Fluid Exchange

Capillaries are composed of a single layer of squamos epithelium surrounded by a thin basement membrane. Most capillaries (except those servicing the nervous system) have pores (spaces) between the individual cells that make up the capillary wall. Plasma fluid and small nutrient molecules leave the capillary and enter the interstitial fluid through these pores, in a process called bulk flow. Bulk flow facilitates the efficient transfer of nutrient out of the blood and into the tissues. However, blood cells and plasma proteins, which are too large to fit through the pores, do not filter out of the capillaries by bulk flow.

Together, blood plasma and interstitial fluid make up the extracellular fluid (ECF). Plasma constitutes 20%, while interstitial fluid constitutes 80% of the ECF. The distribution of extracellular fluid between these two compartments is determined by the balance between two opposing forces: hydrostatic pressure and osmotic pressure.

The beating of the heart generates hydrostatic pressure, which, in turn, causes bulk flow of fluid from plasma to interstitial fluid through walls of the capillaries. In other words, the pressure in the system forces plasma to filter out into the interstitial compartment. The composition of the interstitial fluid and the plasma is essentially the same except that plasma also contains plasma proteins not found in the interstitial fluid. Because of the presence of plasma proteins, the plasma has a higher solute concentration than does the interstitial fluid. Consequently, osmotic pressure causes interstitial fluid to be absorbed into the plasma compartment. In other words, the plasma proteins drive the reabsorption of water back into the capillaries via osmosis.

 
 

The magnitudes of filtration and absorption are not equal. The net filtration of fluid out of the capillaries into the interstitial compartment is greater than the net absorption of fluid back into the capillaries. The excess filtered fluid is returned to the blood stream via the lymphatic system. In addition to its roles in digestion and immunity, the lymphatic system functions to return filtered plasma back to the circulatory system. The smallest vessels of the lymphatic system are the lymphatic capillaries (shown in yellow). These porous, blind-ended ducts form a large network of vessels that infiltrate the capillary beds of most organs. Excess interstitial fluid enters the lymphatic capillaries to become lymph fluid.

Lymphatic capillaries converge to form lymph vessels that ultimately return lymph fluid back to the circulatory system via the subclavian vein. The presence of one-way valves in the lymph vessels ensures unidirectional flow of lymph fluid toward the subclavian vein.

If excess fluid cannot be returned to the blood stream then interstitial fluid builds up, leading to swelling of the tissues with fluid, this is called oedema.

Causes of Oedema

1. Reduced concentration of plasma proteins. When the concentration of plasma proteins drops, the osmotic potential of plasma drops, thus less interstitial fluid is absorbed into the capillaries. The rate of filtration, however, remain unchanged. Therefore, the ratio of filtration to absorption increases, leading to a build up of interstitial fluid. Any condition that would lead to a reduction in plasma proteins could potentially cause edema. Examples of conditions that reduce plasma proteins include:

a) Kidney disease can result in the loss of plasma proteins in the urine.
b) Liver disease can decrease the synthesis of plasma proteins.
c) A protein-deficient diet will decrease plasma proteins.
d) Severe burns result in a loss of plasma proteins (albumin) at the burn site

2. Increased capillary permeability. During an inflammatory response, tissue damage leads to the release of histamine from immune cells. Histamine causes an increase in the size of capillary pores. As capillaries become more permeable, the rate of filtration increases.

3. Increase in venous pressure. If venous pressure is increased then blood dams up in the upstream capillary bed, resulting in excess filtration. Examples of this condition include:

a) Left heart failure. The left half of the heart drains blood from the lungs. When the left ventricle fails to adequately pump blood, venous pressure in the lungs increases. This increases in hydrostatic pressure causes an increase in the rate of filtration of fluid out of the capillaries and into the interstitial compartment. As a result, the lungs fill with fluid, a condition called, pulmonary oedema.

b) Standing still. If one stands still for long period of time, then blood will pool in the veins of the legs. This will increase venous pressure and lead to weeping of fluid into the tissues. You can actually feel your feet swell if you stand motionless for a long time.

4. Blocked Lymphatic Vessels. If lymph vessels become blocked, then lymph fluid will not be drained from the affected area and the area will swell. Any condition that causes blockage or removal of lymph vessels can lead to oedema. Examples of this condition include:

a) Filaria round worms are transmitted to humans by some species of mosquitoes. The worms migrate to the lymph vessels and block them. This causes dramatic swelling of the affected area, a condition called elephantiasis.

b) Treatment for breast cancer may include removal of lymph vessels from breast and arms. This is done to limit the metastasis (spread) of cancerous cells to other parts of the body through the lymph. Removal of lymph vessels results in swelling of the affected area.