Chemical Coordination and Integration

Syllabus Unit: Human Physiology

Why it matters

Chapter overview

Alongside the fast, wired signalling of the nervous system, the body runs a slower, broadcast signalling network: ductless endocrine glands releasing hormones into blood to act on distant target tissues. NEET tests the location and secretion of every major endocrine gland, the specific feedback loops regulating thyroid and pancreatic hormones, the hypothalamus-pituitary axis that controls most other glands, and the disorders caused by hormone excess or deficiency.

Biology Syllabus Human Physiology 20%–22% weightage

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Endocrine System

1Hypothalamus & PituitaryMaster regulators

2Thyroid & ParathyroidMetabolism, Ca2+

3Adrenal GlandsStress, salt balance

4Pancreas & GonadsGlucose, reproduction

1Endocrine Glands & Hormones

Ductless glands secrete chemical messengers directly into blood, acting on distant target cells that carry the matching receptor.

Learning Objectives

Define hormone and distinguish endocrine (ductless) glands from exocrine (duct-bearing) glands.
List the major endocrine glands of the human body and their general location.
Explain why a hormone only affects cells bearing its specific receptor.
Distinguish steroid hormones from peptide/protein hormones by general mechanism.

Key Terms

HormoneA chemical messenger secreted directly into blood by an endocrine gland, acting on distant target cells bearing a specific receptor.

Endocrine GlandA ductless gland that releases secretions (hormones) directly into the bloodstream, e.g. thyroid, adrenal, pituitary.

Target CellA cell bearing the specific receptor for a given hormone, allowing it — and only it — to respond to that hormone.

Heterocrine GlandA gland with both endocrine and exocrine function, e.g. the pancreas (insulin/glucagon endocrine; digestive enzymes exocrine).

The **endocrine system** is the body's second major coordination network alongside the nervous system — slower but longer-lasting, using **hormones** released into blood to reach target cells anywhere in the body.

Major endocrine glands and their location

Hypothalamus — base of the brain; the master regulator, controls the pituitary via releasing/inhibiting hormones.
Pituitary — pea-sized gland below the hypothalamus; 'master gland', secretes hormones controlling most other endocrine glands.
Thyroid — front of the neck, either side of the trachea; secretes T3, T4 (metabolism) and calcitonin (Ca2+ regulation).
Parathyroid — four small glands on the back of the thyroid; secrete PTH, raising blood Ca2+.
Adrenal glands — atop each kidney; cortex secretes corticosteroids, medulla secretes adrenaline/noradrenaline.
Pancreas (islets of Langerhans) — a heterocrine gland; endocrine cells secrete insulin and glucagon.
Gonads — testes (testosterone) and ovaries (oestrogen, progesterone); both reproductive and endocrine roles.
Pineal gland — deep in the brain; secretes melatonin, regulating circadian rhythm.
Thymus — behind the sternum; secretes thymosin, important for T-lymphocyte maturation.

Key PointA hormone travels throughout the whole body in blood but affects only cells carrying its matching receptor — this receptor specificity, not location, is what determines a hormone's target.

Two broad chemical classes

Steroid hormones (e.g. cortisol, oestrogen, testosterone) — lipid-soluble, cross the cell membrane, bind intracellular/nuclear receptors, directly affect gene transcription.
Peptide/protein and amine hormones (e.g. insulin, growth hormone, adrenaline) — bind cell-surface receptors, act via second messengers (e.g. cAMP) without entering the cell.