We have seen that O₂ and CO₂ move between capillaries and tissues by diffusion. In addition, water, certain ions, and other small molecules, but not proteins and blood cells, move from capillaries into the surrounding interstitial fluid (the extracellular fluid surrounding vessels) forced by blood pressure. The blood is thus filtered as it passes through the capillary wall.

Why doesn’t the blood plasma lose all its water and ions over time? As water is filtered out of the capillaries, some ions, cells, proteins, and other large compounds remain inside, increasing in concentration (Fig. 39.19). As a result, there is a tendency of water to flow back into the capillaries by osmosis (Chapter 5). At the same time, blood pressure decreases because of the flow resistance imposed by the capillaries. When the pressure is no longer high enough to counter osmosis, water moves back into the capillaries.

In vertebrates, excess interstitial fluid is also returned to the bloodstream by means of the lymphatic system, a network of vessels distributed throughout the body. The fluid that enters the lymphatic system is called lymph. Small lymphatic vessels merge with progressively larger thin-walled vessels, draining the lymph into lymphatic ducts, which empty into the venous system and then the heart. Lymphatic vessels have one-way valves, similar to those in the veins, which assist the return of lymph to the circulatory system. Local muscle contractions also help to pump the lymph toward the thoracic duct, the final lymphatic vessel through which lymph returns to the circulatory system of vertebrates. In addition to its role in returning plasma to the bloodstream, the lymphatic system has important functions in the immune system (Chapter 43).