Adrenal glands are called the ‘life-saving glands’ or ‘essential endocrine glands’. It is because the absence of adrenocortical hormones causes death within 3 to 15 days and absence of adrenomedullary hormones, drastically decreases the resistance to mental and physical stress.
FUNCTIONAL ANATOMY OF ADRENAL GLANDS
There are two adrenal glands. Each gland is situated on the upper pole of each kidney. Because of the situation, adrenal glands are otherwise called suprarenal glands. Each gland weighs about 4g.
PARTS OF ADRENAL GLAND
Adrenal gland is made of two distinct parts:
- Adrenal cortex: Outer portion, constituting 80% of the gland
- Adrenal medulla: Central portion, constituting 20% of the gland.
These two parts are different from each other in development, structure and functions.
Adrenal medulla develops from the neural crest, which gives origin to sympathetic nervous system. So, its secretions and functions resemble that of sympathetic nervous system.
Adrenal cortex develops from the mesonephros, which give rise to the renal tissues. It secretes entirely a different group of hormones known as corticosteroids.
HISTOLOGY OF ADRENAL CORTEX
Adrenal cortex is formed by three layers of structure. Each layer is distinct from one another.
- Outer zona glomerulosa
- Middle zona fasciculata
- Inner zona reticularis.
HORMONES OF ADRENAL CORTEX
Adrenocortical hormones are steroids in nature, hence the name ‘corticosteroids’. Based on their functions, corticosteroids are classified into three groups:
- Sex hormones.
SYNTHESIS, TRANSPORT AND FATE OF ADRENOCORTICAL HORMONES SYNTHESIS
All adrenocortical hormones are steroid in nature and are synthesized mainly from cholesterol that is absorbed directly from the circulating blood. Small quantity of cholesterol is also synthesized within the cortical cells from acetylcoenzyme A (acetyl-CoA).
A) Mineralocorticoids: Mineralocorticoids are transported in blood by binding with plasma proteins, especially globulins. 50% of these hormones are present in free form.
B)Glucocorticoids: Glucocorticoids are transported by a special plasma protein known as glucocorticoids-binding globulin or transcortin. Ninety four percent of glucocorticoids are transported by this protein, whereas about 6% of them are found free in plasma. Albumin plays a very little role in glucocorticoid transport.