The capillary system and mechanisms of edema formation.

 A.  Normal :   The differential between the hydrostatic and oncotic pressure at the arterial end of the capillary system is responsible for the filtration into the interstitial space of approximately 14 ml of fluid per minute. The fluid is reabsorbed at the venous end at the rate of 12 ml/min. It is also drained through the lymphatic capillaries at a rate of 2 ml/min. Proteins are removed by the lymphatics from the interstitial space.

B. Hydrostatic edema :  If the hydrostatic pressure at the venous end of the capillary system is elevated, reabsorption is decreased. As long as the lymphatics are able to drain the surplus fluid, no edema results. If their capacity is exceeded, however, edema fluid accumulates.

C. Oncotic edema :  Edema fluid also accumulates if reabsorption is diminished by a decrease in the oncotic pressure of the vascular bed, owing to a loss of albumin.

D. Inflammatory and traumatic edema: Edema, either local or systemic, results in the vascular bed becomes leaky following injury to the endothelium.

E. Lymphedema:  Lymphatic obstruction causes the accumulation of interstitial fluid because of insufficient reabsorption and deficient removal of proteins, the latter increasing the oncotic pressure of the fluid in the interstitial space.