Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test 2026 – All-in-One Study Guide for Exam Success!

During gas exchange in the lungs, oxygen is the gas that diffuses into the capillaries. This process occurs primarily in the alveoli, which are tiny air sacs in the lungs where oxygen from inhaled air enters the bloodstream. The oxygen concentration in the alveoli is higher than in the deoxygenated blood flowing through the surrounding capillaries, creating a concentration gradient. As a result, oxygen molecules move from the alveoli into the capillaries, where they bind to hemoglobin in red blood cells and are transported throughout the body.

Carbon dioxide, in contrast, is present in higher concentrations in the blood compared to the alveoli, which is why it diffuses from the blood into the alveoli to be exhaled. Nitrogen and helium are not involved in the gas exchange process relevant to respiratory function under normal physiological conditions since they are not utilized for metabolism in the body. Thus, oxygen is the correct answer, as it is essential for cellular respiration and energy production.