Human Anatomy — Endocrine System
Introduction
The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. In ICSE Class 10 Biology, you study the major endocrine glands, their hormones, functions, disorders, and the feedback mechanism controlling hormone secretion.
Endocrine Glands and Their Hormones
Pituitary Gland (Master Gland)
Located at the base of the brain (attached to the hypothalamus).
| Hormone | Target | Function |
|---|---|---|
| Growth hormone (GH) | Bones, muscles | Body growth |
| Thyroid-stimulating hormone (TSH) | Thyroid | Stimulates thyroxine secretion |
| Adrenocorticotropic hormone (ACTH) | Adrenal cortex | Stimulates cortisol secretion |
| Follicle-stimulating hormone (FSH) | Ovaries/testes | Gamete development |
| Luteinising hormone (LH) | Ovaries/testes | Sex hormone production |
| Prolactin | Mammary glands | Milk production |
| Antidiuretic hormone (ADH) | Kidneys | Water reabsorption |
| Oxytocin | Uterus | Childbirth contractions |
Thyroid Gland
Located in the neck (below the larynx).
| Hormone | Function | Deficiency | Excess |
|---|---|---|---|
| Thyroxine (T₃, T₄) | Regulates metabolism, growth, brain development | Hypothyroidism (cretinism in children, myxoedema in adults) | Hyperthyroidism (Graves' disease, goitre) |
Goitre: Enlargement of the thyroid due to iodine deficiency (iodine is needed to make thyroxine).
Parathyroid Glands
Four small glands embedded behind the thyroid.
| Hormone | Function |
|---|---|
| Parathyroid hormone (PTH) | Regulates calcium and phosphate balance |
Pancreas (Islets of Langerhans)
Located behind the stomach.
| Hormone | Target | Function |
|---|---|---|
| Insulin (β cells) | Liver, muscles, fat cells | Lowers blood glucose (promotes glucose uptake) |
| Glucagon (α cells) | Liver, muscles | Raises blood glucose (promotes glycogen breakdown) |
Diabetes mellitus: Insufficient insulin or insulin resistance → high blood glucose.
Adrenal Glands
Located on top of each kidney.
| Part | Hormone | Function |
|---|---|---|
| Adrenal cortex (outer) | Cortisol | Stress response, metabolism |
| Adrenal cortex | Aldosterone | Salt and water balance |
| Adrenal medulla (inner) | Adrenaline (epinephrine) | 'Fight or flight' response — increases heart rate, breathing, blood sugar |
Reproductive Glands
| Gland | Hormones | Function |
|---|---|---|
| Ovaries (female) | Oestrogen, Progesterone | Female secondary sexual characteristics, menstrual cycle |
| Testes (male) | Testosterone | Male secondary sexual characteristics, sperm production |
Feedback Mechanism
Hormone secretion is controlled by negative feedback.
Example (Thyroid):
- Hypothalamus secretes TRH → Pituitary secretes TSH → Thyroid secretes thyroxine.
- When thyroxine levels rise in blood, it inhibits further TSH and TRH release (negative feedback).
- This maintains thyroxine within a narrow range.
Positive feedback example: Oxytocin during childbirth — contractions stimulate more oxytocin, which strengthens contractions.
Comparison: Endocrine vs Nervous System
| Feature | Endocrine system | Nervous system |
|---|---|---|
| Signal | Hormones (chemical) | Nerve impulses (electrical) |
| Speed | Slow (through blood) | Fast (through nerves) |
| Duration of effect | Long-lasting | Short-lived |
| Mode of transmission | Bloodstream | Neurons |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Calling the pituitary the 'master gland' but forgetting it's controlled by the hypothalamus | The hypothalamus is the true master regulator |
| Confusing insulin and glucagon | Insulin ↓ blood sugar; Glucagon ↑ blood sugar |
| Thinking adrenaline is only from the cortex | Adrenaline is from the MEDULLA, not the cortex |
| Forgetting that iodine is needed for thyroxine | Iodine deficiency → goitre |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: glands and their hormones, functions and disorders, feedback mechanism, comparison with nervous system.
Marks Blueprint: Gland-hormone table — 2 marks, Functions/disorders — 2 marks, Feedback — 1 mark.
Self-Test Questions
-
Name the gland that secretes insulin. What is the function of insulin?
-
What is the role of the thyroid gland? Name two disorders caused by thyroid dysfunction.
-
List four hormones secreted by the pituitary gland and state their functions.
-
Explain the negative feedback mechanism with reference to the thyroid gland.
-
What is the 'fight or flight' response? Which hormone is responsible?
-
Differentiate between the endocrine and nervous systems.
