What surrounds the alveoli to facilitate gas exchange?

The correct answer is blood capillaries, which surround the alveoli in the lungs and play a crucial role in facilitating gas exchange. The alveoli are tiny air sacs where oxygen enters the bloodstream and carbon dioxide is expelled from it. The close proximity of the blood capillaries to the alveoli allows for efficient diffusion of gases: oxygen from the air inside the alveoli diffuses into the blood, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. This process is fundamental to respiration and the overall functioning of the respiratory system, ensuring that the body's tissues receive the oxygen they need and that carbon dioxide, a waste product of metabolism, is effectively removed.

Other options do not have a direct role in the gas exchange process within the alveoli. Muscle fibers are involved in the contraction and expansion of the lungs but do not facilitate gas transfer themselves. Nerve endings may be present in the respiratory system to provide sensory feedback but do not participate in the exchange of gases. Nasal passages are part of the airway system, helping to filter and humidify air, but while they are important for bringing air into the lungs, they are not involved in the actual exchange that occurs at the alveolar level.