Edema is the medical term for excessive fluid accumulation within the interstitial space (between cells) or within the cavities of the body. This does not include fluid within the bladder or in the gastrointestinal tract (gut). Edema may be localized, affecting only one organ or area of the body or it can be generalized where it affects many areas or the whole body simultaneously.
Fluid in the body exists within cells (intracellular fluid), within the tissue space between cells (interstitial fluid), within the blood vessels (blood) or lymphatic vessels (lymph or lymphatic fluid). This fluid is not just water but there are also cells, nutrients, electrolytes and waste products existing with water in these areas.
Two forces are responsible for maintaining the fluid in specific areas or ‘pulling’ and ‘pushing’ fluid into other areas. These forces are known as hydrostatic pressure and osmotic pressure. Hydrostatic pressure is the force that pushes fluid from an area of high pressure to low pressure. Osmotic pressure is the force that draws fluid from an area of low electrolyte concentration to one of a higher electrolyte concentration.
Fluid within our blood vessels are at a higher pressure than the fluid in the tissue spaces. This is due to the pumping heart that pushes the blood with force within the vessels. Blood does not just ooze out of the vessels unless the vessel wall becomes permeable and allows it to exit. Fluid within the tissue spaces also have a hydrostatic force but this is usually smaller than the pressure within the vessels so very little fluid flow from the tissues spaces into the vessel due to hydrostatic pressure. Usually fluid from the tissue spaces enters the blood vessel due to a difference in osmotic pressure.
Fluid is pushed away from an area of high hydrostatic pressure but is pulled into an area of high osmotic pressure. Therefore the blood vessels which have a higher hydrostatic and a higher osmotic pressure will push out some fluid into the tissue spaces and draw other fluids from the tissue spaces. This allows for the exchange of nutrients, gases and wastes. The lymphatic system also plays an essential part here as the interstitial fluid is pulled from the tissue spaces into the lymphatic vessels which then empties into the blood vessels.
Anasarca is the medical term for simultaneous swelling throughout the body (generalized edema). The fluid accumulates within the subcutaneous tissue of the skin and gross swelling of the body is evident. There may also be other types of edema occurring simultaneously like ascites, which is fluid accumulation within the abdominal cavity (swollen stomach). Other cavities and organs may also be affected by edema.
There are various diseases that cause edema, some of which result in localized swelling while others can lead to anasarca. This may vary from cardiovascular (heart and blood vessels) to kidney and liver diseases, certain pharmaceutical drugs and different types of autoimmune diseases like SLE (lupus). Anasarca usually indicates severe illness and may often be seen in the end stages of serious chronic conditions.
Anasarca gives the appearance of being swollen or bloated throughout most of the body and should not be mistaken for weight gain. Immediate medical attention is required if anasarca is evident.
Causes of Edema
Edema will occur under these circumstances :
- Increased hydrostatic pressure will push fluid out of the vessels into tissue spaces. This results in edema.
- Reduced osmotic pressure within the vessels will not pull fluid from the tissue spaces into the vessel. The fluid accumulates within the tissue space and results in edema.
- Fluid retention (water retention) where there is excessive fluid within the blood vessel and tissue spaces. If the body is not able to pass out this excess fluid, it will be retained within the tissue spaces thereby resulting in edema.
- Increased vascular permeability is when blood vessel wall allows fluid to pass out of the blood vessel unabated. Fluid from the tissue spaces are not drawn into the blood vessel fast enough and fluid remains in the tissue space thereby resulting in edema.
- Lymphatic obstruction is where the lymph vessels are blocked at some point and the interstitial fluid cannot be drained from the tissue spaces. Fluid accumulates in the tissue space and the result is edema.
There are different medical terms for edema in specific areas or organs of the body.
- Anasarca is the term for severe generalized edema.
- Ascites is the term for excessive fluid accumulation within the peritoneal cavity. This is the area between the lining of the abdomen and organs within the abdominal cavity.
- Pleural effusion is the term for edema in the pleural space between the outer layers of the lung. It is also known as a hydrothorax.
- Pericardial effusion is the term for edema within the pericardial space between the outer layers of the heart. It is also known as a hydropericardium.
- Pulmonary edema is the term for edema within the lungs.
- Cerebral edema is the term for edema within the brain.
- Lymphedema is the excessive fluid accumulation within tissues because the tissue fluid cannot be drained by the lymphatic vessels in that area.
- Hepatic edema is the term for excessive fluid accumulation in tissues due to a liver dysfunction.
- Cardiac edema is the term for excessive fluid accumulation in tissues due to heart failure.
- Renal edema is the excessive fluid accumulation in they body’s tissues due to kidney disease or failure.
Types of Edema
There are two types of edema that can be identified upon examination.
- Pitting edema is the swelling of a body part where an indentation will persist after pressure is applied to the area. This indentation will slowly disappear over time.
- Non-pitting edema is where there is swelling of a body part with no indentation upon applying pressure.
Pitting edema is the more common type of edema.