Biology - Animal Physiology

Air enters in insects happens through their SPIRRACLES... 

Insects have a unique TRACHEAL SYSTEM, which is made of a NETWORK of TUBES connecting air from the OUTSIDE straight to RESPIRING TISSUES...

The SPIRRACLES lead to the TRACHEA which branch out into TRACHEOLES...

The NETWORK of TUBES and the RESPIRING TISSUES have a SHORT DIFFUSION PATHWAY for MAXIMUM GAS EXCHANGE...

This TRACHEAL SYSTEM can only work at a CERTAIN SIZE...

The TRACHEA have a WALL to SUPPORT them, however the TRACHEOLES have a THIN PERMEABLE MEMBRANE for efficient GAS EXCHANGE...

Insects can OPEN or CLOSE their SPIRRACLES to alter their level of VENTILATION...

WATER is DENSER than AIR and it contains LESS OXYGEN...

FISH are COLD BLOODED so they need LESS OXYGEN than WARM BLOODED ANIMALS...

GILLS are covered in FEATHERY LAMELLAE and FILAMENTS to give a LARGE SURFACE AREA...

The FILAMENTS have a RICH BLOOD SUPPLY and the SHORT DIFFUSION PATHWAY so BLOOD is in CLOSE CONTACT with WATER...

The LAMELLAE have many CAPILLARIES and are COVERED by a SINGLE LAYER of CELLS this ensures a SHORT DIFFUSION PATHWAY...

The DIRECTION of WATER and the DIRECTION of BLOOD FLOW isCONCURRENT (they flow in opposite directions) to MAINTAIN a CONCENTRATION GRADIENT...