Hemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs. Hemoglobin is made up of four protein molecules (globulin chains) that are connected together. The hemoglobin test is often used to check for anemia, usually along with a hematocrit or as part of a complete blood count (CBC).
The molar hemoglobin concentration given in SI units therefore has to be in a range between about 2 and 3 mmol/l.