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.
Thus, CO2 stops leaving your blood faster if the lungs are empty, meaning the CO2 levels in your blood reach critical faster and you need to breath sooner. Oxygen levels in your blood are likewise not rising fast enough (or at all, since you just exhaled), but it's the rise in CO2 that makes you gasp.