Review – factors needed for efficient gas exchange Large surface area:volume Moist surface to dissolve gases Thin membranes for a short diffusion path Concentration gradient is maintained

Gas Exchange in Fish Fish have internal gills over which water continually flows. The water has oxygen dissolved in it. Bony fish have a ventilation mechanism. Cartilaginous fish need to continuously move, or sit in flowing water.

Fish Anatomy Mouth (Buccal Cavity) Operculum Buccal Floor

…lie the gills – the fish’s equivalent of our lungs Behind the Operculum… …lie the gills – the fish’s equivalent of our lungs

Gill Structure Gill rakers Gill arches/bars Gill filaments Lamellae

Structure of Gills The gills are made of numerous thin filaments supported by a bony arch.

Adaptations of the gills for efficient gas exchange – which features? Large surface area : volume allows more diffusion of gases Permeable membranes allows gases to diffuse through tissues Thin (flattened cells) short diffusion distance Good vascular (blood) supply maintains concentration gradients

Each filament is made of thin delicate plates (lamellae) containing many capillaries, so they look dark red.

Ventilation The fish opens its mouth and lowers its buccal floor, increasing the volume in the mouth and so decreasing the pressure Water rushes in The fish closes its mouth and raises its buccal floor This decreases the volume, and so increases the pressure in the mouth Water is forced over the gills and out of the operculum Gas exchange happens in the gills

Gas Exchange O2 into capillaries CO2 into water Capillaries and lamellae have thin walls. O2 and CO2 diffuse through easily. Gases move by diffusion. O2 into capillaries CO2 into water

How does it work? In a concurrent (parallel) flow system, blood and water flow in the same direction. O2 concentration will reach an equilibrium.

CONCURRENT FLOW

Counter-Current Exchange The blood flow in the capillaries is opposite to the flow of water over the gills. This means that the blood in the capillary is always meeting new water with a full load of oxygen. Counter current exchange of water flow and bloodstream maintains concentration gradients for diffusion.

COUNTER-CURRENT FLOW

Why don’t fish drown? Oxygen is 21% of air; < 1% of water More water can flow over (external) gills than in and out of (internal) lungs. Countercurrent flow in gills extracts more of the oxygen from the water in a low O2 environment. Fish demand for O2 supply and removal of CO2 is lower than larger animals

The gill gas exchange system limits fish to a watery environment. The watery environment does not require the gas exchange surface to be internal.

How is the structure of the gills related to its function? Think: Specialised exchange surface SA/V ratio Gas Exchange Blood flow

Bring a fish head to the lesson on Wednesday. Homework Bring a fish head to the lesson on Wednesday.