What type of blood cells carry oxygen?

Red blood cells carry oxygen through the human bloodstream.

These cells, also called erythrocytes, contain hemoglobin, an iron-containing protein that binds oxygen in the lungs. As blood travels through the circulation, hemoglobin releases that oxygen to tissues that need it for cellular respiration and energy production.

Mature human red blood cells have a flexible biconcave shape and lack a nucleus. Losing the nucleus leaves more room for hemoglobin and helps the cells squeeze through narrow capillaries. They also lack mitochondria, so they do not consume the oxygen they transport for their own aerobic metabolism.

Plasma carries substances dissolved in blood, platelets help form clots, and white blood cells support immune defense. Those components are important, but they are not the blood’s principal oxygen carriers. Hemoglobin also helps transport some carbon dioxide, although most carbon dioxide returns to the lungs as bicarbonate dissolved in plasma.