Gaseous exchange in animals

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Problems associated with increase in size

  • Animals have evolved special gas exchange surfaces so diffusion of gas in and out of cells happens rapidly and efficiently.
  • Respiratory surfaces such as gills of a fish, alveoli in the lungs of mammals, trachea of insects, spongy mesophyll layer in the leaves of a plant - are excellent gas exchange surfaces.
  • To achieve maximum rate of diffusion of a respiratory surface:
    • Have sufficiently large surface area relative to volume of organism - speed up rate of exchange and satisfy needs of organism.
    • Be thin, so diffusion paths are short.
    • Be permeable to allow respiratory gases to diffuse easily.
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Simple unicellular organisms

  • Amoeba:
    • cell membrane gas exchange surface.
    • organism lives in water - diffusion of gases occurs over whole body surface.
    • single cell has large surface area compared to its volume - large surface area to volume ratio.
    • membrane is thin and moist - diffusion paths are short.
    • limit to cell size - point reached where length of diffusion path limits efficiency of process of diffusion.
  • Larger the organism, smaller the surface area to volume ratio.
  • Materials need to be exchanged between different organs as well as between organs and environment.
  • Gaseous requirements provided by diffusion through cell surface - insufficient to meet needs of organism - process of diffusion is too slow.
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Multicellular animals

  • larger organisms have low surface area to volume ratio.
  • Higher rate of metabolism - require more oxygen to satisfy their needs.
  • Body surface may become toughened and impervious - in some animals, body enclosed in protective shell.
  • Gradients for diffusion must be maintained - movement of air and movement of blood.
  • Respiratory surfaces thin and easily damaged for protection located inside organism.
  • Ventilating of the lungs - a means of moving air over the surface.
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  • Flatworms
    • Aquatic organisms evolved flattened shape to overcome increase in size.
    • Increases surface area to volume ratio - no part of the body is far from the surface.
  • Earthworms
    • terrestrial organism developed tubular shape.
    • restricted to damp environment of soil.
    • Having an elongated shape provides large surface area to volume ratio.
    • requires special surface for gaseous exchange.
    • needs to keep skin moist by secreting mucus on to surface.
    • Has modest need for oxygen as slow moving - low metabolic rate.
    • O2 and CO2 diffuse across skin surface - no special gas exchange organs.
    • O2 diffuses into blood capillaries beneath skin surface, carried in vessels to cells.
    • CO2 transported opposite direction - blood maintainsdiffusion gradient at respiratory surface
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Other multicellular animals

  • Larger and more advanced - insects, fish, reptiles and mammals - higher metabolic rate requiring more energy, so require high amount of O2.
  • Increase in size and specialisation - tissues and organs more dependent on each other.
  • Specialised surfaces:
    • Aquatic insects and fish - exchange surfaces for respiratory gases are gills.
    • Common ancestor of birds, reptiles and mammals developed lungs.
    • Terrestrial inescts have air-filled tubes called tracheae.
  • For efficient gaseous exchange:
    • Ventilation mechanism
    • Internal transport system - provided by blood circulation system to move gases between respiring cells and respiratory surface.
    • Respiratory pigment in blood - increase oxygen-carrying capacity.
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Gas exchange in fish

  • Fish are active need a good supply of oxygen.
  • Gaseous excahnge takes place over gill - one-way current of water kept flowing by specialised pumping mechanism.
  • Density of water prevents gills from collapsing and laying on top of each other - reducing surface area.
  • Gills made up of many folds providing large surface area which water can flow and gases exchanged.
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Types of fish

  • Cartilaginous fish
    • Skeleton made of cartilage.
    • Behind head on both sides are five gill clefts which open at gill slits.
    • Water taken in at mouth forced out through gill slits - floor of mouth raised.
    • Blood travels through gill capillaries in same direction as sea water
    • Gas exchange in parallel flow - inefficient.
  • Bony fish
    • Internal skeleton made from bone
    • Gills covered by a flap called operculum.
    • Inhabit fresh and sea water.
    • Gas exchange in a counter-current flow - blood capillaries flow in opposite direction to water flowing over gill surface.
    • Four pairs of gills in pharynx - each gill supported by gill arch.
    • Along each gill arch many thin plates called lamellae and on these gill plates.
    • Out of water, gill collapses as gill lamellae lie on top of each other and stick together.
    • In water when supported provide large surface area.
    • Gill plates contain blood capillaries and oxygen passes through gill plates into capillaries and CO2 passes out into water.
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Gills and Ventilation

  • Gills provide:
    • Specialised area rather than whole body surface.
    • Large surface extended by gill filaments.
    • Extensive network of blood capillaries - allow efficient diffusion and haemoglobin for oxygen carriage.
  • Water forced over gill filaments by pressure differences - maintaining continuous, undirectional flow of water.
  • Lower pressure maintained in opercular cavity than in bucco-pharynx.
  • Operculum acts as a valve permitting water out and as pump - drawing water past gill filaments, mouth also acts as pump.
  • Ventialtion mechanism for forcig water over gill filaments:
    • Mouth opens.
    • Operculum closes.
    • Floor of mouth lowered.
    • Volume inside mouth cavity increases.
    • Pressure inside cavity decreases.
    • Water flows in as extrernal pressure higher than pressure in mouth.
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Counter-current flow

  • Orientation of gas exchange surface as water passes from pharynx into opercular chamber, flows between gill plates in opposite direction to blood flow.
  • Increase efficiency of gas exchange - gradient between adjacient flows maintained over whole length of gill filament.
  • Blood always meets water with higher oxygen contents.
  • Gills of a bony fish allowed to remove 80% of oxygen from water, three times rate of extraction of oxygen from air in human lungs.
  • High level of extraction essential to fish as there are around 25 times less oxygen in water than in air.
  • Counter-current system maintains diffusion gradient along whole length of gill.
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Adaptations of vertebrate groups to gaseous exchan

  • Terrestrial organism - organisms that lives on land
  • Problem for all terrestrial organisms is water evaporates from the body surface resulting in dehydration.
  • Gas exchange surfaces need to be thin, permeable surfaces with a large area - these features conflict with the need to conserve water.
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  • Frogs, toads, newts.
  • Frog -
    • gaseous exchange trhough skin and lungs.
    • when inactive skin acts as surface of gaseous exchange with water or air.
    • skin moist and permeable with well developed capillary network.
    • lungs simple elastic sacs with good blood supply.
    • no diaphragm or rib cage.
    • lungs inflated by forcing air into them by movements of the floor of the mouth.
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  • Crocodiles, lizards and snakes.
  • Better suited to life on land than amphibians.
  • move on all four limbs without trunk of body touching ground.
  • pairs of ribs project from vertebrae.
  • ribs provide support and protection to organs in body cavity.
  • ribs involved in ventilation of lungs.
  • lung more complex internal structure than amphibians with in-growth tissues increasing surface area for gas exchange.
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  • Lungs of birds have internal structure similar to mammals.
  • Large volume of oxygen needed to ptovide energy for flight.
  • ventilation of lungs in birds more efficient than in other vertebrates - assissted by system of air sacs which function as bellows.
  • ventilation of lungs brough by movement of ribs.
  • during flight action of flight muscles ventilates lungs.
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Gaseous exchange in insects

  • most insects terrestrial.
  • water evaporates from body surface where dehydration may occur.
  • efficient gas exchange requires thin, permeable surface with large area - conflicts with need to conserve water.
  • to reduce water loss - waterproof coverings over body surfaces.
  • insects have rigid exoskeleton covered by cuticle.
  • Insects have relatively small surface area to volume ratio - cannot use body surface to exchange gases by diffusion.
  • gas exchange occurs through paired holes, spiracles, run along side of body.
  • spiracles lead to system of branched chitin lined air-tubes called tracheae.
  • spiracles open and close like valves - allows gaseous exchange to take place reduces water loss.
  • resting insects rely on diffusion to take in oxygen and remove carbon dioxide.
  • during periods of activity - flight - movements of abdomen ventialte tracheae.
  • ends of tracheal branches called tracheoles - gaseous exchange takes place resulting in oxygen passing directly into cells.
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The human respiratory system

  • Lungs:
    • supply large surface area.
    • increased by alveoli.
    • lined with moisture for dissolving gases
    • thin walls shorten diffusion path
    • extensive capillary network for rapid diffusion and transport - maintain diffusion gradients.
    • enclosed in airtight compartment - thorax.
    • at base is the diaphragm.
    • lungs supported and protected by rib cage.
    • ribs moved by intercostal muscles.
    • enables lungs to be ventilated so air is constantly being replenished.
    • air drawn to lungs via trachea.
    • consist of branching network of tubes called bronchioles arising from a pair of bronchi.
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Gas exchange in alveolus

  • gas exchange surfaces are air sacs or alveoli which provide a large surface area relative to volume of the body.
  • well suited as gas exchange surface because walls are thin - short diffusion path.
  • each alveolus covered by extensive capillary network to maintain diffusion gradients - blood always taking oxygen away from alveolus and returning with carbon dioxide.
    • deoxygenated blood enters capillaries surround alveolus.
    • oxygen diffuses out alveolus into blood in capillary.
    • carbon dioxide diffuses out of capillary into air of the alveolus.
  • Surfactant - moisture between alveoli and capillary - O2/CO2 dissolves in layer so diffusion more rapid.
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Ventilation of the lungs

  • mammals ventilate their lungs by negative pressure breathing - forcing air down into lungs.
  • if air enters lungs pressure inside must be lower than atmospheric pressure.
  • Higher pressure from atmosphere or elsewhere
    • greater number of collisions
    • higher kPa
    • faster rate of diffusion
  • less atmospheric atmosphere (further away from land)
    • less collisions between O2 molecules
    • slower diffusion due to lower kPa
  • Smoking lengthens diffusion path
    • causes epithelium around alveoli to thicken.
    • tar blocks the bronchi, narrowing them.
    • smokers cough can lead to alveoli being destroyed.
    • CO blocks haemoglobin and stops binding to CO2.
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  • Maintains O2 concentration gradient (high in alveoli)
  • Breathing is active process since muscle retraction requires energy.
    • external intercostal muscles contract.
    • ribs pulled upwards and outwards.
    • diphragm muscles contract causing it to flatten.
    • both actions increase volume of thorax.
    • results in reduction of pressure in lungs.
    • atmospheric air pressure greater than pressure in lungs - air forced into lungs.
  • Diaphragm - contracting - pulled down and flattened.
  • Intercostal muscles - contracting - pull ribs up and out.
  • Volume of thoratic cavity (space in chest) - Increases
  • Pressure relative to air of thoratoc cavity and lungs - decreased pressure.
  • Purpose - air moves into lungs down pressure gradient so increasing O2 concentration.
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  • maintains CO2 concentration gradient (low in alveoli)
  • Breathing out passive process - opposite of inspiration.
  • surrounding each lung and lining the thorax and pleural membranes between which is a cavity containing pleural fluid.
  • when breathing fluid acts as a lubricant - friction free movement against inner wall of thorax.
  • to prevent alveoli from collapsing when breathing out - surfactant - covers their surfaces and reduces surface tension.
  • Diaphragm - relaxing moves up in dome shape.
  • Intercostal muscles - relax ribs move in and down
  • volume of thoratic cavity - decreases.
  • pressure relative to air of thoratic cavity and lungs - increases.
  • Purpose - air moves out of lungs down pressure gradient maintaining CO2 gradienr eliminating CO2
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