How do red blood cells carry so much oxygen?

Blood carries much more oxygen than could simply dissolve in its watery liquid. Red blood cells contain large amounts of hemoglobin, a protein that binds oxygen. This adds a substantial carrying capacity without requiring all the oxygen to remain freely dissolved.

A typical adult hemoglobin molecule has four protein subunits. Each holds a heme group containing an iron ion, and each heme can bind one oxygen molecule. One hemoglobin can therefore carry up to four oxygen molecules. The attachment is reversible: binding does not permanently use up the oxygen, so it can be delivered elsewhere.

In the lungs, oxygen entering from the air favors loading onto hemoglobin. In tissues, cells continually use oxygen, helping maintain conditions that favor release from the blood. Released oxygen can move toward those cells. Warmer, more acidic conditions in active tissues can also favor oxygen release. The protein is therefore a changing carrier, not a sealed container transported from place to place.

Hemoglobin returning from tissues need not be completely empty. A molecule with four occupied sites carries four oxygen molecules; the example with two occupied sites carries two. These snapshots make the capacity visible without giving a whole-blood oxygen measurement. The consequential connection is between reversible molecular binding and transport over body-sized distances: blood can pick oxygen up where it is available and release some where it is being used.