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 returning to the lungs is usually only down to about 75% oxygen, while the blood leaving your heart has about 100% oxygen. If you stop breathing but your heart keeps pumping blood to your brain, the oxygen content of your blood will decrease over time until you pass out. If your heart stops pumping, there will be no oxygen delivered to the brain and you die very quickly.