Exchange surfaces

Buccal cavity: the mouth.

Countercurrent flow: where two fluids flow in opposite directions.

Filaments: slender branches of tissue that make up the gill. (Also called primary lamellae).

Lamellae: (or secondary lamellae) are folds of the filament to increase SA. Also called gill plates.

Operculum: A bony flap that cvers and protects the gills.

Spiracle: an external opening or pore that allows air in/out od the tracheae.

Tracheal fluid: the fluid found at the end of the tracheoles in the tracheal system.

Tracheal system: a system of air-filled tubes in insects.

Bony fish must exchange gases with the water in wich they live. They use gill sin order to absorb oxygen dissolved in oxygen and release carbon dioxide into the water. The oxygen concentration will be typically lower than in the air. Most bony fish have 5 pairs of gills which are covered by a bony plate called the operculum.

Each gill conssts of 2 rows of gill filaments (primary lamellae) attatched to a bony arch. The filaments are very thin and thir surface is folded into many secondary lamellae (or gill plates). This provides a very large SA. Blood capillaries carry deoxygenated blood close to the surface of the secondary lamellae where exchange occurs.

Blood flows along the gill arch and out along the filaments to the secondary lamellae. The blood then flows through the capillaries in the opposite direction to the flow of water over the lamellae. This arrangement creates a countercurrent flow that absorbs the maximum amount of oxygen from the water.

Bony fish can keep water flowing over the gills by using a buccal-opercular pump. The buccal cavity can change volume. The floor of the mouth moves downwards, drawing water nto the buccal cavity. The mouth closes and the floor is raised, pushing water through the gills.

Movements of the operculum are coordinated with the movements of the buccal cavity. As water is pushed from the buccal cavity, the operculum moves outwards. The movement reduces the pressure in the opercular cavity (space under the operculum), helping the water to flow through the gills.

They don't transport oxygen in blood. Insects have an open circulatory system in which the body fluid acts as both blood and tissue fluid. Circulation is low and can be affected by body movements.

Insects possess and air-filled tracheal system, which supplies air directly to all the respirig tissues. Air enters the system via a pore in each segment, called a spiracle. The air is transported into the body through a series of tubes called tracheae. These divide into smaller and smaller tubes called tracheoles.

The ends of the tracheoles are open and filled with tracheal fluid. Gaseous exchange occurs between the air in the tracheole and the tracheal fluid. Some may also occur across the thin tracheole walls.

Many insects are very active, so need a good oxygen supply. When tissues are active, the tracheal fluid can be withdrawn into the body fluid in order to incraese the SA of the tracheole wall exposed to air. This means more oxygen can be absorbed when it is active.

Larger insects can also ventilate their tracheal system by movements of the body. This can be achieved in a number of ways.

• In many insects, sections of the tracheal system are expanded and have flexibe walls. These act as air scas which can be squeezed by the action of the flight muscles. Repetetive expansion and contraction of these sacs ventilate the tracheal system.

• In some insects, movements of the wings alter the thorax volume. As the thorax volume decreases, air in the tracheal system is put under pressure and pushed out of the tracheal system.

• Some insects have developed this ventilation further. Locusts can alter the volume of their abdomen by specialised breathing movements. These are coordinated with opening and closing valves in the spiracles. As the abdomen expands, spiracles at the front end of the body opens and air enters the tracheal system.