Which of the following describes the composition of air moving in and out of the lungs?

• A. Expired air has more oxygen than inspired air
• B. Inspired air has more carbon dioxide than expired air
• C. Expired air has higher carbon dioxide concentration than inspired air
• D. Inspired air is always at higher pressure than expired air

Answer: (C)

The statement that expired air has a higher carbon dioxide concentration than inspired air is accurate because of the gaseous exchange that occurs in the lungs. When air enters the respiratory system, it consists primarily of nitrogen and oxygen, with only a small percentage of carbon dioxide.

As this air travels through the lungs, oxygen is absorbed into the bloodstream, while carbon dioxide, which is a waste product of cellular metabolism, is released from the blood into the alveoli. Consequently, the air that is expelled from the lungs after gas exchange—expired air—contains a significantly higher concentration of carbon dioxide compared to the air that was originally inhaled—inspired air. This physiological process is fundamental to respiratory function and is essential for maintaining the body’s acid-base balance.

In contrast, inspired air has a notably lower concentration of carbon dioxide and a higher concentration of oxygen. Therefore, the differences in gas concentrations and the processes involved in respiration clearly affirm that expired air is indeed richer in carbon dioxide.