The structural differences between arteries and veins are all based in their relationship to the heart. You see, since arteries receive blood from the heart, the blood they receive is under a lot of pressure. Can you picture how much pressure the arteries near the heart (like the pulmonary trunk and the aorta) have to withstand every time the ventricles squirt out a load of blood? At the same time, this pressure helps the blood move through the arteries-- even when the arteries are running in opposition to gravity (like the common carotid artery running toward the head).
The veins only receive the blood after it has travelled quite far from the heart. The blood pressure in the veins is thus much less; the blood is certainly much less likely to burst through walls of the veins than arteries. Also, because the blood pressure is small in the veins, it is not going to be enough to return all that blood to the heart; in fact, the blood could easily back up or collect in these vessels, right?
Knowing these facts, you should be able to imagine the following needs of the two types of blood vessels:
These are the very conditions that the different structures of arteries and veins reflect.
Arteries have thick walls
Arteries and arterioles have thicker walls than veins and venules, respectively. The increased amount of smooth muscle that is found in arteries gives them extra strength and elasticity to deal with the surges of blood from the heart.
The smallest arterioles cannot have thick walls. But they must also be able to deal with this pressure. So, even the smallest arterioles do have some supportive smooth muscle. This is shown in this figure from your book... here you can see that when smooth muscle cannot be thick enough to even form sheets, it can still enwrap the small arterioles.
Veins have valves and wider inner diameters
Veins have valves within them to prevent backflow of blood. These valves are formed from the endothelium, in places where the endothelial cells bend into the lumen of the vein. These are one-way valves, as shown in this figure from your book. Blood is allowed to go toward the heart, but it cannot fall back the other way. These types of valves are especially important in getting blood back from places like your legs, all the way back to the heart.
You'll notice that veins tend to have wider inner diameters, too. That means that they can hold more blood. I talked above about how blood could potentially accumulate in veins. Well, our bodies use that feature of veins in an interesting way. Since it would be impossible anyway to get all the blood back to the heart as quickly as it left the heart, the veins have to have more blood in them than the arteries. So, the veins have more room inside them for blood. And they really do hold a lot of blood. In fact, the veins provide a nice storage area for our blood. If you look to see where blood is in our bodies at any given time, you'll see that most of it is in our veins. That's what is shown in this graph from your book.
It should make sense to you that not all of our blood is necessary at any given instant. If we get hurt and lose some blood, we're still OK, right? That is only possible if we have extra blood. The "extra" blood is stored in our veins.
You have had experience with the structural differences between arteries and veins...
When a health professional draws blood from you, they draw blood from your veins. You may have known that already, but whether or not you knew that, now you should know why...
I hope this makes sense!
Arterioles and Venules
When an artery branches into smaller and smaller vessels, eventually the blood vessel is too small to see with the naked eye. At that point, it is called an arteriole. Likewise, a venule is a microscopic vein.
Each artery branches into many, smaller arteries. These continue to branch and eventually these branches are arterioles. The arterioles continue to branch, until the tiniest blood vessels, the capillaries, are what are formed. Capillaries fuse into larger vessels, the venules. These continue to fuse together into larger and larger vessels, until they eventually make veins.
The small arterioles and venules do not necessarily have all three tunics. The tunica media can be extremely reduced. This was shown in Figure 15.26, inserted above, in an arteriole. The tunica externa, in these conditions, is also extremely reduced or even absent.
© 2011 STCC Foundation Press