GENERAL PATHOLOGY

Hemodynamics

• In a normal blood vessel like capillary, there are two forces acting on the fluid in the circulation.
• The hydrostatic pressure causes fluid movement from inside the vessel to outside and the colloid osmotic pressure (mostly due to proteins) is responsible for the reverse movement of fluid from outside the vessel to the inside.
• The capillary hydrostatic and osmotic forces are normally balanced so that there is no net loss or gain of fluid across the capillary bed. However, increased hydrostatic pressure or diminished plasma osmotic pressure leads to a net accumulation of extravascular fluid (edema).

In edema, the excessive interstitial fluid can be either an exudate or a transudate.

• A transudate is a fluid with low protein content (most of which is albumin) and a specific gravity of less than 1.012. It is essentially an ultrafiltrate of blood plasma that results from osmotic or hydrostatic imbalance across the vessel wall without an increase in vascular permeability.
• An exudate is an inflammatory extravascular fluid that has a high protein concentration, cellular debris, and a specific gravity above 1.020. It is formed mainly due to alteration in the normal permeability of small blood vessels in the area of injury.