The Adrenal Glands - Anatomy

The adrenal glands are also known as the suprarenal glands, as they lie on superior aspect of the kidneys, in the retroperitoneal space. The glands are enclosed in a connective tissue capsule, with the whole encapsulated structure surrounded by renal fascia.

Each adrenal gland is divided into two distinct regions:

• Adrenal Cortex
• Adrenal Medulla

The adrenal cortex constitutes the outer region of the adrenal gland (i.e. is peripherally located) and also makes up 80-90% of the gland tissue itself. 

In contrast, the adrenal medulla is located centrally and is small in comparison to the cortex.

As mentioned previously, the outer adrenal cortex is encapsulated by a layer of connective tissue.

Adrenal Cortex

The outer adrenal cortex is made up of 3 functionally and histologically distinct zones:

(Outermost - Innermost)

• Zona Glomerulosa    - Mineralocorticoids
• Zona Fasciculata       - Glucocorticoids
• Zona Reticularis        - Gonadocorticoids (Androgens)

The zona glomerulosa is the outer most region of the adrenal cortex, and is responsible for the release of mineralocorticoids, the primary one being Aldosterone. Broadly speaking, aldosterone is responsible for blood pressure and electrolyte balance regulation.

The middle layer is known as the Zona Fasciculata, and is the site of glucocorticoid production and release. In humans, the primary glucocorticoid circulating in the blood is Cortisol, and functions to increase blood sugar levels and suppress the immune system.

Gonadocorticoids, or Androgens, are produced in the innermost region of the adrenal cortex, known as the Zona Reticularis. The predominant androgen released in this region of the cortex is Dehydroepiandrosterone (DHEA), and is responsible for the development of secondary sexual characteristics.

Adrenal Medulla

Centrally located, the adrenal medulla is composed of special cells called Chromaffin cells (also known as Phaeochromocytes), which function to produce the Catecholamines Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine). In short, catecholamines prepare the body for the 'fight-or-flight' response, by mediating an increase in heart rate, respiratory rate, metabolic rate and inducing blood vessel constriction.

(Physiology of the Adrenal Hormones will be discussed later)

Summary

The adrenal glands lie on top of the kidneys and are encapsulated by a layer of connective tissue.

Grossly speaking, the adrenal glands are made up of an outer cortex, and an inner medulla.

The outer cortex is made up of 3 parts:

• Zona Glomerulosa - Mineralocorticoids (Aldosterone)
• Zona Fasciculata - Glucocorticoids (Cortisol)
• Zona Reticularis - Androgens (Dehydroepiandrosterone, DHEA)

The inner medulla is made up of Chromaffin cells, which release catecholamines:

• Adrenaline (Epinephrine)
• Noradrenaline (Norepinephrine)