While designing and constructing solutions to problems is at the heart of engineering, in biology, many of the “problems” or challenges in living systems have been solved through evolution. By designing and constructing a circulatory system, you can identify the challenges of moving fluid to and from different parts of the body, identify possible solutions, and develop an understanding of the relationship between structure and function.
Models have inherent limitations, and no model is going to behave identically to a true circulatory system. The power of this activity is in evaluating the model and how design elements link to the actual circulatory system. Below are some common design strategies that can offer some insight into this design challenge.
Modeling the heart
In your model, the pumping device you create represents the human heart. The most common “hearts” people tend to construct are single chambers—either a water bottle with vessels entering either side, or a balloon or glove with vessels entering the openings (click to enlarge an example below).
In both cases, the fluid flows in a back-and forth-motion, and there’s not enough pressure to maintain fluid flow throughout the system. When the heart is squeezed, the fluid moves outward; when it’s released, the fluid moves back into the heart (click to enlarge the diagram below).
This back-and-forth motion would create a significant problem for the body because the blood needs to move in a clear, unidirectional path in order to retrieve new materials and deliver them to the cells and to remove waste products from the cells. Otherwise, the same blood carrying the same materials will keep reaching the same cells.
In a real human heart, there are four one-way valves that prevent the blood pushed out during a contraction from flowing back into the heart after the heart relaxes. The diagram below shows a simplified version of how valves prevent backflow (click to enlarge).
Can you design a new and creative way to prevent backflow in your engineered heart?
Modeling the blood vessels
There are three main types of blood vessels: arteries, veins, and capillaries. Each has a structure that is closely related to its function. Most people will have used tubing to represent blood vessels in their models.
Arteries carry blood from the heart to the lungs, and on to the cells of the body. People often use thick-walled, wide tubing to create the bulk of their model’s circulatory system. Because the arteries carry blood away from the heart, their walls are thick and withstand the high pressure. While the quantity of blood carried by this vessel is great, you may notice that the tubing is not sufficiently flexible, and would not actually be able to come into contact with individual cells.
Veins carry blood from the cells of the body back to the heart or the lungs. Because the pressure in your engineered system is relatively low, you may not have chosen to distinguish between the structures of arteries and veins in your design. In reality, vein walls are thinner than arterial walls; they do not need to withstand the high blood pressure that arteries do.
Sometimes, people who find they have significant backflow in their model’s veins add more pumps to make up for the loss in pressure. Think about the benefits and disadvantages of those extra pumps as you reengineer your model. To prevent a backflow of blood, many of the veins of the body are equipped with one-way valves that work much like those in the heart. See photos below for examples of how you might add this to your model.
You may also have chosen to manually force the blood upwards with your hands. In the human body, skeletal muscles such as those in your legs do some of that work. Because the majority of veins are moving blood from parts of the body that lie below the heart, they’re not only fighting low pressure due to the slowdown of blood in the capillaries, they’re also working against gravity.
You might have used the smallest tubing to reach the extremities, but found that it wasn’t small enough, or flexible enough, to reach all the cells. Capillaries cannot be easily modeled with ordinary materials. Because real capillaries are approximately the width of a human hair, they allow only a single-file line of red blood cells to pass at any given time.
It’s estimated that there are up to 60,000 miles of blood vessels in the human body, most of which are capillaries. Because the sheer number of capillaries and the blood volume they contain is so large, the overall pressure is very low, and the blood moves very slowly. Capillaries surround the cells and allow small molecules such as carbon dioxide, oxygen, water, glucose, and nutrients to be exchanged between the blood and the cells.