The heart pumps blood through the lungs. In the lungs the blood receives oxygen and releases carbon dioxide. Then the heart pumps this oxygen-rich blood from the lungs to your cells. When the oxygen-rich blood gets to the cells, the cells receive the oxygen and release the carbon dioxide.
The cells in our bodies need oxygen to stay alive. Carbon dioxide is made in our bodies as cells do their jobs. The lungs and respiratory system allow oxygen in the air to be taken into the body, while also letting the body get rid of carbon dioxide in the air breathed out.
It is a mode of natural selection where an extreme phenotype is favored by the other phenotypes causing the allele frequency to shift over time in the direction of that phenotype
