Endocrine System
1. What is the Endocrine System?
The endocrine system is a system of DUCTLESS GLANDS that secrete HORMONES directly into the BLOODSTREAM.
Hormones are chemical messengers that are PRODUCED in one part of the body and TRAVEL through the blood to target organs, where they REGULATE specific functions.
'The word 'hormone' comes from the Greek meaning 'to SET IN MOTION.' Hormones are SLOW-acting but have LONG-LASTING effects compared to the nervous system.'
2. Endocrine vs Exocrine Glands
| Feature | Endocrine Glands | Exocrine Glands |
|---|---|---|
| Ducts | NO ducts (DUCTLESS) | Have DUCTS |
| Secretion | Secreted into BLOOD | Secreted through DUCTS |
| Examples | Pituitary, thyroid, adrenal | Sweat glands, salivary glands, pancreas (EXOCRINE part) |
3. Major Endocrine Glands and Their Hormones
Pituitary Gland
- Location: Base of the BRAIN
- Called the 'MASTER GLAND' because it controls OTHER endocrine glands
- Important hormones:
- Growth Hormone (GH): Controls BODY growth
- Thyroid Stimulating Hormone (TSH): Stimulates THYROID
- Adrenocorticotropic Hormone (ACTH): Stimulates ADRENAL glands
Thyroid Gland
- Location: In the NECK (below the larynx)
- Hormone: THYROXINE (contains IODINE)
- Functions:
- Regulates METABOLISM (rate of energy release)
- Controls GROWTH and development
- Affects BRAIN development
- Disorders:
- HYPOTHYROIDISM (low thyroxine) → weight gain, lethargy, GOITRE
- HYPERTHYROIDISM (high thyroxine) → weight loss, hyperactivity
Pancreas
- Location: Near the STOMACH
- Endocrine cells: ISLETS OF LANGERHANS
- Hormones:
- INSULIN: LOWERS blood sugar (promotes glucose uptake by cells)
- GLUCAGON: RAISES blood sugar
- Disorder: DIABETES MELLITUS (insufficient insulin or insulin resistance)
Adrenal Glands
- Location: On TOP of each KIDNEY
- Hormone: ADRENALINE (epinephrine)
- Functions — 'Fight or Flight' response:
- Increases HEART RATE
- Increases BREATHING RATE
- DIVERTS blood to muscles
- Raises BLOOD SUGAR
- Dilates PUPILS
Gonads (Reproductive Glands)
| Gland | Male (Testes) | Female (Ovaries) |
|---|---|---|
| Hormone | TESTOSTERONE | OESTROGEN, PROGESTERONE |
| Function | Sperm production, male characteristics | Egg development, menstrual cycle, female characteristics |
4. Feedback Mechanism
The endocrine system uses FEEDBACK MECHANISMS to maintain HOMEOSTASIS.
Negative Feedback (MOST COMMON)
When hormone levels are HIGH → signal to REDUCE production. When hormone levels are LOW → signal to INCREASE production.
Example — Thyroid regulation:
- Hypothalamus releases TRH → Pituitary releases TSH → Thyroid releases THYROXINE
- If thyroxine is HIGH → Pituitary REDUCES TSH → Thyroid reduces thyroxine
- If thyroxine is LOW → Pituitary INCREASES TSH → Thyroid increases thyroxine
5. Hormonal Imbalance and Conditions
| Condition | Cause | Symptoms |
|---|---|---|
| Diabetes mellitus | Low insulin or insulin resistance | High blood sugar, frequent urination, thirst |
| Goitre | IODINE deficiency → low thyroxine → TSH increases → thyroid ENLARGES | Swelling in neck |
| Dwarfism | LOW growth hormone (childhood) | SHORT stature (normal proportions) |
| Gigantism | EXCESS growth hormone (childhood) | VERY TALL stature |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'All glands in the body are endocrine glands' | Exocrine glands (sweat, salivary) have DUCTS. Endocrine glands are DUCTLESS |
| 'The pancreas is only an endocrine gland' | The pancreas is BOTH endocrine (Islets of Langerhans → insulin) and EXOCRINE (digestive enzymes) |
| 'Adrenaline is only produced during danger' | Adrenaline is also produced during EXCITEMENT, stress, exercise — any situation needing heightened alertness |
| 'Hormones work instantly like nerves' | Hormones are SLOWER but have LONGER-LASTING effects compared to nerve impulses |
ICSE Exam Focus (5–7 marks)
- 2-mark questions: Define hormones, name endocrine glands
- 3-mark questions: Functions of specific hormones (insulin, thyroxine, adrenaline)
- 4-mark questions: Feedback mechanism — thyroid regulation
- 6-mark questions: Diabetes — causes, symptoms, and management
Self-Test
Q1. Name the master gland of the endocrine system. Why is it called so? A1. The PITUITARY gland. It is called the master gland because it SECRETES hormones that CONTROL the functioning of other endocrine glands (thyroid, adrenals, gonads).
Q2. What is the function of insulin? What happens when insulin is deficient? A2. Insulin LOWERS blood sugar by promoting glucose uptake by cells. Insulin deficiency causes DIABETES MELLITUS — high blood sugar, frequent urination, and thirst.
Q3. Describe the 'fight or flight' response. Which hormone is responsible? A3. ADRENALINE prepares the body for emergency action. Effects: increased heart rate, faster breathing, more blood to muscles, higher blood sugar.
Q4. What is negative feedback in the endocrine system? A4. Negative feedback is a mechanism where high levels of a hormone INHIBIT further release, and low levels STIMULATE release. This maintains BALANCE (homeostasis).
Q5. Why does iodine deficiency cause goitre? A5. Iodine is needed to make THYROXINE. With insufficient iodine, the thyroid cannot produce enough thyroxine. The pituitary responds by releasing MORE TSH, which makes the thyroid GLAND ENLARGE (goitre).
Q6. Name the hormones produced by the gonads and state their function. A6. Testes produce TESTOSTERONE (sperm production, male secondary sexual characteristics). Ovaries produce OESTROGEN and PROGESTERONE (egg development, menstrual cycle, female characteristics).
