Revision: Class 11 >> Chemical Coordination and Integration NEET (UG) Chemical Coordination and Integration

The endocrine system is defined as a group of organs performing an overall common function. The endocrine glands secrete certain chemicals called hormones.

Hormone is a molecule that is created and secreted in very minute quantities into the blood stream by an endocrine gland or a specialized nerve cell that governs the growth or functioning of a specific tissue organ in a distant area of the body, e.g., insulin

A group of glands and glandular cells that secrete hormones to regulate and coordinate various functions in the body through chemical control.

Myxoedema is a condition that affects an adult if his thyroid does not function properly; in this condition, the person becomes sluggish with swelling of the face and hands.

• Glands that discharge secretions into ducts are known as exocrine glands.
• Sebaceous glands in the skin, salivary glands in the buccal cavity, etc. are examples of exocrine glands.

• Endocrine system controls body activities through chemical messengers called hormones, which are released directly into the bloodstream (no ducts).
• Hormones act as intercellular messengers — they inhibit, stimulate, or modify specific body processes like growth and development, and act only on specific target organs.
• Types of Hormones — Steroid (aldosterone, sex corticoids); Amine (thyroxine, adrenaline); Peptide (ADH, GnRH); Protein (TSH, FSH); Fatty acid derivatives (prostaglandins).
• Endocrine glands are ductless glands — they secrete hormones directly into the blood. Examples: thyroid, pituitary, pancreas, hypothalamus, adrenal glands.
• All endocrine glands work in a coordinated manner, activating each other to bring about overall chemical coordination in the body.

• Controls Pituitary — Hypothalamus regulates the anterior pituitary via Releasing (RH) and Inhibiting (IH) hormones.
• Oxytocin — Stored in posterior pituitary; causes uterus contractions during childbirth and milk ejection.
• ADH (Vasopressin) — Stored in the posterior pituitary; promotes water reabsorption in the kidneys, reducing urine loss.
• GnRH — Stimulates the pituitary to release gonadotropins (reproductive hormones).
• Somatostatin — Inhibits the release of growth hormone from the pituitary.

• Pineal gland is a small endocrine gland located on the roof of diencephalon, between the two cerebral hemispheres, on the dorsal side of the forebrain.
• It consists of neuroglia and secretory cells called pinealocytes.
• It secretes Melatonin — an amine hormone derived from tryptophan, also called the sleep hormone.
• Melatonin regulates the body's biological clock (circadian rhythm) — the 24-hour diurnal rhythm, including the sleep-wake cycle and body temperature.
• It also influences pigmentation, metabolism, defense capability, and reproductive/menstrual cycle.

1. The thyroid gland is a butterfly-shaped gland located in the neck and secretes thyroxine and calcitonin.

2. Thyroxine controls basal metabolic rate, influences growth, mental development, bone formation, and body temperature.

3. Hypothyroidism may cause:

• Simple goitre (due to iodine deficiency),
• Cretinism (dwarfism and mental retardation in children),
• Myxoedema (sluggishness and swelling in adults).

4. Hyperthyroidism leads to exophthalmic goitre with symptoms like protruding eyes, neck swelling, high metabolism, rapid heartbeat, and breathlessness.

5. Use of iodised salt helps prevent thyroid-related issues caused by iodine deficiency.

• Four glands on posterior surface of thyroid; secrete peptide hormone Parathormone (PTH) / Collip's hormone.
• PTH Function — Increases blood Ca²⁺ by bone resorption, renal Ca²⁺ reabsorption, and intestinal Ca²⁺ absorption.
• PTH and Calcitonin (TCT) are antagonistic — together maintain calcium-phosphate balance. PTH is under feedback control of blood Ca²⁺.
• Hyposecretion → low Ca²⁺ → muscle twitch/spasm = Hypocalcaemic Tetany.
• Hypersecretion → excess bone demineralisation → softening/fracture = Osteoporosis (common in post-menopausal women).

• Located in the upper thorax, dorsal side of the heart; soft, pinkish, bilobed mass of lymphoid tissue.
• Prominent at birth but gradually atrophies in adults — hence called a temporary gland.
• Secretes hormone Thymosin, which plays a key role in the development of the immune system.
• Thymosin promotes differentiation and maturation of T-lymphocytes → provides cell-mediated immunity.
• Also promotes production of antibodies → provides humoral immunity.

• Pancreas functions both as an exocrine gland (secreting pancreatic juice into the duodenum) and an endocrine gland (secreting hormones into the blood).
• The Islets of Langerhans contain beta cells (secrete insulin) and alpha cells (secrete glucagon), which regulate blood sugar levels.
• Insulin lowers blood glucose by promoting glucose uptake by cells and storing excess glucose as glycogen in the liver and muscles.
• Under-secretion of insulin causes diabetes mellitus (hyperglycemia), with symptoms like excessive urination, thirst, weakness, and sugar in urine.
• Glucagon raises blood sugar by converting glycogen to glucose in the liver; overdose of insulin can lead to hypoglycemia or insulin shock.

• Hormones bind to specific receptors in target tissues only, forming a hormone-receptor complex → causes biochemical changes.
• Two receptor types — Membrane-bound (cell surface) and Intracellular (inside cell/nucleus).
• Membrane Receptor — Water-soluble hormones bind surface receptor → adenylate cyclase → ATP converts to cAMP (2nd messenger) → enzyme activation. Other 2nd messengers: Ca²⁺, cGMP, IP₃.
• Intracellular Receptor — Lipid-soluble hormones cross membrane → bind cytoplasm receptor → complex enters nucleus → activates DNA → mRNA → protein synthesis.
• Membrane receptor = faster, short-lived; Intracellular receptor = slower, long-lasting.