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How does hemoglobin carry oxygen from the lungs to body tissues?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

Hemoglobin is an iron-containing protein found in red blood cells that facilitates the transport of oxygen throughout the body. Each hemoglobin molecule contains four iron-bearing heme groups, allowing it to bind up to four oxygen molecules.12

In the lungs, where the partial pressure of oxygen is high, oxygen diffuses into the red blood cells and binds to the iron atoms in hemoglobin, forming oxyhemoglobin. As this oxygenated blood travels to the tissues, the release of oxygen is governed by the local environment.13

In tissues where oxygen tension is low, hemoglobin releases its bound oxygen. This process is regulated by the Bohr effect, where increased concentrations of carbon dioxide and hydrogen ions lower the blood pH, decreasing hemoglobin's affinity for oxygen and promoting its release. Additionally, 2,3-diphosphoglycerate helps liberate oxygen in peripheral circulation, ensuring tissues receive the oxygen necessary for their metabolic demands.31