Edema

Edema (also spelled oedema, formerly known as dropsy) is swelling due to accumulation of excess fluid in any biological tissue. Edema has many root causes, but the mechanism is simple; fluid is drawn from the blood into the tissues when there is a higher osmotic pressure in the tissues than in the blood.

Osmosis is the diffusion of a solvent through a semipermeable membrane from a region of low solute concentration to a region of high solute concentration. The semipermeable membrane is permeable to the solvent, but not to the solute, resulting in a chemical potential difference across the membrane which drives the diffusion. That is, the solvent flows from the side of the membrane where the solution is weakest to the side where it is strongest, until the solution on both sides of the membrane is the same strength or concentration.

Osmosis is an important topic in biology because it provides the primary means by which water is transported into and out of cells.  Solutes, such as proteins or simple ions, dissolve in a solvent such as water. This raises the concentration of the solute in these areas. The solvent then diffuses to these areas of higher solute concentration to equalize the concentration of the solute throughout the solution. Osmosis can be opposed by increasing the pressure in the region of high solute concentration with respect to that in the low solute concentration region.

Blood normally has a higher osmotic pressure than the tissues due to the contribution of the oncotic pressure:  a portion of its total osmotic pressure  due to the presence of large protein molecules. Because large plasma proteins can't easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will, to some extent, balance out the tendency for fluid to leak out of the capillaries.

Higher osmotic pressure in the tissues may be due to an actual increase (e.g., salt retention due to kidney failure) or it may be a relative increase, in conditions where blood plasma proteins are reduced (e.g., edema due to low serum protein in the blood due to nutritional deficiency).

Obstruction to venous blood flow also results in edema due to the mechanically caused increase in blood pressure in upstream capillaries.

Capillary damage due to infection, bacterial toxins, or other trauma will also allow fluids to move from the blood into tissues, and the exudation of fluid into extracellular spaces is part of the general process of inflammation.

When the tissue space collects an abnormally large amount of fluid through edema, the fascial tissue adapts by increasing the space between the collagen fibers, and the elastin fibers stretch to allow expansion required to accommodate the higher fluid content.  If the edema increases beyond the fascia's ability to adapt by stretching, pressure will mount inside the tissue, causing nerve endings to react by sending pain signals.

Common conditions causing or characterized by edema are congestive heart failure, some renal problems, varicose veins, cirrhosis, malnutrition, hypothyroidism and allergic conditions such as angioneurotic edema.

 

Tags: excess fluid, osmotic pressure, venous blood flow, Blood plasma, congestive heart failure

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