How Does Oxygen Travel to Every Cell in the Body?

Lungs

When air comes into the lungs, it flows into small structures called the alveoli. Each alveolus is wrapped in a web of capillaries (tiny blood vessels). The alveolar wall separating the air passage and the capillary wall is so thin (about one micron, the thickness of a single cell) that gases can easily pass between them. Oxygen enters the capillary, and carbon dioxide enters the alveolus to be exhaled.

Blood

Red blood cells have a thin membrane and are filled with a protein called hemoglobin. The red color comes from the iron present in hemoglobin. Iron reacts very easily with oxygen so when the oxygen from the alveoli get inside the narrow capillaries, the blood cells absorb the oxygen, making the cell a brighter shade of red. The blood cells move through the capillaries, which eventually become arteries that transport the blood to every part of the body. Eventually the arteries narrow to capillaries so they can make their way through and around every cell. The capillaries are so narrow that the red blood cells must squeeze through one at a time. (Red blood cells are around 25 percent bigger than the capillary passageway.) The capillary walls are flexible enough to let the larger blood cells through and thin enough that the oxygen can pass to the cell membranes. As they give up their oxygen, the blood cells turn a darker red as they absorb carbon dioxide from the cell to take through the veins back to the lungs.

Diffusion

A process called diffusion (which includes osmosis) is how the oxygen passes from the capillary to the cell. When the concentration of oxygen in the red blood cell is higher than the concentration inside the cell, it will be drawn to where the concentration is lower. Because the cells are always producing more carbon dioxide, the concentration of that will usually be higher in the cell, so it passes into the red blood cell where the concentration is lower.