordinate and regulate the body's physiological functions.
• Endocrine glands have no ducts, so they are called ductless glands. What they secrete is called a hormone.
• The classical definition: a hormone is a chemical made by an endocrine gland, released into the blood and carried to a target organ some distance away.
• The current definition is broader: hormones are non-nutrient chemicals, made in trace amounts, that carry messages between cells. It takes in many molecules that do not come from organised endocrine glands.
• Invertebrates have very simple endocrine systems with only a few hormones, while in vertebrates a large number of chemicals act as hormones.

2. The human endocrine system and hypothalamus

NCERT §19.2 and §19.2.1

• The endocrine system is made up of the endocrine glands together with hormone-producing diffuse tissues and cells found in different parts of the body.
• The organised endocrine bodies are eight: pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and the gonads (testes in a male, ovaries in a female). Some other organs, such as the gastrointestinal tract, liver, kidney and heart, also make hormones.
• The hypothalamus is the basal part of the diencephalon of the forebrain and regulates a wide range of body functions.
• It contains several groups of neurosecretory cells, called nuclei, that make hormones. These hormones control the synthesis and secretion of pituitary hormones.
• Hypothalamic hormones are of two kinds: releasing hormones, which stimulate pituitary secretion, and inhibiting hormones, which inhibit it.
• Examples: gonadotrophin releasing hormone (GnRH) stimulates the pituitary to make and release gonadotrophins; hypothalamic somatostatin holds back the pituitary's release of growth hormone (GH).
• These hormones are made in hypothalamic neurons, travel down their axons and are released at the nerve endings. They reach the pituitary through a portal circulatory system and regulate the anterior pituitary.
• The posterior pituitary, by contrast, is under direct neural regulation by the hypothalamus.

3. The pituitary gland

NCERT §19.2.2

• The pituitary sits in a bony cavity, the sella turcica, and is attached to the hypothalamus by a stalk. Anatomically it has two parts: the adenohypophysis and the neurohypophysis.
• The adenohypophysis is made of the pars distalis and the pars intermedia. The pars distalis, usually called the anterior pituitary, makes six hormones: growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), follicle stimulating hormone (FSH) and luteinizing hormone (LH).
• The pars intermedia secretes only one hormone, melanocyte stimulating hormone (MSH). In humans it is almost merged with the pars distalis.
• The neurohypophysis (pars nervosa), or posterior pituitary, stores and releases oxytocin and vasopressin. Both are actually made in the hypothalamus and carried down axons to the neurohypophysis.
• Too much GH causes abnormal growth of the body, gigantism; too little causes stunted growth, pituitary dwarfism. Excess GH in adults, especially in middle age, causes acromegaly: severe disfigurement, mostly of the face, which can lead to serious complications and early death if unchecked. It is hard to detect early and often goes unnoticed for years until outward features change.
• PRL controls the growth of the mammary glands and milk formation in them. TSH stimulates the thyroid to make and secrete thyroid hormones. ACTH stimulates the adrenal cortex to make and secrete steroid hormones called glucocorticoids.
• LH and FSH stimulate the gonads and are called gonadotrophins. In males, LH stimulates the testis to make and secrete androgens, and FSH together with androgens regulates spermatogenesis.
• In females, LH induces ovulation of fully mature (Graafian) follicles and maintains the corpus luteum, which forms from what is left of the Graafian follicle after ovulation. FSH stimulates the growth and development of the ovarian follicles.
• MSH acts on melanocytes, the melanin-containing cells, and regulates skin pigmentation. Oxytocin makes smooth muscles contract; in females it drives the vigorous contraction of the uterus at childbirth and milk ejection from the mammary gland.
• Vasopressin acts mainly on the kidney: it stimulates the distal tubules to resorb water and electrolytes, reducing water lost in urine (diuresis), so it is also called anti-diuretic hormone (ADH). If ADH synthesis or release is impaired, the kidney cannot conserve water well, causing water loss and dehydration: diabetes insipidus.

4. Pineal and thyroid glands

NCERT §19.2.3–§19.2.4

• The pineal gland lies on the dorsal side of the forebrain and secretes melatonin.
• Melatonin is central to the body's 24-hour (diurnal) rhythm, for example the normal rhythms of the sleep-wake cycle and body temperature. It also influences metabolism, pigmentation, the menstrual cycle and the body's defence capability.
• The thyroid has two lobes, one on either side of the trachea, joined by a thin flap of connective tissue, the isthmus. It is made of follicles and stromal tissue; each follicle is a ring of follicular cells around a cavity.
• Follicular cells make two hormones: tetraiodothyronine or thyroxine (T₄) and triiodothyronine (T₃). Iodine is essential for making them at the normal rate.
• Too little iodine in the diet causes hypothyroidism and an enlarged thyroid, called goitre.
• Hypothyroidism during pregnancy disturbs the development and maturation of the growing baby, causing cretinism: stunted growth, mental retardation, low intelligence quotient, abnormal skin, deaf-mutism and so on. In adult women, hypothyroidism may make the menstrual cycle irregular.
• Cancer of the thyroid or nodules in it can push hormone synthesis and secretion to abnormally high levels: hyperthyroidism, which harms body physiology. Exophthalmic goitre, also called Graves' disease, is a form of hyperthyroidism with an enlarged thyroid, protruding eyeballs, raised basal metabolic rate and weight loss.
• Thyroid hormones regulate the basal metabolic rate, support red blood cell formation, control the metabolism of carbohydrates, proteins and fats, and influence water and electrolyte balance.
• The thyroid also secretes a protein hormone, thyrocalcitonin (TCT), which regulates blood calcium by lowering it.

5. Parathyroid gland and thymus

NCERT §19.2.5–§19.2.6

• Humans have four parathyroid glands on the back of the thyroid, one pair in each of its two lobes.
• They secrete a peptide hormone, parathyroid hormone (PTH). How much PTH is secreted is controlled by the level of calcium ions circulating in the blood.
• PTH raises blood Ca²⁺ in three ways: it stimulates bone resorption (dissolution or demineralisation of bone), it stimulates reabsorption of Ca²⁺ by the renal tubules, and it increases Ca²⁺ absorption from digested food. PTH is therefore a hypercalcemic hormone.
• PTH and TCT together play a major part in keeping the body's calcium in balance.
• The thymus is a lobular gland between the lungs, behind the sternum, on the ventral side of the aorta. It has a major role in developing the immune system.
• It secretes peptide hormones called thymosins, which have a major role in the differentiation of T-lymphocytes, the cells that give cell-mediated immunity. Thymosins also promote antibody production, giving humoral immunity.
• The thymus degenerates in old people, so less thymosin is made and their immune responses become weak.

6. Adrenal gland

NCERT §19.2.7

• There is one pair of adrenal glands, one sitting above each kidney. Each has a central tissue, the adrenal medulla, and an outer tissue, the adrenal cortex.
• The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine), together called catecholamines.
• Any kind of stress or emergency makes the medulla pour them out quickly, which is why they are called the emergency hormones, or the hormones of fight or flight.
• Their effects: more alertness, pupillary dilation, piloerection (raising of hairs) and sweating; a faster heartbeat, stronger heart contraction and faster respiration; breakdown of glycogen, raising blood glucose; and breakdown of lipids and proteins.
• The adrenal cortex has three layers: zona glomerulosa (outer), zona fasciculata (middle) and zona reticularis (inner). Its many hormones are called corticoids.
• Glucocorticoids are corticoids involved in carbohydrate metabolism; cortisol is the main one. Mineralocorticoids regulate the body's water and electrolyte balance; aldosterone is the main one.
• Glucocorticoids stimulate gluconeogenesis, lipolysis and proteolysis and inhibit the cellular uptake and use of amino acids. Cortisol also helps maintain the cardiovascular system and kidney function, produces anti-inflammatory reactions, suppresses the immune response and stimulates RBC production.
• Aldosterone acts mainly on the renal tubules. It makes them take back Na⁺ and water and pass out K⁺ and phosphate ions, and so it helps maintain electrolytes, body fluid volume, osmotic pressure and blood pressure.
• The cortex also secretes small amounts of androgenic steroids, which have a role in the growth of axial, pubic and facial hair at puberty.
• Underproduction of adrenal cortex hormones alters carbohydrate metabolism, causing acute weakness and fatigue: Addison's disease.

7. Pancreas

NCERT §19.2.8

• The pancreas is a composite gland: it is both exocrine and endocrine. Its endocrine part is the Islets of Langerhans.
• A normal human pancreas has about 1 to 2 million islets; together they are only 1 to 2 per cent of its tissue.
• The two main cell types in an islet are α-cells, which secrete glucagon, and β-cells, which secrete insulin.
• Glucagon is a peptide hormone that acts mainly on liver cells (hepatocytes). It stimulates glycogenolysis and gluconeogenesis, both raising blood sugar (hyperglycemia), and it reduces cellular uptake and use of glucose. Glucagon is a hyperglycemic hormone.
• Insulin is a peptide hormone central to glucose homeostasis. Its main targets are hepatocytes and adipocytes (fat cells of adipose tissue), whose uptake and use of glucose it increases, so glucose moves rapidly out of the blood and blood glucose falls (hypoglycemia).
• Insulin also stimulates the conversion of glucose to glycogen (glycogenesis) in its target cells.
• Blood glucose is thus kept steady by insulin and glucagon acting together.
• Prolonged hyperglycemia leads to diabetes mellitus, a complex disorder in which glucose is lost in the urine and harmful ketone bodies form. It results from insulin deficiency and/or insulin resistance, and patients are successfully treated with insulin therapy.

8. Testis and ovary

NCERT §19.2.9–§19.2.10

• A pair of testes lies in the scrotal sac, outside the abdomen. The testis has two jobs: it is the primary male sex organ and an endocrine gland.
• The testis is made of seminiferous tubules and stromal (interstitial) tissue. The Leydig or interstitial cells, in the spaces between the tubules, produce androgens, mainly testosterone.
• Androgens control how the male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra and others) develop, mature and work. They also build muscle, bring on facial and axillary hair, and give aggressiveness and a low-pitched voice.
• Androgens have a major stimulatory role in spermatogenesis. They act on the central neural system to influence male sexual behaviour (libido), and they have anabolic (synthetic) effects on protein and carbohydrate metabolism.
• A pair of ovaries lies in the female abdomen. The ovary is the primary female sex organ, producing one ovum each menstrual cycle, and it also makes two groups of steroid hormones: estrogen and progesterone.
• The ovary is made of ovarian follicles and stromal tissue. Estrogen is made mainly by the growing follicles. After ovulation the ruptured follicle becomes the corpus luteum, which secretes mainly progesterone.
• Estrogens stimulate the growth and activity of the female secondary sex organs, the development of growing ovarian follicles, the appearance of female secondary sex characters (such as a high-pitched voice) and mammary gland development. They also regulate female sexual behaviour.
• Progesterone supports pregnancy. It also acts on the mammary glands, stimulating the formation of alveoli, the sac-like structures that store milk, and milk secretion.

9. Hormones of heart, kidney and gut

NCERT §19.3

• Some tissues that are not endocrine glands also secrete hormones.
• The atrial wall of the heart secretes a peptide hormone, atrial natriuretic factor (ANF), which lowers blood pressure. When blood pressure rises, ANF is released and makes the blood vessels dilate, which brings the pressure down.
• The juxtaglomerular cells of the kidney make a peptide hormone, erythropoietin, which stimulates erythropoiesis, the formation of RBCs.
• Endocrine cells in different parts of the gastrointestinal tract secrete four major peptide hormones: gastrin, secretin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP).
• Gastrin acts on the gastric glands, which then secrete hydrochloric acid and pepsinogen. The exocrine pancreas is the target of secretin, and responds by secreting water and bicarbonate ions.
• CCK acts on both the pancreas and the gall bladder, stimulating secretion of pancreatic enzymes and of bile juice respectively. GIP inhibits gastric secretion and motility.
• Growth factors are hormones made by several other non-endocrine tissues; tissues need them to grow normally and to repair and regenerate.

10. Mechanism of hormone action

NCERT §19.4

• A hormone acts on a target tissue by binding to specific proteins, hormone receptors, which are present only in target tissues.
• Receptors on the cell membrane of target cells are membrane-bound receptors; those inside the cell are intracellular receptors, mostly nuclear receptors found in the nucleus.
• A hormone binding to its receptor forms a hormone-receptor complex. Each receptor is specific to one hormone only.
• Forming the complex sets off biochemical changes in the target tissue; this is how hormones regulate target tissue metabolism and hence physiological functions.
• By chemical nature hormones fall into four groups: (i) peptide, polypeptide and protein hormones, such as insulin, glucagon, pituitary and hypothalamic hormones; (ii) steroids, such as cortisol, testosterone, estradiol and progesterone; (iii) iodothyronines, the thyroid hormones; (iv) amino-acid derivatives, such as epinephrine.
• A hormone that binds a membrane-bound receptor normally stays outside the target cell. The binding produces second messengers (such as cyclic AMP, IP₃ and Ca⁺⁺) inside the cell, and these regulate cellular metabolism.
• Hormones that bind intracellular receptors, such as steroid hormones and iodothyronines, mostly regulate gene expression or chromosome function: the hormone-receptor complex interacts with the genome. The combined biochemical actions produce physiological and developmental effects.

Must-know facts

1. Endocrine glands are ductless; hormones are non-nutrient intercellular messengers made in trace amounts.
2. Hypothalamus: basal part of the diencephalon; GnRH stimulates gonadotrophin release, somatostatin inhibits GH release.
3. Hypothalamic hormones reach the anterior pituitary through a portal circulation; the posterior pituitary is under direct neural control.
4. Pars distalis: GH, PRL, TSH, ACTH, LH, FSH. Pars intermedia: MSH only. Pars nervosa: stores and releases oxytocin and vasopressin, made in the hypothalamus.
5. GH excess in childhood: gigantism; GH deficiency: pituitary dwarfism; GH excess in adults: acromegaly.
6. ADH (vasopressin) acts on the distal tubules; its impairment causes diabetes insipidus.
7. LH: ovulation of Graafian follicles and maintenance of corpus luteum in females; androgen secretion in males.
8. Melatonin from the pineal regulates the 24-hour (diurnal) rhythm.
9. Thyroid: T₄ and T₃ need iodine; iodine deficiency gives goitre; hypothyroidism in pregnancy gives cretinism.
10. Exophthalmic goitre (Graves' disease): hyperthyroidism with protruding eyeballs, raised BMR and weight loss.
11. TCT lowers blood Ca²⁺; PTH raises it (hypercalcemic): bone resorption, renal reabsorption, intestinal absorption.
12. Four parathyroids on the back of the thyroid; thymosins drive T-lymphocyte differentiation.
13. Adrenal cortex from outside in: zona glomerulosa, zona fasciculata, zona reticularis.
14. Cortisol: main glucocorticoid. Aldosterone: main mineralocorticoid (Na⁺ and water in, K⁺ and phosphate out).
15. Adrenaline and noradrenaline (catecholamines) from the adrenal medulla are the fight-or-flight hormones.
16. Addison's disease: underproduction of adrenal cortex hormones.
17. About 1 to 2 million islets, only 1 to 2 per cent of pancreatic tissue; α-cells glucagon (hyperglycemic), β-cells insulin (hypoglycemic).
18. Leydig cells make androgens; growing follicles make estrogen; corpus luteum makes mainly progesterone.
19. ANF from the atrial wall lowers blood pressure; erythropoietin from juxtaglomerular cells stimulates RBC formation.
20. Steroids and iodothyronines use intracellular receptors and act on genes; protein hormones use membrane receptors and second messengers (cAMP, IP₃, Ca⁺⁺).

Common traps

Key terms

Endocrine gland

A ductless gland that releases its secretion, a hormone, without a duct.

Hormone

A non-nutrient chemical made in trace amounts that carries a message between cells.

Neurosecretory cells

Hypothalamic nerve cells, grouped as nuclei, that make and release hormones.

Portal circulation

The vessels that carry hypothalamic hormones to the anterior pituitary.

Adenohypophysis

Pars distalis plus pars intermedia; the glandular part of the pituitary.

Neurohypophysis

Pars nervosa or posterior pituitary; stores and releases oxytocin and vasopressin.

Gonadotrophins

LH and FSH, the pituitary hormones that stimulate the gonads.

Acromegaly

Disfigurement, mostly of the face, from excess GH in adults.

Diabetes insipidus

Water loss and dehydration when ADH synthesis or release is impaired.

Goitre

Enlargement of the thyroid, as in iodine deficiency.

Cretinism

Stunted growth and impaired development of a baby from hypothyroidism in pregnancy.

Hypercalcemic hormone

A hormone that raises blood Ca²⁺, such as PTH.

Thymosins

Thymus peptide hormones that drive T-lymphocyte differentiation and promote antibody production.

Catecholamines

Adrenaline and noradrenaline, the fight-or-flight hormones of the adrenal medulla.

Corticoids

Hormones of the adrenal cortex: glucocorticoids and mineralocorticoids.

Addison's disease

Weakness and fatigue from underproduction of adrenal cortex hormones.

Islets of Langerhans

The endocrine part of the pancreas, with glucagon-making α-cells and insulin-making β-cells.

Glycogenolysis

Breakdown of glycogen to glucose, stimulated by glucagon and catecholamines.

Leydig cells

Interstitial cells between the seminiferous tubules that make androgens.

Corpus luteum

The ruptured follicle after ovulation, which secretes mainly progesterone.

Atrial natriuretic factor

A peptide from the atrial wall that dilates vessels and lowers blood pressure.

Second messenger

A molecule such as cAMP, IP₃ or Ca⁺⁺ made inside the cell when a hormone binds a membrane receptor.