In this video, we're going to complete the comparison table down below that is showing the key differences between arteries and veins. Notice that the left-hand side of the table is all about arteries, whereas the right-hand side of the table is all about veins. Now when it comes to arteries and veins, the direction of blood flow with respect to the heart is critically important. Arteries will always carry blood away from the heart, whereas veins will always carry blood towards the heart. Recall that you can think that the "a" in artery stands for the "a" in away from the heart, and the "v" in vein for the "v" in visiting the heart. Arteries almost always carry oxygenated blood or blood that has relatively high concentrations of oxygen. The only notable exception to this is the pulmonary artery, which carries deoxygenated blood away from the heart and toward the lungs so that it can be reoxygenated. Veins, on the other hand, almost always carry deoxygenated blood, or blood that has relatively low concentrations of oxygen and higher concentrations of carbon dioxide gas. The only notable exception to this is the pulmonary vein, which carries oxygenated blood towards the heart from the lungs.

Because arteries usually carry oxygenated blood, the blood that they carry is a brighter red color, since oxygenated blood is brighter red. Veins, on the other hand, because they usually carry deoxygenated blood, their blood is darker red in color or purple, due to deoxygenated blood having a darker red or purple color. Because arteries carry blood away from the heart and the heart generates very forceful contractions that eject blood out with relatively high pressure, arteries need to have thick walls to withstand that relatively high pressure. They also need to contain significantly higher proportions of both muscle and elastin. Recall that the muscle facilitates the change in diameter via vasoconstriction and vasodilation. Arteries have a greater ability to vasoconstrict and vasodilate. The elastin imparts more elasticity, giving arteries the ability to stretch and also recoil back to its original shape.

Veins, on the other hand, are further away from the forceful contractions of the heart that eject blood out at high pressure. Because of this, their walls are thinner than those of arteries and contain significantly less muscle and elastin, meaning they have less ability to vasoconstrict, vasodilate, and be elastic. Veins lack internal and external elastic lamina, which, as shown in the image on the left-hand side, are highlighted yellow structures looking somewhat like Swiss cheese, made of elastic connective tissue, giving arteries quite a bit of elasticity.

Arteries typically have a more narrow lumen, meaning the size of their lumen is smaller compared to veins, which have significantly larger lumens. This allows veins to hold larger volumes of blood. If you had a foot-long artery and a foot-long vein, the foot-long vein would be able to hold more volume of blood because of the larger lumen. At any given time, most of the blood in our body is found in veins due to their larger lumens. Because arteries have relatively high pressures from the forceful contractions of the heart that ejects blood out with relatively high pressures, these arteries do not need to have valves. The high blood pressure keeps the blood moving in the right direction away from the heart. On the other hand, veins, having relatively lower pressure, may have venous valves in them to prevent the backflow of blood and ensure that blood is always moving towards the heart. These venous valves are especially found in the limbs, such as our arms and legs, where the blood needs to fight against gravity even more so than other areas of our body. You can see the venous valve on the right, which allows blood to flow in the correct direction towards the heart but prevents backflow due to the relatively low pressure.

Last but not least, the idea of pressure again highlights that arteries, which are closer to the forceful contractions of the heart that eject blood with high pressures, are part of a relatively higher pressure system. They have higher blood pressure because they receive blood directly from the heart. On the other hand, veins have relatively lower blood pressure because they are further away from the forceful contractions of the heart that eject blood out, and these veins are delivering the blood back to the heart. These are the key differences between arteries and veins. Moving forward, we'll be able to apply some of these concepts in practice problems. I'll see you all in our next video.