Final answer:
A countercurrent gas exchange system is expected in animals that breathe a dense, viscous medium with a low oxygen partial pressure. This system is efficient for maximizing oxygen uptake due to the maintenance of a gradient for oxygen transfer across the gas exchange surface. It is particularly suitable for aquatic animals, such as fish, with gills that have a large surface area for diffusion.
Step-by-step explanation:
In an animal that breathes a dense and viscous medium with low oxygen partial pressure, one would expect a countercurrent (c) gas exchange system. The countercurrent gas exchange is efficient in maximizing oxygen uptake in environments where oxygen levels are low. This system facilitates gas exchange by flowing blood in the opposite direction to the water, thus maintaining a gradient for oxygen transfer from the water to the blood over the entire length of the exchange surface.
In this system, diffusion plays a critical role as gases move from regions of higher concentration to regions of lower concentration, thereby ensuring the transfer of oxygen into the blood and carbon dioxide out of it. The folded surfaces of the gills in fish, which are a prime example of countercurrent exchange, provide a large surface area that maximizes this diffusion process.
It contrasts with other systems such as tidal respiration which is typical of humans where air comes in and out of the same passages, concurrent exchange where the blood and water flow in the same direction but the gradient for exchange diminishes along the path, and cross-current systems where the blood flow intersects the airflow at various angles.