Answer:
Alveoli
Step-by-step explanation:
The circulatory system helps to pump blood throughout the body. This is important because red blood cells work to carry oxygen throughout the body. The respiratory system is what allows us to breathe in this oxygen that is later absorbed by red blood cells. The respiratory system also extracts carbon dioxide from the blood which we later breathe out.
In the lungs, gas exchange occurs between red blood cells and the alveoli. Alveoli give oxygen to red blood cells so they can distribute this oxygen to other cells in the body. Alveoli also take carbon dioxide that cells may be carrying which we breathe out of our body.