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.
Describe what happens when blood flows through the lungs. Blood flows through tiny capillaries that are touching the air sacs in the lungs. It brings oxygen to the red blood cells and gets rid of carbon dioxide. Describe what happens when blood in capillaries flows past cells. Gas exchange takes place when the red blood cells is sliding past a cell.