Which of the following is a characteristic of capillaries in the pulmonary system?

The characteristic of capillaries in the pulmonary system being a single cell layer thick is crucial for efficient gas exchange. This thinness facilitates the rapid diffusion of gases, such as oxygen and carbon dioxide, between the alveoli in the lungs and the blood. The single layer of endothelial cells reduces the distance molecules must travel, enhancing the process of gas exchange. In the pulmonary system, where the primary function is the exchange of gases, this structural adaptation is essential.

The other options do not apply to the unique characteristics of pulmonary capillaries. For instance, thick walls would impede gas exchange, while a connective tissue matrix would complicate the capillary structure and function. Additionally, a large diameter would decrease the surface area available for gas exchange and could slow down the flow of blood, which contradicts the efficiency needed for rapid gas transfer in the lungs. Therefore, the single cell layer thick structure is vital for the capillaries' role in facilitating efficient gas exchange in the pulmonary system.