How does the cardiovascular system work?

The cardiovascular system is a complex network of blood vessels that keep our cells supplied with the oxygen and nutrients necessary for everyday functions. Think of the system as a well-organized, speedy delivery company much like FedEx or UPS, with blood vessels as the one-way streets making it all possible.

The process starts with individual homes, or tissue cells, placing orders for the oxygen stored in a large warehouse (the lungs). Red blood cells in the heart—the distribution center of the body—set out to satisfy these orders. The blood cells, like delivery trucks, travel to the nearby warehouse along pulmonary-artery expressways and pick up their oxygen packages for shipment. Upon retrieving their loads from the lungs, the delivery trucks then return to the distribution center via other expressways called pulmonary veins.

From the center, the oxygen-rich blood cells hop on bodily highways, or arteries, to deliver their packages. There are many types of arteries in the human body, but they all serve the same function: to carry blood away from the heart. When they near their destination, the delivery trucks exit the highways and enter capillaries. Capillaries, similar to narrow roads in a neighborhood, allow the trucks to reach homes in need of the oxygen shipments. As a way of signing for the oxygen delivery, tissue cells hand over cellular waste like carbon dioxide to the red blood cells.

Order fulfilled, the blood cells get back on the capillary roads and exit onto a second set of highways called veins, which lead the cells back to the heart. From here, the minute-long cycle begins anew: red blood cells take a trip to the lungs, pick up an oxygen shipment while dropping off their carbon-dioxide signature, travel back to the distribution center, then go off to the tissue cells for delivery.

As is sometimes the case with human-made roads, the blood-vessel streets are not always clear and easily traveled. Along with red blood cells, other substances also travel through our arteries, such as calcium, fat, and, notably, bad cholesterol. When bad cholesterol, commonly known as LDL, enters the arteries, white blood cells race to digest the cholesterol, like police officers rounding up disruptive criminals. The amount of LDL battling white blood cells in the arteries at a particular moment depends on an individual’s diet and exercise habits. Over time, if you don’t keep your poor habits in check, the accumulating mass of cops and criminals start to clog up your highways, their jumbled mess forming plaque.

When plaque builds, it thickens artery walls and creates a bottleneck (a condition known as atherosclerosis), making it more difficult for red blood cells to navigate the once freely flowing arteries. Oxygen shipments slow, UPS becomes USPS, and a slew of health issues can develop, like cardiovascular disease, the leading cause of adult deaths in the United States.

And while I sit here, painfully and anxiously awaiting a shipment of books from the United States Postal Service, I can’t help but worry that my cells are feeling the same way.