Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

Which of the following substances primarily diffuses out of the alveoli during gas exchange?

1. Nitrogen

2. Oxygen

3. Carbon dioxide

4. Water vapor

The correct answer is: Carbon dioxide

During gas exchange in the alveoli of the lungs, carbon dioxide is the primary substance that diffuses out of the alveoli into the bloodstream. This process is a critical component of respiration, as the body needs to expel carbon dioxide, a waste product of cellular metabolism. In the alveoli, oxygen enters the bloodstream through diffusion based on concentration gradients; oxygen is abundant in the alveolar air and less concentrated in the blood. Conversely, carbon dioxide is at a higher concentration in the blood because it has been produced by cells during metabolism. This gradient facilitates the movement of carbon dioxide out of the blood and into the alveoli, where it can then be exhaled. While water vapor does diffuse out as well, especially in exhaled air, the main focus of gas exchange involves the critical removal of carbon dioxide and the intake of oxygen. Nitrogen, on the other hand, is not actively involved in the gas exchange process in significant amounts because it does not participate in metabolic processes and typically remains in the bloodstream without a significant influx or efflux during respiration. Thus, carbon dioxide is the substance that primarily diffuses out of the alveoli during gas exchange.