Adrenal Glands: Location, Structure and Types

In this article we will discuss about:- 1. Location and Structure of Adrenal Glands 2. Types of Adrenal Glands.

Location and Structure of Adrenal Glands:

Adrenal Glands are paired structures located on the top of the kidneys. Each adrenal gland has two parts exter­nal adrenal cortex and internal ad­renal medulla. The cortex is sur­rounded by a fibrous capsule. Both adrenal cortex and medulla have dif­ferent embryonic origin, structure and functions.

Types of Adrenal Glands:

(A) Adrenal Cortex:

Origin:

The adrenal cortex is derived from the mesoderm of the embryo. Structure (Fig, 22. 12).

The adrenal cortex is subdivided into three zones:

(i) Zona glomerulosa (zona- belt, glomerul- little ball):

This is the outer zone that lies just below the capsule. It constitutes about 15% of the gland. Its cells are closely packed and arranged in spheri­cal clusters and arched columns which secrete hormones called mineralocorticoids because they affect mineral homeostasis.

(ii) Zona fasciculata (fascicul-little bundle):

This is the middle zone which is widest of the three zones. It constitutes about 50% of the gland. It consists of the cells arranged in long, straight col­umns. The cells of this zone secrete mainly glucocorticoids, which are named because they affect glucose homeostasis.

(iii) Zona reticularis (reticul- network):

This is the inner zone that constitutes about 7% of the gland. The cells are arranged in branching cords which secrete gonado-corticoids (e.g., androgens that have masculinizing effects).

The cells of the zona fasciculata and zona reticulata contain ascorbic acid (vitamin C).

Hormones:

All hormones of adrenal cortex are synthesized from cholesterol. Corticos­teroids (corticoids—hormones of adrenal cortex) are grouped into three categories : min­eralocorticoids, glucocorticoids and gonad corticoids.

(i) Mineralocorticoids:

These hormones are secreted by the cells of zona glomerulosa of adrenal cortex. As the name indicates, they are responsible for the regulation of mineral metabolism. Aldosterone (salt-retaining hormone) is the principal mineralocorticoid (90 to 95%) in humans.

Like all other hormones of the adrenal cortex, aldosterone is a steroid. Its main function is to regulate the sodium content of the body. It is secreted when the sodium level is low.

It acts on the kidneys to cause more sodium to be returned to the blood and more potassium to be excreted. As the sodium concentration in the blood increases, water follows it by osmosis, so the blood volume also increases. Thus the effect of aldosterone is to increase both sodium and water in the blood.

Target cells:

Mineralocorticoids act on the cells of the kidneys.

(ii) Glucocorticoids:

As their name suggests, they affect carbohydrate metabolism, however, they also affect the metabolism of proteins and fats. Glucocorticoids include three main hormones: cortisol (= hydrocortisone), corticosterone and cortisone. Of the three, cortisol is the most abundant (about 95%). It stimulates the liver to synthesize carbohydrates from non-carbohydrates such as amino acids and glycerol.

Thus increases level of glucose in the blood. Cortisol also stimulates the degradation of proteins within cells and amino acids in the blood, therefore, increases level of amino acids in the blood. A third effect of cortisol is to stimulate the break-down of fats in adipose tissue and release fatty acids into the blood. Thus cortisol has anti insulin effect. It also helps in reducing pain. Cortisol is anti-inflammatory.

It retards phagocytic activities of WBCs and thus suppresses ‘inflammation reaction’. This hormone also reduces the number of mast cells, reducing secretion of histamine. This is also an anti-inflammatory effect. Cortisol is also “immunosuppressive”. It suppresses synthesis of antibodies by inhibiting the production of lymphocytes in the lymphoid tissues.

That is why, cortisol is used for treatment of allergy. It is also used in transplantation surgery to suppress the formation of antibodies in the body of recipients so that the latter may accept the transplanted organs. This hormone increases RBC count, but decrease the WBC count of blood.

It also elevates blood pressure. Cortisol has the capacity to cope with stress. When we are under stress our body secretes cortisol that is why this hormone is called “stress hormone”.

Target Cells:

Glucocorticoids act on the cells of the liver.

(iii) Gonad corticoids (Sex-corticoids):

They are also called sex hormones of adrenal glands. Large quantities of male than female sex-corticoids (sex hormones) are produced. These male sex hormones are called androgens which are important in the development of a male foetus.

Although the genetic sex is determined by the chromosomes in a fertilized egg, a male foetus develops normal male characteristics only if the foetal gonads and adrenal glands produce sufficient quantities of androgens.

Therefore, androgens stimulate the de­velopment of male secondary sexual characters like distribution of body hair. Female sex hormones secreted by the adrenal cortex are oestrogens which maintain the development of female secondary sexual characters.

Target cells:

Gonad corticoids act on the cells of gonads (testes and ovaries).

The adrenal cortex is essential for life. Its removal or destruction is fatal unless the hormones produced by it are supplemented artificially.

Disorders of the Adrenal cortex:

(i) Addison’s disease:

This disease is caused by the deficiency of mineralocorticoids and glucocorticoids. It is also caused by the destruction of adrenal cortex in disease such as tuberculosis. Its symptoms include low blood sugar, low plasma Na⁺, high K⁺ plasma, increased urinary Na⁺, nausea, vomiting, diarrhoea and a bronze-like pigmentation of skin. Severe dehydration is also common in the person suffering from this disease.

(ii) Cushing’s Syndrome (Fig. 22.13):

It is caused by excess of cortisol which may be due to a tumour of the adrenal cortex. It is characterised by high blood sugar, appearance of sugar in the urine, rise in plasma Na⁺, fall in plasma K⁺, rise in blood volume, high blood pressure, obesity and wasting of muscles of thighs and pectoral and pelvic girdles.

(iii) Aldosteronism (Conn’s Syndrome):

Excessive production of aldosterone from an adrenal cortical tumour causes this disease. Its symptoms include a high plasma Na⁺, low plasma K⁺, rise in blood volume, high blood pressure and polyurea.

(iv) Adrenal Virilism (Fig. 22.13):

Appearance of male characters in female is called virilism. Excessive production of male sex-corticoids (androgens) produces male secondary sexual characters like beard, moustache, hoarse voice in woman.

(v) Gynaecomastia:

It is the development of enlarged mammary glands (breasts) in the males. It is due to excessive secretion of female sex hormones (oestrogens) in males. Decreased testosterone may also lead to gynaecomastia.

(B) Adrenal Medulla:

Origin:

The adrenal medulla develops from the neuroectoderm of the embryo.

Structure:

The adrenal medulla consists of rounded groups of relatively large and granular cells. These cells are modified postganglionic cells of sympathetic nervous system which have lost normal processes and have acquired a glandular function.

These cells are called chromaffin cells or phaeochromocytes. These cells are connected with the pregan­glionic motor fibres of the sympathetic nervous system. Obviously, the adrenal medulla is simply an extension of the sympathetic nervous system, therefore, these are discussed together as sympatheticoadrenal system.

Hormones:

The medulla of the adrenal glands secretes two hormones: norepinephrine (norad­renaline) and epinephrine (adrenaline). Norepinephrine and epinephrine are derived from tyrosine amino acid.

(i) Norepinephrine (= Noradrenaline):

It regulates the blood pressure under normal condition. It causes constriction of essentially all the blood vessels of the body. It causes increased activity of the heart, inhibition of gastrointestinal tract, dilation of the pupils of the eyes and so forth.

(ii) Epinephrine (= Adrenaline):

It is secreted at the time of emergency. Hence it is also called emergency hormone.

It causes almost the same effects as those caused by norepi­nephrine, but the effects differ in the following respects:

First, epinephrine has a greater effect on cardiac activity than norepinephrine.

Second, epinephrine causes only weak con­striction of the blood vessels of the muscles in comparison with a much stronger constriction that results from norepinephrine.

A third difference between the action of epinephrine and norepinephrine relates to their effects on tissue metabolism. Epinephrine probably has several times as great a metabolic effect as norepinephrine.

Target Cells:

Both adrenaline and noradrenaline acts on the cells of skeletal, cardiac and smooth muscles and blood vessels and fat cells. Because of the role of their hormones, the adrenal glands are also called ‘glands of emergency’.

Sympatheticoadrenal System:

Stimulation of the sympathetic nerves to adrenal me­dulla causes large quantities of epinephrine (adrenaline) and norepinephrine (noradrenaline) to be released into the blood circulation and then these hormones are carried to all the tissues of the body.

Both the hormones (epinephrine and norepinephrine) and sympathetic nervous system act on the same organs and produce similar effects on them (e.g., accelerates heart beat, raises blood pressure, slows peristalsis, etc.).

Since the sympathetic nervous system and the adrenal medulla function as an integrated system, it is called sympatheticoadrenal system. Adrenaline hormone is responsible for “fight or flight response”.