In multicellular organisms, various physiological processes must be carefully coordinated and regulated to maintain proper functioning of the body. This coordination is achieved through two main systems: the nervous system and the endocrine system. While the nervous system works through electrical impulses and provides rapid responses, the endocrine system regulates body functions through chemical messengers called hormones.
Chemical coordination and integration refer to the process by which hormones regulate activities such as growth, metabolism, reproduction, and homeostasis.
Endocrine System
The endocrine system is a network of glands that secrete hormones directly into the bloodstream. These hormones travel through blood to reach specific target organs where they produce their effects.
Unlike exocrine glands, endocrine glands do not have ducts. Therefore, they are also called ductless glands.
Characteristics of Hormones
Hormones possess several important features that make them effective regulators of body functions:
- Hormones are produced in very small quantities, yet they have significant effects on target tissues.
- They are transported through blood from the site of secretion to the site of action.
- Each hormone acts only on specific target cells that have appropriate receptors.
- Hormones regulate processes slowly but their effects are long-lasting compared to nervous control.
Human Endocrine Glands
The major endocrine glands in the human body include:
- Hypothalamus
- Pituitary gland
- Pineal gland
- Thyroid gland
- Parathyroid glands
- Thymus
- Adrenal glands
- Pancreas
- Gonads (testes and ovaries)
Hypothalamus
The hypothalamus is a part of the brain that plays a crucial role in linking the nervous system with the endocrine system. It secretes releasing and inhibiting hormones that regulate the activity of the pituitary gland.
For example, the hypothalamus produces hormones like TRH (Thyrotropin-releasing hormone) and GnRH (Gonadotropin-releasing hormone) which control the secretion of pituitary hormones.
Pituitary Gland (Master Gland)
The pituitary gland is often referred to as the “master gland” because it controls the activity of many other endocrine glands.
It is divided into two main parts:
1. Anterior Pituitary (Adenohypophysis)
This part secretes several important hormones:
- Growth Hormone (GH): It stimulates growth of bones and muscles. Excess secretion can lead to gigantism, while deficiency causes dwarfism.
- Thyroid Stimulating Hormone (TSH): It regulates the activity of the thyroid gland.
- Adrenocorticotropic Hormone (ACTH): It stimulates the adrenal cortex.
- Follicle Stimulating Hormone (FSH): It regulates gamete formation.
- Luteinizing Hormone (LH): It controls reproductive processes such as ovulation and testosterone production.
- Prolactin (PRL): It stimulates milk production in females.
2. Posterior Pituitary (Neurohypophysis)
This part stores and releases hormones produced by the hypothalamus:
- Antidiuretic Hormone (ADH): It regulates water balance by increasing water reabsorption in kidneys.
- Oxytocin: It stimulates uterine contractions during childbirth and milk ejection during lactation.
Pineal Gland
The pineal gland secretes the hormone melatonin, which regulates the biological clock of the body. It controls sleep-wake cycles and is influenced by light exposure.
Thyroid Gland
The thyroid gland is located in the neck and produces hormones such as thyroxine (T4) and triiodothyronine (T3).
These hormones play an important role in regulating metabolism, growth, and development.
Another hormone secreted by the thyroid gland is calcitonin, which helps in lowering blood calcium levels.
Disorders of Thyroid
- Hypothyroidism: Reduced secretion of thyroid hormones leads to conditions such as cretinism (in children) and myxedema (in adults).
- Hyperthyroidism: Excess secretion causes increased metabolism, weight loss, and nervousness.
Parathyroid Glands
The parathyroid glands secrete parathyroid hormone (PTH), which increases blood calcium levels. It acts opposite to calcitonin and is important for maintaining calcium balance in the body.
Thymus Gland
The thymus gland secretes thymosin, which plays a key role in the development of the immune system, particularly in the maturation of T-lymphocytes.
Adrenal Glands
Each adrenal gland consists of two parts:
1. Adrenal Cortex
The adrenal cortex secretes:
- Mineralocorticoids (e.g., aldosterone): They regulate salt and water balance.
- Glucocorticoids (e.g., cortisol): They regulate metabolism and help in stress response.
- Sex hormones: They influence secondary sexual characteristics.
2. Adrenal Medulla
The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline, which are involved in the fight or flight response. These hormones increase heart rate, blood pressure, and energy availability during emergencies.
Pancreas (Islets of Langerhans)
The pancreas functions as both an endocrine and exocrine gland.
Hormones Secreted:
- Insulin: Lowers blood glucose level by promoting uptake and storage.
- Glucagon: Raises blood glucose level by stimulating breakdown of glycogen.
Disorder:
- Diabetes mellitus: Caused due to deficiency or improper functioning of insulin, leading to high blood sugar levels.
Gonads
Testes (Male)
- Secrete testosterone
- Responsible for development of male secondary sexual characters and spermatogenesis
Ovaries (Female)
- Secrete estrogen and progesterone
- Regulate menstrual cycle, pregnancy, and development of female secondary sexual characters
Mechanism of Hormone Action
Hormones exert their effects by binding to specific receptors.
1. Protein Hormones
These hormones bind to receptors on the cell surface and activate secondary messenger systems inside the cell.
2. Steroid Hormones
These hormones pass through the cell membrane and bind to receptors inside the cell, directly influencing gene expression.
Feedback Mechanism
Hormone secretion is regulated by feedback mechanisms.
Negative Feedback
In this mechanism, an increase in hormone level inhibits further secretion. For example, high levels of thyroxine inhibit the secretion of TSH.
Important NEET & CBSE Points
- Pituitary gland is called the master gland because it controls other glands.
- Hormones act in very small quantities but have powerful effects.
- Insulin lowers blood glucose, while glucagon increases it.
- Adrenaline prepares the body for emergency situations.
- Negative feedback regulates hormone secretion.
Quick Revision Summary
- Endocrine system uses hormones for regulation
- Hormones travel through blood
- Pituitary controls other glands
- Thyroid regulates metabolism
- Pancreas controls blood sugar
- Adrenal gland handles stress
Frequently Asked Questions (FAQs)
Q1. What is chemical coordination?
It is the regulation of body functions through hormones.
Q2. Why is the pituitary gland called the master gland?
Because it controls the activity of other endocrine glands.
Q3. What is the role of insulin?
It lowers blood glucose level.
Q4. What is the function of adrenaline?
It prepares the body for emergency situations.
Conclusion
Chemical coordination and integration ensure that different organs of the body work together in a coordinated manner. The endocrine system, through hormones, plays a vital role in maintaining internal balance and regulating long-term processes. This chapter is extremely important for CBSE exams and NEET, especially for understanding physiological regulation and disorders.