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.
When the right atrium contracts, blood flows through a valve into the heart's right ventricle. When the right ventricle contracts, blood is pumped through a valve into the pulmonary artery and into the lungs where it picks up oxygen. It works this way for an important reason. Blood returning from the body is poor in oxygen. It has to be full of oxygen before returning to the body. So, the heart's right side pumps blood to the lungs first to gather oxygen before going to the heart's left side where it is sent back to the body full of oxygen.