The Veins Always Come Last Right Before The Heart

The ''little circle" of the human circulation takes its final curve through the veins. A vein, also called a vena (VEE-nah) in Latin, is a vessel that returns blood back towards the heart. For this reason, veins come last in sequence within the vascular network, right before the heart.

A patient glance back at Figure 11.4 shows that the veins have the same three tunicas as do the arteries. As you can see, though, veins have wider lumens compared to arteries, but thinner walls, overall. In particular, veins have a thinner tunica media than arteries. This means that they are much less able to vasoconstrict or vasodilate by contraction-relaxation of smooth muscle in their walls. (This also means that veins in a cadaver are much less stiff to the touch than are arteries, and that their lumens are readily collapsible.)

Although their total wall is thinner, veins have a thicker tunica externa (adventitia) than do arteries. This gives the veins a much higher proportion of collagen fibers. In addition, the large veins have an added network of elastic fibers.

Large veins, therefore, function mainly as blood storage depots. Having so many elastic fibers (as well as collagen fibers) in their tunica externa, they are able to expand and store considerable amounts of blood volume. (At any given time, more than 50% of the blood is being slowly carried through the veins.)

The two largest veins in the body can be seen back in Figure 11.2, which revealed the surface anatomy around the heart. The superior vena cava (KAY-vah) and the inferior vena cava - the ''upper and lower cave or hollow veins'' - are shown entering the right-hand side of the heart. The superior vena cava returns venous blood from the area above the heart, while the inferior vena cava returns blood from the entire body below the heart. Being so far from the pumping end of the heart, the two vena cavae (KAY-vigh) have an extremely low blood pressure, and their rate of blood flow is very slow. [Study suggestion: Picture each vena cava as a very wide, dark, slowly-moving river that stores a huge volume of fluid.]

In medium-sized veins of the arms and legs, there are venous valves. Each valve is a thin flap that opens in only one direction - towards the heart. Because veins are such low-pressure vessels, the venous valves help keep the blood from pooling (due to the effects of gravity) in the body extremities.

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