In human physiology, nitrogen is the gas exhaled in the greatest amount by volume.
Air entering the lungs is mostly nitrogen, about 78% by volume, and the body does not significantly consume nitrogen during ordinary breathing. As a result, exhaled air still contains roughly 78% nitrogen. Oxygen falls from about 21% in inhaled air to around 16% in exhaled air, while carbon dioxide rises from about 0.04% to roughly 4%.
The lungs exchange gases across the thin alveolar-capillary membrane. Oxygen diffuses into the blood, and carbon dioxide diffuses from blood into the alveoli before exhalation. Water vapor also increases in exhaled air because the respiratory surfaces are moist.
Carbon dioxide is the body's principal gaseous waste product of aerobic metabolism, but it is not the largest component of exhaled air. The largest component remains nitrogen, which is mostly an inert passenger through the respiratory system.