Edema is defined as the swelling caused by excessive accumulation of fluid in the tissues. It may be generalized or local. Edema that involves the entire body is called generalized edema. Local edema is the one that occurs is specific areas of the body such as abdomen, lungs and extremities like feet, ankles and legs. Accumulation of fluid may be inside or outside the cell.


Edema is classified into two types, depending upon the body fluid compartment where accumulation of excess fluid occurs:

1. Intracellular edema

2. Extracellular edema.


Intracellular edema is the accumulation of fluid inside the cell. It occurs because of three reasons:

1. Malnutrition

2. Poor metabolism

3. Inflammation of the tissues.

1. Edema due to Malnutrition

Malnutrition occurs because of poor intake of food or poor circulatory system, through which the nutritive

substances are supplied. Due to the lack of nutrition, the ionic pumps of the cell membrane are depressed leading to poor exchange of ions. Especially, the sodium ions leaking into the cells cannot be pumped out. Excess intracellular edema.

2. Edema due to Poor Metabolism

Poor metabolism is caused by poor blood supply. Poor blood supply leads to lack of oxygen. It results in poor function of cell membrane and edema, as explained above.

3. Edema due to Inflammation of Tissues

During inflammation of the tissues, usually the permeability of cell membrane increases. This causes

the movement of many ions, including sodium into the cells resulting in endosmosis and intracellular edema.


Extracellular edema is defined as the accumulation of fluid outside the cell.

Causes for extracellular edema

1. Abnormal leakage of fluid from capillaries into interstitial space.

2. Obstruction of lymphatic vessels that prevents fluid return from interstitium to blood.

Conditions which lead to extracellular edema

1. Heart failure.

2. Renal disease.

3. Decreased amount of plasma proteins.

4. Lymphatic obstruction.

5. Increased endothelial permeability.

1. Edema due to Heart Failure

Edema occurs in heart failure because of various reasons such as:

i. Failure of heart to pump blood: Failure of the heart to pump blood from veins to arteries increases

venous pressure and capillary pressure. This leads to increased capillary permeability and leakage of fluid from blood into interstitial fluid, causing extracellular edema.

ii. Fall in blood pressure during heart failure: It decreases the glomerular filtration rate in the

kidneys, resulting in sodium and water retention. So, the volume of blood and body fluid increases.

This in turn increases the capillary hydrostatic pressure. These two factors together increase the accumulation of fluid causing extracellular edema.

iii. Low blood supply to kidneys during heart failure: It increases renin secretion, which in turn

increases aldosterone secretion. Aldosterone increases the reabsorption of sodium and water

from renal tubules into ECF resulting in the development of extracellular edema.

Pulmonary Edema

Pulmonary edema is the accumulation of fluid in pulmonary interstitium. In left heart failure, the blood

is easily pumped into pulmonary circulation by right ventricle. However, the blood cannot return from lungs to left side of the heart because of weakness of this side of the heart. This increases pulmonary vascular pressure leading to leakage of fluid from capillaries into pulmonary interstitium. It causes pulmonary edema which can be life threatening.

2. Edema due to Renal Diseases – Generalized Edema

In renal disease, the kidneys fail to excrete water and electrolytes particularly sodium, leading to retention of water and electrolytes. So, the fluid leaks from blood into interstitial space causing extracellular edema. Initially, the edema develops in the legs, but later it progresses to the entire body (generalized edema).

3. Edema due to Decreased Amount of Plasma Proteins

When the amount of plasma proteins decreases, the colloidal osmotic pressure decreases. Because of this, the permeability of the capillary increases, resulting in increased capillary filtration. So, more amount of water leaks out of the capillary. It accumulates in the tissue spaces resulting in extracellular edema.

Amount of plasma proteins decreases during the conditions like malnutrition, liver diseases, renal

diseases, burns and inflammation.

4. Edema due to Lymphatic Obstruction – Lymphedema

Lymphedema is the edema caused by lymphatic obstruction. It is common in filariasis. During this

disease, the parasitic worms live in the lymphatics and obstruct the drainage of lymph. Accumulation of lymph along with cellular reactions leads to swelling that is very prominent in legs and scrotum. Repeated obstruction of lymphatic drainage in these regions results in fibrosis and development of elephantiasis.


Elephantiasis is a disorder of lymphatic system, characterized by thickening of skin and extreme enlargement of the affected area, most commonly limbs (legs), genitals, certain areas of trunk and parts

of head.

5. Edema due to Increased Endothelial Permeability

The permeability of the capillary endothelium increases in conditions like burns, inflammation, trauma, allergic reactions and immunologic reactions, which lead to oozing out of fluid. This fluid accumulates leading to development of edema.


Interstitial fluid is present in the form of a gel that is almost like a semisolid substance. It is because the

interstitial fluid is not present as fluid but is bound in a proteoglycan meshwork. It does not allow any free

space for the fluid movement except for a diameter of about a few hundredths of a micron. Normal volume of interstitial fluid is 12 L and it exerts a negative pressure of about 3 mm Hg. It applies a slight

suction effect and holds the tissues together. However, in abnormal conditions, where the interstitial fluid volume increases enormously, the pressure becomes positive. Most of the fluid becomes free fluid that is not bound to proteoglycan meshwork. It flows freely through tissue spaces, producing a swelling called edema. This type of edema is known as pitting edema because, when this area is pressed with the finger, displacement of fluid occurs producing a depression or pit. When the finger is removed, the pit remains for few seconds, sometimes as long as one minute, till the fluid flows back into that area. Edema also develops due to swelling of the cells or clotting of interstitial fluid in the presence of fibrinogen.

This is called non-pitting edema because, it is hard and a pit is not formed by pressing.

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