Introduction
Chemical coordination in humans is regulated by the endocrine system, which releases hormones to control growth, metabolism, reproduction, and homeostasis. Unlike the rapid responses of the nervous system, chemical coordination is slower but longer-lasting. This post explains the major endocrine glands, their hormones, and the integration of body functions.
1. Endocrine Glands and Hormones
Endocrine glands secrete hormones directly into the blood. Major glands include:
a) Pituitary Gland (Master Gland)
- Location: Base of the brain, below hypothalamus.
- Hormones:
- Growth hormone (GH): Stimulates growth of bones and muscles.
- Thyroid-stimulating hormone (TSH): Stimulates thyroid gland.
- Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex.
- Prolactin: Milk production.
- Follicle-stimulating hormone (FSH) & Luteinizing hormone (LH): Reproduction.
b) Thyroid Gland
- Location: Neck, near larynx.
- Hormones:
- Thyroxine (T4) & Triiodothyronine (T3): Regulate metabolism, growth, and development.
- Calcitonin: Lowers blood calcium levels.
c) Parathyroid Glands
- Hormone: Parathormone (PTH) – increases blood calcium.
d) Adrenal Glands
- Location: On top of kidneys.
- Hormones:
- Adrenal cortex: Cortisol (stress, metabolism), Aldosterone (salt balance).
- Adrenal medulla: Adrenaline & Noradrenaline (fight or flight).
e) Pancreas
- Hormones:
- Insulin: Lowers blood glucose.
- Glucagon: Raises blood glucose.
f) Gonads
- Ovaries: Estrogen, Progesterone (female reproduction).
- Testes: Testosterone (male reproduction).
g) Pineal Gland
- Hormone: Melatonin – regulates sleep-wake cycles.
2. Mechanism of Hormone Action
- Hormones bind to specific receptors on target cells.
- They regulate metabolic activities, growth, and homeostasis.
- Types:
- Steroid hormones: Pass through cell membrane (e.g., cortisol, estrogen).
- Peptide hormones: Bind to surface receptors (e.g., insulin, GH).
3. Feedback Mechanisms
- Most hormone secretion is controlled by negative feedback.
- Example: High thyroxine levels inhibit TSH release from pituitary.
- Positive feedback is less common (e.g., oxytocin during childbirth).
4. Differences Between Nervous and Endocrine Systems
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Signal type | Electrical impulses | Chemical hormones |
| Speed | Fast | Slow |
| Duration | Short | Long-lasting |
| Target | Specific cells via neurons | Any cell with receptor |
| Control | Rapid responses | Sustained control of body functions |
5. Disorders of Endocrine System
- Hyperthyroidism (Graves’ disease): Excess thyroxine.
- Hypothyroidism (Goiter): Low thyroxine; iodine deficiency.
- Diabetes Mellitus: Insufficient insulin or resistance to insulin.
- Cushing’s Syndrome: Excess cortisol.
- Gigantism / Dwarfism: Abnormal GH secretion.
MCQs for Exam Preparation
- The master gland of the body is:
- A) Thyroid
- B) Pituitary
- C) Adrenal
- D) Pancreas
- Answer: B) Pituitary
- Which hormone regulates blood calcium levels?
- A) Insulin
- B) Calcitonin & Parathormone
- C) Cortisol
- D) Adrenaline
- Answer: B) Calcitonin & Parathormone
- Insulin is secreted by:
- A) Thyroid
- B) Adrenal cortex
- C) Pancreas
- D) Pituitary
- Answer: C) Pancreas
- Which hormone controls sleep-wake cycles?
- A) GH
- B) Melatonin
- C) Thyroxine
- D) Cortisol
- Answer: B) Melatonin
- Cortisol is secreted by:
- A) Adrenal medulla
- B) Adrenal cortex
- C) Thyroid
- D) Pancreas
- Answer: B) Adrenal cortex
- Which of the following is an example of negative feedback?
- A) Oxytocin during labor
- B) Insulin lowering blood glucose
- C) Thyroxine inhibiting TSH
- D) Adrenaline release during stress
- Answer: C) Thyroxine inhibiting TSH
- Estrogen is secreted by:
- A) Testes
- B) Ovaries
- C) Adrenal gland
- D) Pituitary
- Answer: B) Ovaries
- Which hormone prepares the body for fight or flight response?
- A) Cortisol
- B) Adrenaline
- C) Insulin
- D) GH
- Answer: B) Adrenaline
- Gigantism is caused by excessive secretion of:
- A) Thyroxine
- B) Insulin
- C) Growth Hormone
- D) Cortisol
- Answer: C) Growth Hormone
- Hormones act only on cells that:
- A) Are nearby
- B) Have specific receptors
- C) Are muscles
- D) Are in CNS
- Answer: B) Have specific receptors