Hemoglobin refers to a protein, found in red blood cells, that is responsible for carrying oxygen from the lungs to all other tissues of the body. Red blood cells circulate through the lungs and the hemoglobin in these cells pick up (bind with) oxygen. These cells carry the oxygen, in a form called oxyhemoglobin, out to the tissues and cells of the body. Once at the proper destination, the oxyhemoglobin releases the oxygen and becomes hemoglobin again. The red blood cells, carrying hemoglobin (without oxygen), circulate back to the lungs to pick up more oxygen, and the process begins again.
Hemoglobin also refers to a blood test that indicates the amount of hemoglobin in the blood, and thus, how well the red blood cells are able to carry oxygen to the rest of the body. Hemoglobin is a part of a Complete Blood Count (CBC) test. Hemoglobin is used to check for anemia and if low, can indicate conditions ranging from mild, such as not getting enough iron in the diet, to serious, such as abnormal bleeding due to colon cancer or other health problems.