Blood vessels are structures that carry the blood from the heart to all the tissues of the body and then back to the heart. As such, they must be constructed for carrying fluids over long distances, allowing for exchange of materials to and from the blood, and for vasoconstriction/vasodilation to alter blood flow. The walls of the blood vessels allow for all these things, and some more, too.
In lab you were able to look at blood vessels under the microscope, and you identified the major components of a blood vessel: tunica externa, tunica media, and tunica intima. Let's look a little bit more into these three tunics...
As you look at the figure above from your textbook (Figure 15.25 on p. 575), you should be able to distinguish 4 types of tissues that layer over one another to comprise the blood vessel wall. These four types and the tunic in which they lie, from the middle of the vessel to the outside are:
Click here to link to a nice photomicrograph of both a large artery and a large vein from Curtin University of Technology in Perth, Australia.
The endothelium is an epithelial lining that is only one-cell-thick. The cells are also squamous, so this tissue is extremely thin. Therefore, the tunica interna is always very, very thin.
At the outside edge of the vessel, the tunica externa of connective tissue simply provides some support for the blood vessel and attaches it to whatever other tissue it runs through. For example, the tunica externa of blood vessels running through the dermis contains collagenous and elastic fibers that run out from the vessel and into the irregular connective tissue of the dermis; this connects the vessel to the skin. This continuation of connective tissue from the blood vessels into the surrounding area holds the blood vessels in place. But, because the tunica externa is simply a layer of connective tissue, it remains relatively thin.
Since some blood vessels have very thick walls, while others have extremely thin walls, and the tunica interna and tunica externa remain pretty constant in thickness, the difference in blood vessel thickness must mainly depend on the thickness of the tunica media. Right?
The increased thickness of the tunica media in a thick-walled vessel comes from an increase in the amount of smooth muscle. I'm sure you think of smooth muscle mainly as a tissue useful for contraction... like in vasoconstriction. However, smooth muscle in the walls of vessels can also be thought of differently.
Since smooth muscle contains contractile elements that hold its cells at a certain length, when it is not doing an active contraction and shortening, it tends to maintain its length. So, smooth muscle will not stretch out too much, and will work to keep its length. It can stretch a bit, but it will return to its original length afterward. So smooth muscle provides an elastic function. It also will not rupture under pressure, because its stretchiness is limited, so it also provides a strength function for vessels.
Taken all together, the tunica media can provide strength, elasticity, and contractile abilities to the blood vessel wall. A big blood vessel with lots of blood rushing through it will need more strength and elasticity than a smaller blood vessel... that's why the tunica media is the one that is thick in large blood vessels.
This website has images of small arteries and veins. You can go back to the main page to see more.
I have just given you a summary of blood vessels in general. But there are some differences among the types of blood vessels. What types of blood vessels are there?
Arteries (and arterioles) and veins (and venules) look pretty much like I've described blood vessels in general. Yet, there are some differences between arteries and veins, so I have described these differences in a separate web page. Meanwhile, capillaries are the tiniest blood vessels, so they actually lack the tunica media. The interesting properties of capillaries are described on a separate web page.
Meanwhile, the different types of blood vessels are arranged in a very particular order, as written in the bulleted list above. From the heart, blood goes into arteries, then arterioles, capillaries, venules, veins, and then back to the heart. There's no real deviation from this rule, although there are a few exceptions I will try to explain to you later. I have tried to illustrate the order of the blood vessels in this animation.
The only terms that appear in this animation but that I have not explained are metarteriole and precapillary sphincter. These terms will be explained on the capillary page. Also, it may help to imagine that the heart lies to the bottom left of the figure.
© 2011 STCC Foundation Press