Figure 1 | Blood and lymph vascular systems.   a | The cardiovascular system consists of the heart as well as blood vessels (arteries, veins and capillaries) and lymphatic vessels. Arteries deliver oxygenated blood (red) to the capillaries where bidirectional exchange occurs between blood and tissues. Veins collect deoxygenated blood (blue) from the microvascular bed and carry it back to the heart. Lymphatic vessels (yellow) collect extravasated tissue fluid, filter it through lymph nodes and return it to the circulation through the thoracic and lymphatic ducts and the lymphaticovenous anastomoses (not shown). The lymphatic vascular system is not continuous like the blood vascular system. b | Shows extravasation of tissue fluid from blood to the lymphatic vessels. Within the bloodstream, fluid flows rapidly as a plasma suspension of erythrocytes whereas outside the bloodstream, it flows slowly as a tissue fluid–lymph suspension of immune cells through lymphatics. Blood vessels have a continuous basal lamina (black) with tight interendothelial junctions and they are supported by pericytes and smooth muscle cells. By contrast, lymphatic endothelial cells have a discontinuous basal lamina and have gaps between the lymphatic endothelial cells that can open to the adjacent connective tissue. In oedematous tissue, the endothelial cells are pulled by the anchoring filaments. Bileaflet valves prevent backflow of lymphatic fluid. Fluid flow is represented as large arrows.

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