Hormones of adrenal gland

There are two adrenal glands in the human body and each one is situated on top of a kidney. Hence they are also called as suprarenal (above kidney) glands.

The adrenal glands synthesize many hormones regulating metabolism and development of secondary sex characteristics. Some of them play a vital role in fluid and electrolyte balance.

Each adrenal gland consists of two parts-cortex and medulla. The cortex is the outer part while medulla is the inner part.

The cortex a can be further differentiated into 3 layers known as zona glomerulosa, zona fasiculata and zona reticularis. The layers appear different under a microscope.

The cortex is involved in the synthesis of steroid hormones. The steroid hormones are classified as glucocorticoids, mineralocorticoids and sex steroids. The steroid hormones are synthesized from cholesterol.

Glucocorticoids: The major glucocorticoid is cortisol also called as hydrocortisone. This is also referred to as the stress hormone as it is released in response to any stress. Blood level of this hormone is the highest in the early morning and lowest around midnight.

The secretion of glucocorticoids is controlled by the ACTH released by the pituitary gland. Cortisol increases blood sugar and blood pressure but decreases immune response. It is used to treat allergy and inflammation.

Mineralocorticoids: The principal mineralocorticoid is aldosterone. It plays a vital role in the maintenance of the levels of minerals (sodium and potassium) in blood.

It increases the re-absorption of sodium from the tubules of the kidneys. Along with sodium water is also re-absorbed. It increases the excretion of potassium from the kidneys.

The secretion of aldosterone is mainly controlled by two factors.

1) Concentration of potassium ions in the fluid surrounding the cells: Any increase in potassium triggers secretion of aldosterone. Increase in the level of potassium (medically known as hyperkalemia) in the fluid surrounding the cells can cause abnormality of heart function and is very dangerous.

2) Decreased blood flow to kidneys: Whenever the blood flow to a kidney is decreased it releases a substance called renin, which acts on a substance called angiotensinogen in blood and converts it into angiotensin 1. The lungs produce an enzyme called angiotensin converting enzyme (ACE) which converts angiotensin 1 to angiotensin 2 which stimulates release of aldosterone.

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The medulla secretes epinephnine (adrenaline) and nor-epinephrine (nor-adrenaline). There are special cells in the medulla called chromaffin cells which take up an amino acid (amino acids are the building blocks of proteins) called tyrosine and convert it into epinephrine and nor- epinephrine .

The secretion of these hormones is stimulated by the nerves supplying the medulla. Any stress (exercise, decrease in the level of blood glucose, trauma etc.) causes release of these hormones.

Physiologic actions: These two hormones cause constriction of blood vessels and increase in heart rate. The force of contraction of the heart muscle is also augmented.

They cause expansion of the small airways inside the lungs.

The production of glucose is increased which is necessary for providing energy for increased metabolic activity.

The pupils dilate due to the action of these hormones.

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