When a person needs more oxygen, these muscle cells relax in the necessary arterioles, allowing their diameter to increase. Tissues requiring less oxygen see the arterioles serving them constrict a bit diminishing their supply of blood. The diameter of these vessels is just sufficient to allow a red blood cell to pass. It is at the capillary level that exchanges occur between cells and blood. The porous nature of capillary walls allows gases and water to be readily exchanged but prevents cells and most proteins from leaving the capillary.
Blood flows from capillaries into venules which eventually empty into larger thin-walled veins.
Venous blood pressure is very low, its having been dissipated by passage through the extensive capillary system. Skeletal muscle pressure against veins forces blood to move. One-way flaps or valves prevent backflow.