Bony fish use a countercurrent flow system in their gills to absorb dissolved oxygen from the water.
Which of the statements, A to D, correctly describes how the countercurrent system facilitates efficient gas exchange?
Blood and water flow in the same direction, maintaining a constant diffusion gradient.
Blood always meets water with a higher oxygen concentration, maintaining a diffusion gradient along the entire length of the lamellae.
Blood and water reach equilibrium halfway along the lamellae, maximizing the rate of diffusion.
Blood with the lowest oxygen concentration meets water with the highest oxygen concentration at the start of the lamellae.
