Final answer:
Op\tion(a) All gas exchange systems have moist exchange surfaces, a feature essential for the diffusion of gases. This characteristic is vital regardless of the presence of other specific anatomical features and is seen in various organisms with different types of respiratory structures like lungs, gills, and tracheal systems.
Step-by-step explanation:
The common feature that all gas exchange systems have in common is that their exchange surfaces are moist. This moisture is crucial for the diffusion of gases such as oxygen and carbon dioxide. This is true for various types of respiratory systems across different species, including those with lungs such as mammals, gills in fish, and even the tracheal systems of insects. The moisture facilitates the dissolving of gases, which enables them to cross cell membranes more readily. While not all gas exchange systems are enclosed within ribs or are exposed to air, and while they are not necessarily maintained at a constant temperature, hydration of the exchange surface is a universal necessity.
Animal respiratory systems, such as the mammalian lungs, are comprised of numerous structures, including nasal cavities, trachea, bronchi, and alveoli, which provide a large surface area for gas exchange. The presence of mucus and cilia in these systems helps to trap particles and protect the system, but it is the inherent moisture of the alveoli that is critical for the gas exchange process to occur effectively.