Sure!
So it is very important that our body is able to complete the process of respiration (breathing), in order to get rid of C02 which is a product of cellular respiration in the body (a process which every cell uses) as well as get new O2 so that cellular respiration can take place. This process of the exchange of gasses takes place within the alveoli which are located at the ends on each bronchiole (a small branch of a lung). The blood that starts at the beginning of the sac, after it is pumped in the heart is high in C02 as it is bringing the waste product C02, so it can be expelled when you breathe out. When you breathe in and absorb 02, oxygen travels to the alveoli so that it can diffuse over the capillary wall. This process occurs at the same time, meaning that as C02 is leaving O2 is going into the blood. As the blood passes by the alveoli it gains oxygen and looses carbon dioxide so when it returns back to the heart it is high in oxygen.
So in summary, the blood that comes to the alveoli is very low in O2 as all the oxygen has been used up in order for cellular respiration to take place, and high in CO2 because it is a biproduct of cellular respiration that must be expelled from the body.
