Gas Exchange in Fish

Why can’t fish exchange gases across their outer body surface?

• They have waterproof scales!

• Many fish are quite large, so they have a smaller surface area to volume ratio.

The Operculum covers the Gills

Four whole gills

One gill filament with many gill

plates (lamellae)

Gill filaments

The Gill System in a Fish

OperculumRemoved to reveal 4 gills

Fish Gill Filaments showing Gill Lamellae (Gill Plates):

Fish Gills provide a Large Surface Area for Gas Exchange because:

• There are usually 8 gills in a fish head and each gill has about 100 gill filaments.

• Along each gill filament there are loads of gill lamellae (or gill plates). The gill lamellae is the fish gas exchange surface.

Countercurrent Arrangement of Blood flow and Water flow over the Gill Filaments

= Direction of water flow over the gill filament, inbetween the gill lamellae = Direction of blood flow through the capillaries inside the gill lamellae

(gill plates)

Artery (Deoxygenated

Blood)

Vein (Oxygenated

Blood)

Gill lamella at 90o to the main direction of the gill

filament)

Countercurrent flow arrangement in Fish Gills (Animations):

http://mail.dilworth.school.nz/Subjectpages/Science/BIOLOGY/Interactive/fish_countercurrent.swfgood - but American pronunciation!

http://www.kscience.co.uk/animations/anim_3.htm

Important Theory notes about fish now

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Water and blood flow in same direction

Water and blood flow in opposite direction – a countercurrent flow system.

blood

water

blood

water

Equilibrium established – no further change

Not all available oxygen is removed from water

(a) Parallel Flow Arrangement

(b) Countercurrent Flow Arrangement

(a) If the blood and water flowed in the same direction, oxygen would diffuse into the blood until an equilibrium was reached, at which point there would be no further net gas exchange.

(b) However, instead fish have a

countercurrent flow system

•Blood and water flow in opposite directions (blood through the capillary and water over the gill lamellae surface)

•At any point along the gill lamella there is always more oxygen in the water than in the blood

•This means diffusion of oxygen takes place across the whole surface of the lamella•As a result, the blood leaving the gill lamella gains a much higher concentration of oxygen.

Countercurrent Flow

IT IS IMPORTANT TO REMEMBER THAT:It is harder for a fish to get oxygen from water than it is for mammals to get oxygen from the air because: 1. Oxygen is not very soluble in water – it only has 1/30th the oxygen that air does2. Water is higher in density than air, therefore it is harder to move over the respiratory surface during ventilation.

Fish have 2 special adaptations to cope with this…..

• Ventilation in fish is in one direction (water travels over the gills in one direction only)

• This maintains the countercurrent flow system for efficient gas exchange

Countercurrent flow  Water flows across the respiratory surface of the gill in one direction while blood flows in capillary in the opposite direction through.

%sat WaterWith O2

A) Water arrives with high oxygen. As it flows across gill lamella (gill plate) it is exposed to blood (C) with a lower concentration of oxygen.

Oxygen diffuses from water into blood.

Water drops in O2 content (D)Blood increases in O2 content (B)

Blood C D

Distance across gill plate

AB

Very Short diffusion pathway• Very thin surface of gill lamella, one

cell thin – and are made of flattened epithelial cells.

• The blood capillaries in the gill plate are also one cell thin and are made of flattened (endothelial) cells.

• The two surfaces are immediately next to each other.

A Large Concentration Gradient is Maintained as:

• Blood circulation constantly carries newly oxygenated blood away from gill lamellae (gill plates) and brings in more deoxygenated blood.

• Large number of capillaries

• Ventilation brings water high in oxygen concentration to the gill lamellae (gill plates).