Physics of Blood Flow in the Body

Blood flow and the circulation of fluids throughout our bodies is a crucial concept for keeping us alive. Although blood flow is a physiological mechanism, the important physics concepts that make it possible cannot be overlooked. When we think about things like velocity of blood flow, the cross-sectional area of where the blood is moving, and why these measurements vary in different parts of the body, we are actually doing physics. For example, in my vertebrate physiology class when we learned about the velocity of blood flow in the circulatory system, I couldn’t help but think about the importance of physics for proper functioning of the human body. One of the key features of the human body is that the velocity of blood flow is lowest in the capillaries. This is because the velocity of flow is important for material exchange efficiency. In other words, there needs to be enough time for essential nutrients and materials to get to the rest of the body during the exchange of fluids at the capillaries. Thus, from a biological standpoint, it makes sense that velocity of blood flow would be slower at the capillaries because this is where the exchange of oxygen and nutrients into the body takes place. Conversely, the arteries need to deliver oxygen-rich blood to the capillaries at a higher velocity of flow in order to make sure nutrients are getting to the capillaries in a timely manner in the first place. This difference can also be explained using physics if we look at how the velocities of blood flow compare to the cross-sectional area they move through. The primary determinant of velocity of blood flow is the total cross sectional area of the "tube" it moves through. Therefore, the large cross sectional area of the capillaries results in a decrease in velocity of blood flow as it moves through the body in order for there to be sufficient time for materials to be exchanged between the blood and the rest of the body. This figure from my physiology textbook summarizes the relationship well:

As we see from these graphs, capillaries and venules have the greatest cross sectional area and the smallest velocity of blood flow, whereas the aorta and arteries have the smallest cross sectional area and greatest velocity of blood flow. This makes sense, as blood flows quickly when first entering the heart in order to get from areas of high pressure to areas of low pressure in the body to deliver essential nutrients to the rest of the body. In fact, by the time blood has moved from the aorta to the capillary beds, the velocity of flow slows to about 0.026 cm/sec, which is one thousand times slower than the velocity of flow in the aorta! Structure and function is a common theme in physiology, and this holds true here as the physics of blood flow is imperative for proper functioning of circulation of fluids in our bodies. 

Sources:

Human Physiology: An Integrated Approach 8th Edition

https://courses.lumenlearning.com/wmopen-biology2/chapter/blood-flow-and-blood-pressure-regulation/