While the nervous system provides rapid responses to external and internal stimuli, the endocrine system ensures long-term regulation of growth, metabolism, reproduction, and homeostasis. This mode of regulation is known as chemical coordination and integration, and it involves hormones secreted by various glands in the human body. For Class 11 students, this topic holds immense importance, not only for board exams but also for competitive exams like NEET.
The Endocrine System
The endocrine system consists of glands that release hormones directly into the bloodstream, influencing target organs. Unlike exocrine glands, endocrine glands do not have ducts. Hormones act as chemical messengers that control physiology, growth, and behavior.
Hypothalamus and Pituitary Gland
The hypothalamus acts as a link between the nervous and endocrine systems. It regulates the pituitary gland through releasing and inhibiting hormones.
The pituitary gland is called the “master gland” because it regulates the activity of several other endocrine glands. It has three lobes:
- Anterior pituitary: secretes growth hormone (GH), TSH, ACTH, FSH, LH, prolactin.
- Posterior pituitary: releases oxytocin and vasopressin (ADH).
- Intermediate lobe: secretes MSH.
Thyroid Gland
The thyroid gland, located in the neck, secretes thyroxine (T4), triiodothyronine (T3), and calcitonin.
Functions include:
- Regulating metabolism
- Controlling growth and development
- Maintaining calcium balance
Parathyroid Glands
The parathyroid glands secrete parathyroid hormone (PTH), which increases calcium levels in the blood by acting on bones, kidneys, and intestines.
Adrenal Glands
Located above the kidneys, each adrenal gland has two parts:
- Adrenal cortex: secretes cortisol, aldosterone, and androgens.
- Adrenal medulla: secretes adrenaline and noradrenaline, which prepare the body for “fight or flight.”
Pancreas as a Dual Gland
The pancreas functions as both:
- Exocrine gland (secreting digestive enzymes).
- Endocrine gland (Islets of Langerhans secreting insulin and glucagon).
Insulin lowers blood glucose, while glucagon raises it, maintaining glucose homeostasis.
Gonads: Testes and Ovaries
- Testes secrete testosterone, responsible for male secondary sexual characters and sperm production.
- Ovaries secrete estrogen and progesterone, regulating female reproductive cycles and pregnancy.
Pineal Gland
The pineal gland secretes melatonin, which regulates biological rhythms such as sleep-wake cycles.
Hormonal Imbalances
Some common disorders due to hormonal imbalance include:
- Diabetes mellitus – insulin deficiency.
- Goitre – iodine deficiency leading to thyroid enlargement.
- Cushing’s syndrome – excess cortisol.
- Dwarfism/Gigantism – imbalance of growth hormone.
Table: Major Endocrine Glands and Hormones
| Gland | Hormones Secreted | Major Functions |
|---|---|---|
| Hypothalamus | Releasing & inhibiting hormones | Controls pituitary gland |
| Pituitary | GH, TSH, ACTH, FSH, LH, Prolactin, ADH | Growth, metabolism, reproduction |
| Thyroid | T3, T4, Calcitonin | Metabolism, growth, calcium regulation |
| Parathyroid | PTH | Increases blood calcium |
| Adrenal Cortex | Cortisol, Aldosterone, Androgens | Stress response, salt balance |
| Adrenal Medulla | Adrenaline, Noradrenaline | Fight-or-flight response |
| Pancreas | Insulin, Glucagon | Blood glucose regulation |
| Testes | Testosterone | Male reproduction, secondary characters |
| Ovaries | Estrogen, Progesterone | Female reproduction, pregnancy |
| Pineal | Melatonin | Biological rhythms |
Frequently Asked Questions (FAQs)
Q1. What is chemical coordination and integration?
It is the regulation of body functions through hormones secreted by endocrine glands.
Q2. Which gland is known as the “master gland”?
The pituitary gland, because it controls other endocrine glands.
Q3. What is the difference between endocrine and exocrine glands?
Endocrine glands secrete hormones into the blood, while exocrine glands release secretions through ducts.
Q4. Why is insulin important?
Insulin lowers blood glucose levels and prevents diabetes mellitus.
Q5. How do hormones maintain homeostasis?
Hormones regulate metabolism, growth, reproduction, and stress responses, keeping the body in balance.