In the lungs:
$(a)$ What substance is taken into the body?
$(b)$ What substance is removed from the body?

(N/A) Step $1$: During the process of respiration, the lungs facilitate gas exchange.
Step $2$: Oxygen $(O_2)$ is taken into the body from the inhaled air to be transported to the cells.
Step $3$: Carbon dioxide $(CO_2)$, which is a waste product of cellular respiration, is removed from the body through exhalation.
$(a)$ Oxygen $(O_2)$
$(b)$ Carbon dioxide $(CO_2)$