The study of Endocrine glands and their functions is called ‘Endocrinology’ the term introduced by PENDE. Vertebrates have been attained its systems in evolution. The systems are Respiratory system, Circulatory system, excretory system, Digestive system, Nervous system, Endocrine system, and some other systems.

In this, there are two integrated systems functioning in Homeostasis in vertebrates including humans. These are the Nervous system and the Endocrine system. In this post, we can discuss the details about the Endocrine system, Endocrine secretions, and their functions.

What is Endocrine Gland and What are Hormones?

(endonG=within; krineinG=to separate or distinguish). Endocrine glands are also called Ductless Glands. A ductless gland producing hormonal secretions that pass directly into the bloodstream (or) Lymph.

Bayliss and Starling (1902) defined an endocrine gland is a ductless gland that synthesizes and then, upon appropriate stimulation, Release into the Blood Stream 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.

Hormones: Endocrine system and Homeostasis in Vertebrates

How many Glands are Present in Human?

The Principle Endocrine Glands in Human being

  1. Head Region (Two Glands): Pineal and Pituitary Gland
  2. Neck Region (Three Glands): Thymus, Thyroid, and Parathyroid
  3. 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: It has specific local effects, where their nomenclature. E.g.: Acetylcholine, Secretin, Cholecystokinin, Serotonin, Histamine, Angiotensin, Bradykinin & Kallidin.

b) General Hormones: These are secreted by specific endocrine glands and are transported in the cause of physiologic actions at 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:

  1. Steroidal Hormones
  2. Peptide Hormones
  3. 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

  • Insulin – Reduce the Blood Glucose levels
  • Glucagon – Rises the Blood Glucose levels
  • Somatostatin

D) Adrenal Glands:

The 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

E) Gonads:

These are sex hormones, which are categorized 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 steroid nucleolus is phenolic in nature and is devoid of the C-19 methyl group.
  • Progesterone is a C-21 steroid produced during the luteal phase of the menstrual cycle and also during pregnancy.

What are the mechanisms involved in Hormonal action?

Before going to read 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


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