We breathe in air which has a higher concentration of oxygen than carbon dioxide and we breathe out air with a higher concentration of carbon dioxide. Both the gases are present in the inhaled and the exhaled air.
In fact, when breathing in normal air, you only take in about 21% oxygen, the rest of the gasses being mainly nitrogen (about 79%). As for breathing out, it is mainly carbon dioxide, but there are a couple more gasses in there.
