Pressure is high in large arteries, but decreases as they approach the capillaries. Capillaries are about the size of a RBC, and these pass through single file. Pressure is 35 at arterial end and 15 at venous end. Blood enters from capillaries at 15 mm and pressure drops to zero at the heart. The many veins coalesce into one big one that takes blood back to the heart.
Veins are thin-walled and can expand more than arteries.
The ventricle sends it either to the lungs for more oxygen, or to the organs as a source of oxygen for metabolism. It returns from the lungs to the left atrium, and then goes again to the ventricle. Thus the two types of blood mostly stay in separate streams and each stream goes to the appropriate vessel. Mammals and birds Fig. It then goes to the lungs, where it gets oxygen, and returns from the lungs to the left atrium.