Where are carbon dioxide and blood exchanged in the body?

In the lungs the blood will get oxygenated and ##"CO"_2## will get exhaled and returns back to the heart.

To get pumped through out your body, as the blood with ##"O"_2## reach the capillary it decreases its speed to allow more time for the exchange to take place.

After that the blood is becoming deoxygenated, returning back to the heart and the lung and so on... I hope I have answered your question.