Veins play by different rules than arteries. Take a look at Figure 15G on page 587. That picture is in a clinical application box, and we're not really working through this box for lecture (although it will help you understand some of this stuff better). Anyway, in that picture (I don't have that image readily available) you can see that all the arteries and arterioles have systolic and diastolic pressures. But, by the time the blood makes it through the capillaries, there is no difference in pressure that occurs anymore. That means there is no pulse pressure in venules and veins. And, if you continue to look at this figure, you will notice that by the time blood is in small veins, there isn't even any more blood pressure at all!
After you finish this page, you will go on to read about factors that influence blood pressure and controlling blood pressure. Since veins do not have substantial blood pressure, that material doesn't really relate to veins. Because of this, your book describes them (pp. 584 - 590) before it describes venous blood pressure and blood flow (pages 590 - 592).
Since there is little venous blood pressure, it cannot be the sole force to get blood back to the heart... So how does blood get back? The answer to this question takes up the rest of this page. Here we talk about how veins have alternative methods to move blood...
What extra means are used to get blood back to the heart?
There are four main ways that blood gets propelled back to the heart (your book only talks about three). Important for each of these ways is the fact that veins have valves, to prevent any backflow of blood after it has progressed toward the heart.
Skeletal muscles push the blood
We have voluntary muscles, skeletal muscles, all over our bodies. Whenever we make any kind of movement, these muscles undergo contraction and relaxation cycles. Contracting muscles can push on blood vessels, just like a muscular person in a tight t-shirt can stretch their shirt during muscular contractions. Every time the muscles push against the blood vessels (from the outside... they press against the tunica externa), they propel the blood within the veins. Remember, the walls of veins are thinner and more prone to being squished than the walls of arteries. Veins can get squished rather easily.
Any squirting toward the heart propels the blood on its path. We don't have to worry about any blood that might get propelled the wrong way, because valves prevent that.
Contraction of skeletal muscles is the primary means by which blood returns to the heart.
Changes in pressure within the thoracic cavity (from breathing) helps
Every time you take a breath, the pressure within your thoracic cavity decreases and the pressure within your abdominal cavity increases. Why? (simplified answer:) Because during inspiration, your diaphragm contracts-- that pulls your diaphragm caudally... increasing the size of the thoracic cavity and decreasing the size of the abdominal cavity. I have drawn a cartoon of what happens to try to help you see it better.
When you breath in, since the abdominal pressure increases and the thoracic pressure decreases, blood flows from high to low pressure and heads toward the heart. This might take you a little bit of thinking through to make sure you have it straight.
Therefore, everytime you breath, you propel blood back toward the heart. Pretty good, considering you have to breath anyway!
Veins can vasoconstrict, but they do NOT do this in any sort of rhythmic way. Remember, though, that veins are blood reservoirs. So, if your body generally vasoconstricts its veins, there is suddenly a smaller reservoir, so some of the blood is forced out. This can happen when blood pressure drops, especially if it is dropping due to blood loss. But this is not something that occurs every minute throughout the day.
When gravity can pull in the right direction, it aids the return of blood to the heart. So, blood returning from your head to the heart is assisted by gravity. At the same time, gravity can work against blood flow in veins, like, in the legs. I think this is why your book doesn't include gravity in its list. However, in clinical settings, it is not uncommon to raise someone's legs or head to help blood return to the heart. So that makes it important enough to me to add it to the list!
© 2011 STCC Foundation Press