Edema (swelling) is a common manifestation of many diseases. Edema develops as fluid build-up in tissues. The fluid usually leaks from local blood or lymphatic veins and it is good to remember that not only tissues but even body cavities (ascites, pleural effusion) and organs (pulmonary edema, brain edema) may be affected. Edema may be local (restricted to a certain body part) or overall (more body parts or the whole body)
Before mentioning the causes of edema, it is important to know something more about fluid mechanics and its movement between capillaries and tissues. The capillaries are the tiniest blood vessels that are found in all tissues. In capillaries there take places the exchange of nutrients and waste products between blood and tissue cells. The movement of fluid is influenced by three main factors:
Local blood pressure allows blood fluid (with nutrients) to flow through capillary walls from the blood stream into the tissues.
Blood proteins pressure
Proteins in the bloodstream have their own pressure, which is called oncotic. This oncotic pressure acts against the blood pressure, helps to keep the fluid in the blood vessels and to draw fluid from the tissues back into the bloodstream.
Permeability of the capillary walls
Small capillaries’ walls are permeable enough to allow fluid passage in both directions. Vessel permeability may be further increased by local inflammation processes.
Blood pressure in the beginning of the capillaries is higher than the oncotic pressure and the resulting pressure drives the fluid containing nutrients out of the vessels into the surrounding tissue. At the end of the capillaries the blood pressure drops and oncotic pressure is higher. The resulting pressure force drives liquid containing waste products back into the blood vessels. It should be noted, however, that about 10% of the fluid volume does not return into blood vessels and it is removed via lymphatic vessels.
The mechanism of edema build-up
There are only four basic mechanisms that cause edema formation and there is usually their combination. First mechanism is increased blood pressure in the capillary that is in the whole course higher than oncotic pressure and thus prevent the fluid to return from tissue back into the bloodstream. Second possible mechanism is drop of the oncotic pressure that is related to low level of serum proteins. As in the previous case, the oncotic pressure at the end of the capillary is lower than local blood pressure. Third possibility of edema cause is increased capillary permeability, which allows fluid leakage into tissues. Fourth mechanism is related to lymphatic system disorders that block lymph outflow from tissues.
Heart failure means that heart is no longer able to properly ensure its function as a pump. Blood fluid starts to accumulate in vessels as heart can not handle to pump the needed volume. When left part of the heart fails, the blood builds-up in lung vessels and can cause pulmonary edema. In case of right part of heart, the blood accumulates in big veins leading blood towards the heart. Typical result is swelling of lower extremities.
Allergic reactions cause edema by increasing permeability of blood vessel walls. This process is usually local but severe allergic reactions (anaphylactic shock) can cause an overall edema including dangerous swelling of airways that can lead to suffocation. Fluid massively leaking to tissues is missing in the bloodstream and this situation may be easily followed by circulatory collapse and death.
Inflammation processes (except for allergies)
Any inflammation no matter its cause (i.e. infectious and non-infectious) results in dilation of blood vessels and increased permeability of blood vessel walls. Inflamed body parts are therefore reddish and swollen such as in arthritis (noninfectious inflammation) or festering acne (infectious inflammation).
Cirrhosis causes mainly leakage of fluid into the abdominal cavity and this condition is called ascites. There are two combined factors. Blood flows poorly through cirrhotic liver tissue and accumulates in abdominal veins increasing the local blood pressure. In addition, cirrhotic liver ceases to function properly and does not produce enough protein, thereby decreasing protein concentration in the blood. Malnutrition is another reason of protein deficiency as cirrhotic patients are usually chronic alcoholics with inadequate protein intake in their diet.
This condition is present in malnourished people with inadequate protein intake and in patients with some kidney diseases (such as some glomerulonephritides) when damaged kidneys pass large amounts of protein to urine. If the protein loss to urine is greater than 3 grams per day, we talk about the so-called nephrotic syndrome. The oncotic pressure drops and fluid is increasingly filtered into tissues. Conventional subcutaneous swelling and fluid in the abdominal cavity (ascites) are characteristic images of malnourished children in Africa.
Venous system obstruction
Venous obstruction occurs in deep vein thrombosis. The blood can not leave the limb because of thrombotic mass and accumulates. This increases local blood pressure and causes fluid to filtrate into the tissues. Unlike heart failure, usually only one leg is swollen. The swelling in deep vein thrombosis is accompanied by leg pain and redness. It should be also noted that even non-obstructive venous disorders can cause limb swelling (chronic venous insufficiency).