Edema is increased fluid in the interstitial tissue spaces or it is a fluid accumulation in the body cavities in excessive amount. Depending on the site, fluid accumulation in body cavities can be variously designated as:
a) Hydrothorax – fluid accumulation in pleural cavity in a pathologic amount. b) Hydropericardium – pathologic amount of fluid accumulated in the pericardial cavity. c) Hydroperitoncum (ascites) – fluid accumulation in peritoneal cavity. d) Ancsarca – is a severe and generalized edema of the body with profound subcutaneous swelling.
Mechanism of edema formation:
Approximately 60% of the lean body weight is water, two-thirds of which is intracellular with the remainder in the extracellular compartment. The capillary endothelium acts as a semipermeable membrane and highly permeable to water and to almost all solutes in plasma with an exception of proteins. Proteins in plasma and interstitial fluid are especially important in controlling plasma and interstitial fluid volume. Normally, any outflow of fluid into the interstitium from the arteriolar end of the microcirculation is nearly balanced by inflow at the venular end. Therefore, normally, there is very little fluid in the interstitium.
Edema formation is determined by the following factors: 1) Hydrostatic pressure 2) Oncotic pressure 3) Vascular permeability 4) Lymphatic channels 5) Sodium and water retention
1) Hydrostatic and oncotic pressures:
The passage of fluid across the wall of small blood vessels is determined by the balance between hydrostatic and oncotic pressures. There are four primary forces that determine fluid movement across the capillary membrane. Each of them can be listed under the above two basic categories, the hydrostatic pressure and the oncotic pressure. These four primary forces are known as Starling forces & they are: a. The capillary hydrostatic pressure (Pc) This pressure tends to force fluid outward from the intravascular space through the capillary membrane to the interstitium.
b. The interstial fluid hydrostatic pressure (Pif)
This pressure tends to force fluid from the interstitial space to the intravascular space.
c. The plasma colloid osmotic (oncotic) pressure (Пp)
This pressure tends to cause osmosis of fluid inward through the capillary membrane from the interstitium. The plasma oncotic pressure is caused by the presence of plasma proteins.
d. The interstial fluid colloid osmotic (oncotic) pressure (Пif)
This pressure tends to cause osmosis of fluid outward through the capillary membrane to the interstitium. In addition, some fluid is normally drained by the lymphatic channels. Usually, excess fluid will accumulate in the interstitium (i.e. edema is formed) when the capillary hydrostatic pressure is increased or when the plasma oncotic pressure is decreased or when the lymphatic drainage is blocked.