Anatomy Physiology

Water is the major compound in the human body.  It constitutes more than 50% of the total body mass.  Its importance stems from the fact that it can solvate many molecules including proteins and electrolytes in addition to many other compounds such as urea.  Nature had designed a special hormone in the body to regulate water level in the human body. 

This hormone is called antidiuretic hormone and is secreted by the posterior pituitary gland.  It regulates the level of water in the body based on two mechanisms.  These are: Low blood level in the circulation in which case the atidiuretic hormone stimulates the kidneys to conserve water.  In addition an osmolality effect will also determine the amount of water that is conserved by the kidneys.  Hyperosmolality induces the kidney to conserve water more while hypoosmolality induces the body to secrete water more.

Another hormone which is less significant than the antidiuretic hormone in regulating water level in the blood is called atrial natriuretic hormone.  This hormone comes into action when there is water excess in the blood or there exists a state of hypertension.  In this case this hormone stimulates the kidney to secrete more water in the urine until the overload is finished.  This hormone is secreted by the cells of the heart.

Another steroid hormone which exerts its effect on water balance in the blood indirectly is called aldosterone.  This hormone is secreted by the adrenal cortex in response to elevated levels of the hormone renin which is secreted by the cells of the kidney.  Aldosterone main function is to stimulate the conservation of sodium ions in the kidney tubules.  As a result water will also flow in the same direction as the sodium ions by an osmotic effect.  Thus causing a state of edema and hypertension. 

The two kidneys are the site where excretion and conservation of water occurs.  Based on a stimulus from the antidiuretic hormone the kidney can conserve water or excrete it.

Diuretics are drugs which are used to induce loss of water in the urine.  It is used mainly in pathologic states such as in hypertension and edema.  Osmotic diuresis works by the effect of osmosis that a compound in the kidney tubules and which is not reabsorbed into the circulation.  Thus exerting an osmotic effect causing diuresis.

An example is the sugar glucose or mannitol.  Glucose is mainly important in diabetes mellitus in which its abundance in the kidney tubules induces or stimulates diuresis of water.  Mannitol is used sometimes in the case of intracranial pressure due to excess in cerebrospinal fluid in which case diuresis help to lower the pressure within the brain by inducing diuresis. 

Carbonic anhydrase is a protein enzyme in which stimulates the formation of water and carbon dioxide from carbonic acid.  Its inhibition is used clinically to alleviate pressure that is caused by increased amount of water in the body.  It is usually used for example in the medical condition of glaucoma in which case it draws water out of the eye balls by osmotic diuresis.

Side effects of using carbonic anhydrase inhibitors is acidosis due to the excessive secretion of bicarbonate ions and the conservation of acid into the circulation.  Another type of drugs that is used to alleviate hypertension by osmotic diuresis is aldosterone antagonists.  In this case the reabsorption of sodium ions that are secreted into the tubules of the kidney at the glomeruli is not performed. 

As a result water starts to to flow in the same direction into the kidney tubules by the effect of osmosis.  Thus causing increased amount of urine that contains much water.  This in turn leads to alleviation of the hypertension and edema.