7.4 Ventilation and gas exchange in other organisms

why do insects have different mechanisms of gas exchange and ventilation to humans

1) tough exoskeleton = little gas exchange can take place

2) do not have blood pigments that can carry O₂

spiracles

small openings along the thorax and abdomen in which air enters and leaves and water is lost

opened and closed by sphincters

tracheae

large tubes that carry air into the body along the body of the insect

lined with chitin = keeps them open if bent or pressed, impermeable to gases

tracheoles

single, elongated cells with no chitin lining = freely permeable to gases

spread throughout the tissues of the insect

where most gas exchange takes place between air and respiring cells

describe gas exchange in insects

1) insects contract and relax their abdominal muscles so air enters through the spiracles

2) air moves along the tracheae and tracheoles by diffusion and reaches the tissues = vast number of tiny tracheoles gives very large SA for gas exchange

3) O₂ dissolves in moisture on the walls of the tracheoles and diffuses into surrounding cells

tracheal fluid

towards ends of tracheoles

water from the tracheoles

how do insects maintain a large surface area for gas exchange

many branching tracheoles

how do insects maintain a short diffusion distance for gas exchange

many branching tracheoles reach muscle and thin-walled tracheoles

how do insects maintain a steep concentration gradient for gas exchange

when the cells respire they use O₂ and produce CO₂ and the abdominal muscles contract to pump air

describe gas exchange when O₂ demands build up e.g. when flying

1) muscle cells start to respire anaerobically to produce lactate

2) this lowers the water potential of the cells

3) water moves from the tracheal fluid into the cells by osmosis

4) this decreases the vol of liquid in the tracheoles = causes more air from the atmosphere to move in + larger surface area in tracheoles for gas exchange

how do larger insects increase the level of gas exchange

1) mechanical ventilation of the tracheal system = air is actively pumped into the system by muscular pumping movements of the thorax and abdomen = changes the vol of body = changes pressure in tracheae and tracheoles = air is drawn in/out

2) collapsible enlarged tracheae/ air sacs = used to increase the amount of air moved through the gas exchange system = inflated and deflated by the ventilating movements of the thorax and abdomen

why are gills suitable for effect gas exchange in fish

large surface area, good blood supply, thin layers

steps of gas exchange in fish

1) mouth opens = buccal cavity floor is lowered = increases the vol of the buccal cavity

2) the pressure in the buccal cavity decreases = water moves into the buccal cavity

3) opercular valve is shut and opercular cavity expands = lowers pressure in opercular cavity

4) buccal cavity floor moves up = increases pressure inside buccal cavity = water moves from buccal cavity over the gills

5) mouth closes = operculum opens = sides of opercular cavity move inwards = increases pressure in opercular cavity

7) water is forced over the gills and out of the operculum

two adaptations of gills to ensure effective gas exchange

1) tips of adjacent gill filaments overlap = increases resistance to flow of water over the gill surfaces and slows down the movement of the water = more time for gas exchange to take place

2) countercurrent flow of water and blood = ensures that steeper concentrations gradients maintained = O₂ continues to diffuse down concentration gradient so a much higher level of O₂ saturation in the blood is achieved = equilibrium is never reached

gill lamellae

main site of gas exchange in fish

rich blood supply and large surface area

gill filaments

occur in large stacks called gill plates and need a flow of water to keep them apart, exposing the large surface area needed for gas exchange