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Starling Forces

Starling forces are responsible for the movement of fluid across the permeable membranes of the capillary beds.

  • On the capillary side of the membrane
    • Hydrostatic pressure: acts on the fluid within the capillary and forces it out of the capillary and into the interstitium
    • Osmotic pressure: draws fluid from the interstitium and into the capillary
  • On the interstitial side of the membrane
    • Hydrostatic pressure: acts on the fluid within the interstitium and enter the capillary
    • Osmotic pressure: draws fluid into the interstitium from the capillary

The net movement of fluid between the interstitium and the capillary lumen is a result of an imbalance between these pressures. An increase in hydrostatic pressure causes a net fluid movement out of the capillary and into the surrounding interstitium. Pulmonary edema occurs when there is an accumulation of fluid in the interstitium and/or the alveoli of the lungs. Ascites or pleural effusion is also due to increased hydrostatic pressures.


The lymphatic system is responsible for the retrieval of blood plasma that has been lost by the circulatory system. The lymphatic system is made up of lymph vessels that transport the lymph throughout the body and to the thoracic duct, which will drain back into the circulatory system through the subclavian veins to flow into the cranial vena cava and right atrium.

The lymphatic system acts to prevent the accumulation of excessive interstitial fluid especially in the lung. In the absence of lymphatic obstruction, lymph flow can increase 20 to 50 times to remove excess fluid accumulation. Pulmonary edema, ascites or pleural effusion occurs when the lymphatic system is overwhelmed by the amount of lost blood plasma from the circulatory system.