What gas do humans need to breathe in to survive, and what gas do they exhale?

Humans INHALE OXYGEN (needed for cellular respiration to produce energy) and EXHALE CARBON DIOXIDE (a waste product of respiration). ASVAB General Science covers the respiratory system. In the lungs, oxygen from inhaled air passes into the blood, and carbon dioxide passes out of the blood to be exhaled (gas exchange in the alveoli). This is essentially the opposite of plant photosynthesis (which uses CO2 and releases O2). The oxygen-in, carbon-dioxide-out cycle is fundamental, commonly tested biology.