Blood group antigens are carbohydrates that are attached to proteins or lipids. An antigen is a substance foreign to the body that causes an immune response. An immune response occurs when antibodies, which are proteins in your immune system, are summoned to attack an antigen.
Blood that reaches the lungs travels throughout a network of small blood vessels, where oxygen moves into the blood and carbon dioxide moves out of the blood, according to the National Heart, Lung and Blood Institute.
The blood from the lungs reenters the heart through the left atrium. The left atrium them pushes blood into the left ventricle, which in turn pumps the oxygenated blood throughout your body. Notice that blood always enters the heart through an atrium, and always leaves the heart through a ventricle.