Water is forced out of the blood through very tiny blood vessels,
known as capillaries, which, in this case, takes water into the
tissues. The capillaries act as an exchange system that connects
the smallest veins with the smallest arteries. Veins carry blood to
the heart whereas arteries carry blood away from the heart and to
Water also goes from the tissues to the blood. This happens
because certain proteins in the blood attract water away from the
tissues. When this happens, the water goes from the tissues,
through the capillaries, to the smallest veins, and into the circulating
If all goes right, the body is balanced between the correct amount of water in the tissues and the correct
amount of water in the blood. The kidneys (two organs located on each side of the spine, behind the
stomach) also help keep the water level balanced. The kidneys do this by passing extra salt out of the
blood and into the pee so it can leave the body. Too much salt causes the body to retain (keep in) water.
If a part of the water balancing process goes wrong, however, edema results.
Now to some of the specific causes of edema, one of which is heart failure. Heart failure is a disease that
affects the heart's ability to bring blood to the body's tissues. When heart failure occurs, blood will get
clogged up in the veins. Remember that veins are blood vessels that carry blood to the heart. When the
blood gets clogged up in the veins, it makes it more difficult for blood to get through the veins to the heart.
Pressure in the veins now goes in the opposite direction, causing blood to back up into the capillaries.
This causes backwards pressure in the capillaries.
This backwards pressure in the capillaries forces an abnormally high level of fluid to go into tissues at
different places in the body. It is important to note that the backwards pressure does not mean that blood
is no longer going through the veins to heart, but only that less of it is going in this forward direction. This
backwards pressure can also be caused by a tumor pressing on the veins. In this case, however, the
edema only occurs in the area that is drained by the blocked vein. Thrombophlebitis, which is an inflamed
vein accompanied by a blood clot (a thick mass of blood), can also cause edema.
Another cause of edema is a problem with the kidneys known as nephrotic syndrome. The job of the
kidneys is to filter (remove) wastes from the blood. In nephrotic syndrome, the filtering parts of the kidney
are damaged, which causes a severe loss of protein from the blood into the pee. Remember that normally
protein in the blood attracts water away from the tissues. The loss of protein in the blood reduces the
amount of pressure in the blood. This loss of pressure prevents enough fluid from being drawn from the
tissues into the blood. The same problem with a loss of protein can occur in alcoholics because these
individuals often do not get enough protein in their diet due to too much alcohol use. Too little protein in
the blood can also occur in burn injuries because the burns cause blood loss.
Edema can also be caused by cirrhosis of the liver (a type of disease that destroys the liver). The liver is
the largest organ in the body and is responsible for filtering (removing) harmful chemical substances,
producing important chemicals for the body, and other important functions. The damage caused by
cirrhosis of the liver leads the blood to be clogged in the veins of the liver. The clogged up veins leads to
low amounts of protein in the blood, which leads to low pressure in the blood, which leads to too much salt
staying in the body, which leads to too much water building up in the tissues.
Failure of the kidneys to work properly can cause edema in another way. The kidneys normally filter salt
out of the body. If the kidneys are damaged, they do not filter enough salt out of the tissues. Thus, too
much salt builds up in the body. Since salt attracts water, too much salt in the tissues will lead to too much
water in the tissues. Edema can be caused by injuries because of damage that occurs to the capillaries
(see beginning of this section). The damage to the capillaries allows fluid to leak out of them.
Other situations that can cause edema include bleeding, wounds that drain, very bad nutrition,
inflammation and allergic reactions, long-term diarrhea, pressure from casts, wearing tight bandages or
stockings, and administering too many fluids to a patient outside of the digestive system. Fistulas can also
cause edema. Fistulas are abnormal passages between two internal organs or between an internal organ
and a body surface. Blockages in lymphatic vessels (tube shaped structures that carry lymph) can cause
edema. Lymphatic vessels drain lymph from all over the body back into the blood. Lymph is a milky fluid
that contains proteins, fats, and white blood cells (which help the body fight off diseases).
Edema can also be caused by certain medications that act on the kidneys and cause too much salt to
build up. As we have seen in previous examples, too much salt can lead to edema because salt attracts
water. Examples of medications that can cause edema in this way are corticosteroids, androgen drugs,
and high-estrogen birth control pills taken by mouth. Corticosteroids are a group of drugs that act similarly
to a natural chemical in the body known as corticosteroid hormone. Corticosteroid hormones control the
body's use of nutrients and the amount of water and salts in the urine (pee). Hormones are types of
chemicals in the body that affect other cells. Androgen is a type of hormone that promotes the growth of
some male sexual characteristics whereas estrogen is a type of hormone that promotes the growth of
some physical female sexual characteristics. Antidiuretic hormones, which are medications that cause the
kidneys to retain more water, can also cause edema.
WHAT ARE THE SIGNS AND SYMPTOMS OF EDEMA?
Until the extra fluid in the body increases by more than approximately 15%, the only sign of edema may be
weight gain. After a 15% increase in extra fluid, swelling indicates the presence of edema. Edema is
usually present in the lower part of the body, such as the legs, ankles, and lower back because the force
of gravity pulls the liquid downward. Sometimes, if you press your finger against a part of the body
affected by edema, it will make an indentation in the skin that will flatten out as the fluid returns back to
that area. When this happens, it is known as pitting edema. Due to pressure caused by edema, pain may
In cases of severe edema, the fluid will build up in large spaces in the body. For example, in a condition
known as pleural effusion, fluid fills up in a space that contains the lungs. The lungs are two organs in the
body that help people breathe. Fluid buildup in the space that contains the lungs makes it difficult to
breathe because the lungs get pushed in by the fluid. Difficulty breathing can also be caused by pulmonary
edema, in which too much water enters the lung tissue and the air sacs in the lung. In a condition known
as ascites, fluid builds up in the space between layers that line the belly. Build up of fluid in this area will
cause the belly to look swollen.
HOW IS EDEMA TREATED?
As you can tell from the previous section, edema has many causes. To treat edema, one needs to treat
the condition that is causing it. In many situations, however, the condition that is causing the edema
cannot be fixed immediately, if at all. In such cases, the only treatment is to make the body excrete
(release as waste) the extra water and/or salt by increasing the amount of pee produced by the kidneys.
This is done with medications known as diuretics and by decreasing the amount of salt in the diet. Special
diuretics are usually used that do not get rid of potassium while getting rid of the extra fluids in the body.
These drugs are known as potassium-sparing diuretics. The reason that these medications spare
potassium is that it is a very important element for bodily functioning.
Parts of the body that are affected with edma should be protected from possible injury, continued
pressure, and extreme temperatures (too hot or too cold). If an arm or leg has edema that is caused by
abnormally slow blood flow through a vein, the blood flow can be increased by lifting up the arm or leg and
applying an elastic stocking around the leg or an elastic sleeve around the arm.
WHAT IS THE ORIGIN OF THE TERM, "EDEMA"?
Edema comes from the Greek word "oidema" meaning "a swelling."