Pathopedia-India.com: Hemodynamic Disorders 1

Water absorbed from stomach and intestine enters blood some of this water is lost in urine and exhaled air and the rest passes to intercellular fluid, and then to intracellular fluid carrying nutrition. Water moves in reverse direction from intracellular fluid to blood carrying waste products and excreted via excretory mechanism (skin, kidney, lung, intestine etc.).

Three compartments are separated from each another by semi-permeable membranes (endothelium and cell membrane), which allow the movements of water and electrolytes but not the colloids (i.e. protein molecules).

Percentage of distribution of water in blood and in tissue cells is almost constant, which is maintained at the expense of intercellular fluid.

Hence intercellular fluid is variable and may be increased, called edema or diminished, called dehydration.

Body fluid in three compartments is in continuous movement.

This is mainly regulated by two opposing forces i.e. hydrostatic pressure of blood, and osmotic pressure of plasma proteins.

Normally, at the arterial end of the capillary bed, the hydrostatic pressure (32 mmHg.) is greater than osmotic pressure of plasma proteins (25 mmHg.), so that the water and electrolytes diffuse out from the vessels in to the interstitial tissue. At the venous end of the capillary bed osmotic pressure (25 mmHg.) is greater than the hydrostatic pressure (12 mmHg.) due to increased concentration by fluid loss and hence the fluid is reabsorbed from the interstitial tissue into the vessels at this end. Some of the interstitial fluid is drained by the lymph vessels in to the veins.

Normal fluid volume is maintained by two organs:

1. Kidneys: act through two ways:

i) Posterior Pituitary by liberating ADH.

When there is increased plasma osmotic pressure, it acts on the osmoreceptor cells in the supraoptic nucleus of the hypothalamus, causing release of ADH into the general circulation. ADH acts on the collecting tubules of kidney causing more absorption of free water.

ii) Adrenal steroid - aldosterone:

Aldosterone causes tubular absorption of Na+ and excretion of K+.

Retention of Na+ raises osmotic pressure of blood plasma, which in turn activates the osmoreceptors to stimulate the secretion of ADH.

2. Heart:

Expansion or contraction of fluid volume in the body, particularly the blood volume, activates the cardiac stress receptor, situated in the wall of the left atrium.

Cardiac stress receptor influences aldosterone secretion and cause the kidney to increase or decrease the excretion of salt and water.