Review – factors needed for efficient gas exchange

• Large surface area

• Moist surface to dissolve gases

• Concentration Gradient

The Gill Gas Exchange

• Fish have gills over which water continually flows.

• Gills are supported by water flowing over them, so are unsuitable for life on dry land.

• The water has oxygen dissolved in it.

Fish Anatomy

Operculum

Mouth (Buccal Cavity)

Buccal Floor

Behind the Operculum…

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

Gill Structure

• Gill arches

• Gill filaments

• Lamellae

Adaptations of the gills for efficient gas exchange

• 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

Structure of Gills

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

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

• gills are protected by an operculum.

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

Exchange and Diffusion of Gases.

• The capillaries have very thin walls that allow the O2 to move from the water into the blood and CO2 from the blood into the water by diffusion.

• O2 moves from where there is a high concentration in the water to where there is a lower concentration in the blood of the capillaries.

• CO2 moves from where there is a high concentration in the capillary blood to where there is a low concentration in the water.

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 meeting new

water with a full load of oxygen.• Counter current exchange of water flow and blood stream

maintains concentration gradients for diffusion.

Counter Current PrincipleAnimation of counter-current

Efficiency of Counter-current Exchange

• Counter-current exchange removes up to 80% of the oxygen in the water.

Gills greatly increase the surface area for gas exchange.

Thin Exchange Surface

• Thin walls of lamellae and capillaries to ↓ barrier for diffusion of O2 into capillaries and CO2 out

Moist Gas Exchange Surface

• Moist surface maintained by watery environment.

• O2 already dissolved into watery substance for diffusion into capillaries.

Why don’t fish drown?

• Oxygen is 21% of air, < 1% in 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.