What causes edema?

Edema is observable swelling from fluid accumulation in body tissues. Basically, the body fluid is divided into intracellular and extracellular compartments. Further the extracellular fluid compartment is divided into plasma or the blood component and interstitial fluid or fluid in the interstitium or space between cells.

Edema occurs mostly in extracellular fluid compartment but it also can occur in intracellular fluid compartment. In extracellular edema, the fluid is collected into the extracellular spaces due to the following reasons:

• abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries(the smallest division of blood vessels)
• faliure of the lympatics to return the fluid from the interstitium back into the blood often called lymphedema. Like in the case of cancer, surgery, certain infection or absence of lymphatic vessels

The most clinical cause of interstitial fluid accumulation is excessive capillary fluid filtration(leakage). The factors that cause excess capillary fluid filtration are:

• Increased capillary pressure which can be caused by kidney failure, high venous pressure or venous constriction like in the case of heart failure and decreased arteriolar resistance like in case of consumption of vasodilator drugs.
• Decreased plasma proteins which can be caused by excessive loss of protein in urine(proteinuria), impaired production of protein like in case of liver diseases(liver cirrhosis)
• Increased capillary permeability caused by inflammation, toxins, bacterial infection, burns, vitamin C deficiency

Likewise, intracellular edema is caused by the swelling of cells by the accumulation of excess fluid in them. To understand this, one must know what is osmosis. Well, osmosis simply mean the movement of water from the place where there are less solute to the place where there are lots of solute through a semi permeable membrane until there is a balance between both sides. So the cause of intracellular edema are:

• Hyponatrimia meaning there is less sodium or more water in extracellular fluid. When there is this condition, the water moves from compartment of low concentration of solute to the compartment with high concentration of solute that is inside the cell causing swelling of the cell. This can cause brain edemas.
• Depressed metabolic system of the tissue and lack of adequate nutrition to the cells. Normally, little amount of sodium from extracellular fluid always leaks into the cell which is pumped back into the extracellular fluid by the help of protein pumps present in cell membrane. Lack of nutrition or depressed metabolism of the cell cause damage to these pumps causing excess sodium inside the cell. This causes osmosis of water into the cell. This can be seen in ischemic body parts.
• Inflammation causes increase in cell membrane permeability allowing more sodium and other ions to diffuse into the cell causing osmosis of water into the cell.

When edema causes in lungs, it is referred to as pulmonary edema, when in abdomen or peritoneal cavity, its called ascites, when in the plural cavity or space around the lungs, its called plural effusion.There are two types of edema. Pitting and non pitting edema. Simply pitting edema is the one which forms a pit when pressed for sometime after applying the pressure on the swollen areas and non pitting edema is the one which doesn't form a pit when pressed.

Treatment of edema can include diuretic therapy. Loop diuretics which affect the Loop of Henle of the kidneys are most useful. Their function is to decrease the amount of water retention in kidneys and producing more urine.