The smallest blood vessels in the human body are capillaries. They are responsible for the absorption of oxygen into the blood stream and for removing the deoxygenated red blood cells for return to the heart and lungs for reoxygenation. To understand capillaries, you need to understand what happens to a red blood cell as it’s being circulated around the body. A red blood cell both brings oxygen to the body’s tissues and carries away the carbon dioxide byproduct from cellular metabolism to ultimately be breathed out as we exhale. First, a deoxygenated red blood cell enters the heart through the superior and inferior vena cava. The deoxygenated blood traveled there by way of capillaries to veins to reach the vena cava. Next, the heart forces these red blood cells through the pulmonary artery into the lungs. When we breathe in, oxygen is attached to the red blood cells. The lungs send this reoxygenated blood back to the heart. The heart via the ascending and descending aorta sends the blood to the rest of the body for use. Capillaries are the site at which oxygen is diffused into the tissues of the body, such as skeletal muscles. All of the cells of all of the tissues in our body need oxygen to perform normal metabolic function. Once they are finished with the oxygen, carbon dioxide is the byproduct which the capillaries take up and funnel into the veins for circulation back to the heart to repeat the cycle. In summary, capillaries are the site of oxygen exchange between the body’s tissues and the blood stream. Capillaries and arteries carry oxygenated red blood cells, and venules and veins carry the deoxygenated blood cells.

The smallest capillaries in the body are in the brain. The brain uses an incredible amount of oxygen and therefore needs many blood vessels to bring in this oxygen and carry away the waste. About one liter of blood flows through the brain every minute, carrying with it nutrients and oxygen to fuel the neurons and support cells. The brain uses 20% of the food energy that we consume. Consider that we have neurons that are over one meter long. These are the ones that send impulses from the brain via the spine to our feet. Imagine how much energy it takes to conduct nerve signals to the ends of those neurons, and you can understand why the brain requires so many resources.