Your body is constantly moving, even when you are sitting still. One of the busiest journeys happening inside you involves tiny red blood cells traveling through an enormous network of blood vessels. A single red blood cell can travel through the heart, lungs, arteries, and smaller blood vessels as it helps deliver oxygen to your tissues. It also plays a role in carrying carbon dioxide away from tissues so your body can get rid of it. So, what does this microscopic journey actually look like?

It Starts in the Bone Marrow

Before a red blood cell can travel through your bloodstream, it has to be made.  Red blood cells are produced mainly in the bone marrow, the soft tissue found inside certain bones.As a developing blood cell matures, it becomes specialized for transporting gases. Mature red blood cells are packed with hemoglobin, a protein that gives blood its familiar red color and allows the cells to carry oxygen. Once they are ready, these cells enter the bloodstream and begin their journey.

The Red Blood Cell Heads Toward the Heart

After entering circulation, a red blood cell eventually makes its way toward the heart. Blood returning from much of the body has already delivered oxygen to tissues, so it contains less oxygen and more carbon dioxide. This blood enters the right side of the heart. The heart then pumps it toward the lungs, where the red blood cell gets an important opportunity to reload with oxygen.

The Heart Sends It Around the Body

Oxygen-rich blood returns to the left side of the heart. From there, the heart pumps it into the body’s largest artery, the aorta. The blood then travels through progressively smaller arteries and blood vessels. This network reaches practically every part of your body, from your brain and muscles to your skin and internal organs. The heart’s pumping action keeps the blood moving continuously.

The Red Blood Cell Reaches Tiny Capillaries

Eventually, the red blood cell reaches capillaries. These are extremely small blood vessels that form connections between the smallest arteries and veins. Capillary walls are thin enough to allow substances to move between the blood and surrounding tissues. Oxygen carried by the red blood cell can move into nearby cells, where it can be used to support energy production. This is one of the most important parts of the entire journey because your cells depend on a steady supply of oxygen.

The Lungs Give It a Fresh Supply of Oxygen

The lungs are one of the most important stops on the journey. When you breathe in, oxygen enters tiny air sacs called alveoli. These air sacs are surrounded by very small blood vessels. Oxygen moves from the air in the alveoli into the blood, where it attaches to hemoglobin inside red blood cells. At the same time, carbon dioxide moves from the blood into the lungs so it can be removed when you breathe out. The red blood cell is now carrying a fresh supply of oxygen and is ready to leave the lungs.

Carbon Dioxide Begins the Return Trip

After delivering oxygen, the red blood cell begins its return journey. Your cells produce carbon dioxide as a waste product of normal metabolism. Carbon dioxide enters the bloodstream and is transported back toward the lungs in several forms. …

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