vessel

The body has two major vessel systems: blood vessels and lymphatic vessels. Blood vessels include arteries (carry oxygenated blood away from the heart), veins (return deoxygenated blood to the heart), and capillaries (microscopic exchange vessels). Lymphatic vessels collect interstitial fluid, immune cells, and dietary fats from tissues and return them to the venous circulation via the thoracic duct and right lymphatic duct.

Vessel walls are adapted to their function. Arteries have thick walls with extensive smooth muscle and elastic tissue to withstand high pressure. Veins have thinner walls with valves to prevent backflow in low-pressure venous circulation. Capillaries have walls only one cell thick (endothelium) to allow gas and nutrient exchange. Lymphatic capillaries have overlapping endothelial cells that act as one-way valves to draw in interstitial fluid.

The endothelium is the single layer of flat cells lining the interior (lumen) of all blood and lymphatic vessels. Far from being a passive barrier, endothelial cells regulate vascular tone, control substance exchange, prevent clotting under normal conditions (via prostacyclin and nitric oxide), and initiate inflammation by expressing adhesion molecules. Endothelial dysfunction is a key early event in atherosclerosis and many cardiovascular diseases.