Gas Exchange in Bony Fish in a Snap! Unlock the full A-level Biology course at http://bit.ly/2Z7K826 created by Adam Tildesley, Biology expert at SnapRevise and graduate of Cambridge University.



The key points covered of this video include:

1. Structure of the Gills
2. Buccal Pumping
3. Countercurrent Flow

Structure of the Gills

Fish need to perform gas exchange like all other animals except they must do this whilst underwater. Fish are active and can grow quite large so they evolved gills to exchange gasses efficiently. The gills consist of a series of bony gill arches each with two stacks of gill filaments. Gill filaments have protruding rows of very thin lamellae. Each lamellae consists of a network of capillaries covered by a single layer of epithelial cells. Because the gill structures are delicate they are protected by a bony plate called an operculum.

Buccal Pumping

Mammalian lungs ventilate through the processes of inspiration and expiration. Fish also need to ventilate their gills to maintain a strong diffusion gradient for efficient exchange. To ventilate their gills fish coordinate opening of the opercular vents with closing of their buccal cavity. The buccal cavity is the technical term for a fish’s mouth. When the fish open their buccal cavity they close their opercular vents to draw freshwater in which can then be pumped over the gills again. Some sharks don’t use this mechanism and so have to swim constantly to keep a fresh supply of water flowing over their gills.

Countercurrent Flow

The orientation of the gill filaments and lamellae ensures that the water flowing over them moves countercurrent to the flow of blood through the capillaries. Countercurrent flow is a term that describes two liquids in close proximity to each other flowing in opposite directions. The process of counter-current flow ensures maximum efficiency of gas exchange at the gills. As water flows past the capillary it loses oxygen to the blood down a diffusion gradient. Blood that has very little oxygen flows past water with some oxygen and diffusion occurs. Blood that’s partially saturated with oxygen flows past water loaded with oxygen so diffusion can still take place. Parallel flow would result in both media containing equal amounts of oxygen as regular diffusion would occur which is less effective.

Summary

Fish require an efficient gas exchange organ that can work underwater
The gills are a specialised organ designed to perform this process
The gills consist of bony arches with rows of gill filaments containing lamellae
The orientation of the gill structures allow for countercurrent flow of water and blood
Countercurrent flow increases the efficiency of gas exchange in the gills
Buccal pumping allows fish to ventilate their operculum thus maintaining a strong diffusion gradient around their gills