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.
1 of 19
Simple unicellular organisms
- 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.
2 of 19
- 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.
3 of 19
- 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.
- 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
4 of 19
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.
5 of 19
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.
6 of 19
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.
7 of 19
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.
8 of 19
- 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.
9 of 19
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.
10 of 19
- 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.
11 of 19
- 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.
12 of 19
- 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.
13 of 19
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.
14 of 19
The human respiratory system
- 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.
15 of 19
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.
16 of 19
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.
17 of 19
- 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.
18 of 19
- 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
19 of 19