Blood flow refers to the volume of blood flowing through a vessel or group of vessels during a specific time. It is measured in milliliters per minute (mL/min). Blood flow during rest averages three to four mL/minute per 100 grams of muscle tissue, but may increase to 80 mL/minute or more during exercise. The distance blood flows during a specific time period is its velocity. Blood velocity is measured as centimeters/second (cm/s). In general, blood velocity is greatest in larger vessels and decreases in vessels with a smaller diameter. The velocity of blood in the aorta is approximately 30 cm/s, in arterioles 1.5 cm/s, in capillaries 0.04 cm/s, in venules 0.5 cm/s, and in the venae cavae 8 cm/s.