1. Countercurrent System
黄樱樱

2. ☆ Gill Structure Enabling the Countercurrent System

3. • Fish employ a method known as the countercurrent system to extract oxygen from the water. 

4. • This system moves water flowing across the gills, in
an opposite direction to the blood flow creating the maximum efficiency of gas exchange. 
• The lamellae are very important in this process, as this is the only place where countercurrent occurs, they can be found protruding from the gill filaments dorsal and ventral surfaces . 

5. How it works？

6. Ⅰ.The gill arches of a fish support the gill
filaments (also called theprimary lamellae)
and form a curtain through which water
passes as it moves from the buccal cavity
to the opercular cavity.
Ⅱ.As water flows across the filaments of
a teleost, blood flows through the
secondary lamellae in the opposite
direction.

7. Ⅲ. In elasmobranchs, even though septa create some structural
differences in gill filaments,water flow across the secondary lamellae
is still countercurrent to blood flow.

8. Ⅳ.The countercurrent flow of water and blood at the exchange surface of the secondary lamellae ensures that the partial pressure of oxygen in the water
always exceeds that of the blood, thereby maximizing the efficiency of oxygen diffusion into the blood.

9. Thank you