Adrenal gland

Adrenal gland

There are two adrenal glands, one situated on the upper pole of each kidney (so called suprarenal glands) enclosed within the renal fascia. They are about 4 cm long and 3 cm thick.

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.

The arterial blood supply to the glands is by branches from the abdominal aorta and renal arteries.

The venous return is by suprarenal veins. The right gland drains into the inferior vena cava and the left into the left renal vein.

Parts of Adrenal Gland-

Adrenal gland is made of two distinct parts

  • Adrenal cortex- Outer portion, constituting 80% of the gland Adrenal cortex is formed by three layers of structure. Each layer is distinct from one another.
  1. Outer zona glomerulosa
  2. Middle zona fasciculata
  3. Inner zona reticularis
  • Adrenal medulla- Central portion, constituting 20% of the gland.

A. Adrenal cortex

The adrenal cortex produces three groups of steroid hormones from cholesterol. They are collectively called adrenocorticoids (corticosteroids, corticoids).

  • Mineralocorticoids
  • Glucocorticoids
  • Sex hormones (androgens).

  1. Mineralocorticoids-

 Aldosterone is the main mineralocorticoid. Its functions are associated with the maintenance of water and electrolyte balance in the body. It stimulates the reabsorption of sodium (Na+) by the renal tubules and excretion of potassium (K+) in the urine.

Mineralocorticoids are

  • Aldosterone
  • 11-deoxycorticosterone.

Source of Secretion-

Mineralocorticoids are secreted by zona glomerulosa of adrenal cortex.

Function-

Ninety percent of mineralocorticoid activity is provided by aldosterone. Aldosterone is very essential for life and it maintains the osmolarity and volume of ECF. It is usually called life-saving hormone Aldosterone has three important functions.

It increases-

  • Reabsorption of sodium from renal tubules
  • Excretion of potassium through renal tubules
  • Secretion of hydrogen into renal tubules.

Actions of aldosterone are-

  • On Sodium Ions- Aldosterone acts on the distal convoluted tubule and the collecting duct and increases the reabsorption of sodium. During hypersecretion of aldosterone, the loss of sodium through urine is only few milligrams per day.
  • On Extracellular Fluid Volume- When sodium ions are reabsorbed from the renal tubules, simultaneously water is also reabsorbed. Water reabsorption is almost equal to sodium reabsorption so, the net result is the increase in ECF volume.
  • On Blood Pressure- Increase in ECF volume and the blood volume finally leads to increase in blood pressure.
  • On Potassium Ions – Aldosterone increases the potassium excretion through the renal tubules. When aldosterone is deficient, the potassium ion concentration in ECF increases leading to hyperkalaemia. Hyperkalaemia results in serious cardiac toxicity, with weak contractions of heart and development of arrhythmia.
  • On Hydrogen Ion Concentration - While increasing the sodium reabsorption from renal tubules, aldosterone causes tubular secretion of hydrogen ions. To some extent, secretion of hydrogen ions is in exchange for sodium ions. It obviously reduces the hydrogen ion concentration in the ECF.
  • On Sweat Glands and Salivary Glands- Aldosterone has almost the similar effect on sweat glands and salivary glands as it shows on renal tubules. Sodium is reabsorbed from sweat glands under the influence of aldosterone, thus the loss of sodium from the body is prevented.
  • On Intestine- Aldosterone increases sodium absorption from the intestine, especially from colon and prevents loss of sodium through faces. Aldosterone deficiency leads to diarrhoea, with loss of sodium and water.

2. Glucocorticoids-

Glucocorticoids act mainly on glucose metabolism. Glucocorticoids are-

  • Cortisol
  • Corticosterone
  • Cortisone.

Sources of Secretion- Glucocorticoids are secreted mainly by zona fasciculata of adrenal cortex. A small quantity of glucocorticoids is also secreted by zona reticularis.

  Daily output and plasma level of glucocorticoids 

Hormone Daily output (µg)
 		 Plasma level (µg/dL) 
Cortisol 10.0  13.9
Corticosterone   3.0  0.4

Glucocorticoids have widespread effects and these include-

  • Gluconeogenesis (formation of new sugar from, for example, protein) and hyperglycaemia (raised blood glucose level)
  • lipolysis (breakdown of triglycerides into fatty acids and glycerol for energy production)
  • Stimulating breakdown of protein, releasing amino acids, which can be used for synthesis of other proteins, e.g. enzymes, or for energy (ATP) production
  • Promoting absorption of sodium and water from renal tubules (a weak mineralocorticoid effect).

In pathological and pharmacological quantities glucocorticoids-

  • Have an anti-inflammatory action
  • Suppress the immune response
  • Suppress the response of tissues to injury
  • Delay wound healing

3. Sex hormones (androgens)-

Adrenal sex hormones are secreted mainly by zona reticularis. Zona fasciculata secretes small quantities of sex hormones. Adrenal cortex secretes mainly the male sex hormones, which are called androgens. But small quantity of estrogen and progesterone are also secreted by adrenal cortex.

Androgens secreted by adrenal cortex

  • Dehydroepiandrosterone
  • Androstenedione
  • Testosterone

B. Adrenal medullary-

Medulla is the inner part of adrenal gland and it forms 20% of the mass of adrenal gland. It is made up of interlacing cords of cells known as chromaffin cells. Chromaffin cells are also called pheochrome cells or chromophil cells.

Adrenal medulla is formed by two types of chromaffin cells

  • Adrenaline-secreting cells (90%)
  • Noradrenaline-secreting cells (10%)

Hormones of Adrenal Medulla-

Adrenal medullary hormones are the amines derived from catechol and so these hormones are called catecholamines.

  • Adrenaline or epinephrine
  • Noradrenaline or norepinephrine
  • Dopamine.

Plasma level of Catecholamines

  • Adrenaline - 3 µg/dL
  • Noradrenaline - 30 µg/dL
  • Dopamine -3.5 µg/dL

Adrenaline and noradrenaline-

They are structurally very similar and this explains their similar effects. Together they potentiate the fight or flight response after initial sympathetic stimulation by:

  • Increasing heart rate
  • Increasing blood pressure
  • Diverting blood to essential organs including the heart, brain and skeletal muscles by dilating their blood vessels and constricting those of less essential organs, such as the skin
  • Increasing metabolic rate
  • Dilating the pupils.
  • Adrenaline causes contraction of arrector pili. It also increases the secretion of sweat.

Dopamine

Dopamine is secreted by adrenal medulla. Type of cells secreting this hormone is not known. Dopamine is also secreted by dopaminergic neurons in some areas of brain, particularly basal ganglia. In brain, this hormone acts as a neurotransmitter. Injected dopamine produces the following effects-

  • Vasoconstriction by releasing norepinephrine
  • Vasodilatation in mesentery
  • Increase in heart rate via beta receptors 
  • Increase in systolic blood pressure. Dopamine does not affect diastolic blood pressure.

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