Hemoglobin is a globular protein made up of four subunits. Two of these subunits are alpha chains, and the other two are beta chains. Each subunit contains a molecule of heme, which has an iron atom and can bind to oxygen. When an oxygen molecule binds to one heme group, it changes the shape of hemoglobin, making it easier for the other heme groups to bind oxygen as well.

When all four heme groups are bound to oxygen, the resulting molecule is called oxyhemoglobin. As a result, arterial blood is bright red. As blood passes through tissues, hemoglobin releases its bound oxygen, becoming deoxyhemoglobin, giving venous blood a darker appearance than arterial blood.

Hemoglobin can also bind to carbon dioxide, forming carbaminohemoglobin, accounting for about 20% of the carbon dioxide transport in the blood. When blood returns to the lungs, the high oxygen concentration causes hemoglobin to release carbon dioxide and bind to oxygen instead.

In addition to respiratory gases, hemoglobin can also bind to nitric oxide, resulting in vasodilation to improve blood flow.