Which organ in the thoracic cavity is responsible for gas exchange with the environment?

• A. Heart
• B. Lungs
• C. Liver
• D. Stomach

Answer: (B)

Gas exchange with the environment happens in the lungs, which are located in the thoracic cavity. In the lungs, air reaches tiny air sacs called alveoli where oxygen passes through the thin walls into surrounding capillaries and carbon dioxide moves from the blood into the alveolar air to be exhaled. This diffusion occurs because of different partial pressures of gases between the inhaled air and the blood, and the lungs are specialized with a large surface area and very thin membranes to make this exchange efficient.

The heart, while essential for circulating blood, does not exchange gases with the environment. The liver processes various substances, and the stomach digests food. So, the lungs are the organ responsible for exchanging atmospheric gases with the bloodstream.