Expert: John Locke - 5/14/2010

Question
Hi,

I'm sorry for the stupidity of my question in advance! I am just beginning a human biology course and I am studying the circulatory system. I have a diagram where the veins just seem to stop when they get to the end of, for example, the finger even though we were told blood cannot go backwards throught the veins. My question is: if the blood cannot go backwards then how does it get back to the heart if the veins simply stop? Thanks for your help!

Erin

Answer
Thanks for using AllExperts. Your question isn't stupid; I just think that your diagram was somewhat unclear or perhaps you were not understanding it exactly. The direction of blood flow from the heart is to the arteries, then to the capillary beds, then to the veins, and back to the heart. Blood flows only in one direction. The vascular system is continuous: the vessels do not end blindly at any point, but simply connect to the next vessel in the flow stream. Perhaps your diagram showed only the venous half of the circulation, and not the arterial side.

A more complete diagram of the circulatory system would look like this. I've included generic descriptions of the involved vessels, rather than any specific named vessels:

Left atrium of heart-->Left ventricle-->Aorta-->Large muscular arteries-->Small muscular arterioles-->capillary bed-->venules-->small veins-->larger veins-->Superior vena cava/Inferior vena cava-->Right atrium-->Right ventricle-->Lungs-->Left atrium

Veins have a number of mechanisms for preventing backflow. All veins above the level of venules have one-way valves that only allow for flow toward the heart. Veins often lie next to or wrapped around arteries; the pulsations of the artery help promote the return of blood to the heart. In a similar fashion, the large veins of the body lie deep within muscle tissue, where the contraction of the muscle helps return blood to the heart. Finally, it should be noted that there is a pressure gradient between the end of the capillary beds and the heart; blood will flow through the venous system by this gradient alone.