Final answer:
Bulk flow aids in the rapid movement of blood in larger animals, complementing diffusion at respiratory surfaces such as alveoli, allowing for efficient gas exchange to meet higher metabolic demands.
Step-by-step explanation:
Bulk flow and gas exchange by diffusion work together to facilitate efficient gas exchange in larger animals by overcoming the limitations of diffusion alone. Diffusion is sufficient for small organisms because they have high surface area-to-volume ratios, allowing gases to quickly reach all cells. However, as animals grow larger, the diffusion distances become too great, and diffusion alone cannot meet their metabolic demands.
In these alveoli, the exchange of oxygen and carbon dioxide takes place due to the concentration gradient between the air in the lungs and the blood in the pulmonary capillaries. This gradient-driven transport is central to respiratory gas exchange. Concurrently, the circulatory system employs bulk flow to move blood rapidly to and from these gas exchange surfaces, distributing oxygen throughout the body and collecting carbon dioxide for expulsion.