The leakage of plasma from the capillaries of the cardiovascular system creates interstitial fluid which bathes the surrounding cells, supplying them with nutrients and oxygen and collecting carbon dioxide and other waste products. It has been estimated that the total plasma volume of the human body (approximately three liters) leaks from the blood circulation every nine hours, and while some interstitial fluid is reabsorbed directly by the blood vessels, the great majority of this fluid is transported back to systemic circulation through the lymphatic system (Levick and Michel 2010). Once the interstitial fluid enters the lymphatic system, it is given the name lymph.
Unlike the blood, the lymph does not have a pump like the heart to help keep it circulating, although the presence of one-way valves within the lymph vessels do prevent back flow of lymph. The intrinsic driving force, known as the active pump, is created by sections of the lymphatic vessel acting like the ventricles of the heart. Our precise understanding of this process is somewhat limited, although it is believed that coordinated contractions of these sections are initiated by the pacemaker activity of smooth muscle cells (Gashev 2002) and that the contractions spread from one section to the next like a wave that causes contraction along both the length and width of the vessels (Margaris and Black 2012). The contractions can be compared to the rhythmical contraction of the digestive system, known as peristalsis.
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