Gas Exchange

Gas exchange in single-celled organism

Relies on diffusion
Such a small distance between envionment & organism that gas simply diffuses into the body

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Tracheal system (system of air tubes inside insect)
They have holes in the cuticle going from their body surface to the air tubes
The holes are called spiracles
Spiracles lead into the tracheae which are lined with chitin
Chitin supports the tracheae and makes them impermeable to gases
The tracheae branches off into tracheoles where respiratory gases are exchanged
Large surface area for gas exchange due to large number of tracheoles
When insects are active, water in the ends of the tracheole is withdrawn
This minimizes diffusion distance for gases
Also tracheoles are tiny and extend into individual cells e.g muscle fibres
Tracheal system lets gas exchange happen independently of circulatory system so insects blood doesn't transport any respiratory gases
Small insects: air moves along tracheal system by diffusion
Larger/ active insects: air actively pumed by by thorax/abdomen movements
These also have air sacs (collapsible tracheae) which are inflated & deflated by respiratory movements which moves more air through tracheal system when needed
Ventilatory movements are more forceful & frequent when activity increases

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gills= respiratory structures helping gas exchange
fish have 4 pairs of gill arches that support gill filaments
gill filament subdivided into 2 stacks of gill lamellae (main site for gas exchange) whcih lie on top of each other
water keeps lamellae apart exposing surfaces for gas exchange
surface area of gill lamellae maximised by gill plates on upper & lower surfaces
high concentration gradient between water (carrying oxygen to gills) & blood (carrying oxygen from gills to body)
Counter-current principle: water moves across gill plates in opposite direction to blood flowing through them
Causes water to have higher oxygen concentration than the blood it flows past so diffusion can occur over whole surface of gill plate
gill plates are very thin and delicate so blood flowing through them is only a short distance from seawater
Water is taken in through the mouth and then passes over gills

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