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The lungs are primarily responsible for gas exchange in humans because they facilitate the process of bringing oxygen into the body and expelling carbon dioxide. Inside the lungs, tiny air sacs called alveoli provide a large surface area for diffusion. When air enters the alveoli, oxygen passes through the thin walls into the bloodstream, where it binds to hemoglobin in red blood cells. At the same time, carbon dioxide, which is a waste product of cellular respiration, diffuses from the blood into the alveoli to be exhaled. This exchange of gases is vital for maintaining the body's oxygen levels and removing carbon dioxide, highlighting the critical role of the lungs in respiration. The other options, such as the heart, kidneys, and stomach, have their respective functions but do not perform gas exchange. The heart circulates blood, the kidneys filter waste from the blood, and the stomach is part of the digestive system, processing food, rather than performing gas exchange.