Capillaries are the human blood vessels where oxygen and nutrients mainly move from blood into tissues.
These microscopic vessels form networks between arterioles and venules. Their walls are usually only one endothelial cell thick, giving dissolved gases, nutrients, hormones, and waste products a short path for diffusion. Blood flow through capillary beds is slower than in larger vessels, which supports exchange.
Oxygen generally leaves capillary blood and enters cells, while carbon dioxide moves in the opposite direction. Glucose and other nutrients can also cross, and fluid movement is influenced by hydrostatic and osmotic pressures. Not every substance crosses freely; the blood-brain barrier is a specialized example of restricted exchange.
Arteries carry blood away from the heart, and veins return it. Arterioles regulate entry into capillary beds. Oxygen content depends on the circuit: pulmonary capillaries load oxygen, whereas systemic capillaries usually deliver it to tissues.