Which blood protein carries most oxygen in human blood?

Hemoglobin carries most oxygen in human blood.

Hemoglobin is an iron-containing protein found inside red blood cells. Each molecule can bind oxygen reversibly, allowing blood to pick up oxygen in the lungs and release it in tissues where it is needed for cellular respiration. Hemoglobin also helps transport some carbon dioxide and contributes to the blood's acid–base balance.

A hemoglobin molecule is made of four protein subunits, each containing a heme group with an iron atom. In adult humans, the most common form is adult hemoglobin, or HbA, which contains two alpha and two beta chains. The iron in each heme group gives oxygenated blood much of its bright red colour.

Hemoglobin is not the same as plasma albumin, which helps maintain fluid balance and transports various substances. Low hemoglobin concentration is a defining feature of anemia, although anemia has many different causes.