The study of endocrine glands and their functions is called “endocrinology,” a term introduced by Pende. Vertebrates’ systems evolved through natural selection. The systems are the respiratory system, circulatory system, excretory system, digestive system, nervous system, endocrine system, and some others.
There are two integrated systems in vertebrates, including humans, that function in homeostasis. These are the nervous system and the endocrine system. In this post, we can discuss the details of the endocrine system, endocrine secretions, and their functions.
Table of Contents
What is Endocrine Gland and What are Hormones?
(endonG = within; krineinG = to distinguish or separate) Endocrine glands are also called ductless glands. A ductless gland secretes hormonal substances directly into the bloodstream (or) lymph.
Bayliss and Starling (1902) defined an endocrine gland as a ductless gland that synthesises and then, upon appropriate stimulation, releases into the bloodstream a chemical agent or hormone.
The Hormones or the chemical messenger is carried by the blood throughout the body of the animal to target cells that possess specific receptor sites for hormones.
How many Glands are Present in Human?
The Principle Endocrine Glands in Human being
- Head Region (Two Glands): Pineal and Pituitary Gland
- Neck Region (Three Glands): Thymus, Thyroid, and Parathyroid
- Abdominal Region (Four Glands): Pancreas, Gastrointestinal mucosa, Adrenals, and Gonads
How to Classify the Endocrine Gland secretions (Hormone)?
A) Based on their site on the action, the hormones are of two types:
a) Local Hormones: According to their nomenclature, it has specific local effects. E.g.: Acetylcholine, Secretin, Cholecystokinin, Serotonin, Histamine, Angiotensin, Bradykinin & Kallidin.
b) General Hormones: These are secreted by specific endocrine glands and are transported by the cause of physiologic actions to points remote from their place of origin.
- Some general hormones affect almost all cells of the body. E.g.: GH, Thyroid hormones
- Some general hormones affect specific tissues far more than other tissues. E.g.: Adrenocorticotropin, Ovarian hormone
B) A classification of vertebrate hormones, based on their chemical composition:
- Steroidal Hormones
- Peptide Hormones
- Amino acid derivatives
Endocrine Glands, Secretions, and their functions
A) Pituitary Gland:
The pituitary gland is made of 3 lobes namely,
1. Anterior pituitary lobe (largest of all) [70%] –
- Growth Hormone (GH)
- Adrenocorticotrophic hormone (ACTH)
- Luteinizing Hormone (LH)
- Follicular Stimulating Hormone (FSH)
- Thyroid-stimulating Hormone (TSH)
2. Middle lobe (5%): Melanocyte stimulating Hormone (MSH)
3. Posterior pituitary lobe (25%):
- Vasopressin and
- Oxytocin (or) Ocytocin
B) Thyroid Gland:
T3 and T4 molecules (Thyroxine) are the thyroidal gland secretions.
C) Pancreatic Hormones:
Pancreas secrets Three hormones. They are
D) Adrenal Glands:
The adrenal gland has Two-part. They are
- Adrenal Cortex Region: Secrets Mineralocorticoids and Glucocorticoids
- Adrenal Medullary Region: Secrets Epinephrine (or) Adrenalin and Nor-Epinephrine (or) Nor-Adrenalin
The gonads gland secrete six different sex hormones, which are classified into three groups.
- Androgens or male sex hormones which are C-19 steroids.
- Estrogens or female sex hormones which are C-18 steroids. Ring A of the steroid nucleolus is phenolic in nature and is devoid of the C-19 methyl group.
- Progesterone is a C-21 steroid that is made during pregnancy and the luteal phase of a woman’s menstrual cycle.
What are the mechanisms involved in Hormonal action?
Before reading this, read my old notes on hormonal action. Based on the mechanism of action, the hormones may be classified into two types:
- Hormones with cell surface receptors (or) Peptide Hormonal Mechanism
- Hormones with Intra-cellular receptors (or) Steroidal Hormonal Mechanism