CARDIAC CYCLE II
The characteristic "lub-dup" heart sounds are caused by the closing of the valves during the cardiac cycle. The "lub" sound is the closing of the atrioventricular valves at the beginning of ventricular systole. The "dup" sound is the closing of the pulmonary and aortic valves at the beginning of ventricular diastole.

Both blood flow velocity and blood pressure decrease as the blood travels through smaller and smaller arteries. For example, blood travels about 1,000 times faster in the aorta (30 cm/s) than in the capillaries (0.03 cm/s). Thus, the red blood cells remain in the capillary long enough for O₂ and CO₂ to diffuse across the capillary wall. Similarly, systolic blood pressure in the aorta and large arteries is about 110 mm Hg, but only about 20 mm Hg in the capillaries. This great reduction in velocity and pressure occurs because the sum of the cross-sectional areas of the millions of capillaries is almost 5,000 times that of the aorta.