1. First - a mathematical 'extra' on
surface area to volume ratio calculations and implications for exchange
surfaces in living organisms
The greater the surface area the
greater the possible rate of material transfer.
The most compact shape to give the
lowest surface area/volume ratio is a sphere, but that's not
very practical for the working of many specialised cells, tissues or
organs - but very good for single-celled organisms!
However, systems in living
organisms that involve transfer of substances, do need as large a
surface area as possible within the volume the 'system'
To this end, many organs have
evolved to give the maximum surface area as possible
within the volume the 'system' occupies.
A bit of area/volume maths to
illustrate this idea with cubes of various sizes (6 faces):
A 1 cm cube has a volume
of 1 cm3 (1 x 1 x 1), a total surface are of 6 x 1 x 1 = 6 cm2
So the surface area / volume
ratio = 6 / 1 = 6.0 cm-1
A 2 cm cube has a volume
of 8 cm3 (2 x 2 x 2), a total surface are of 6 x 2 x 2 = 24 cm2
So the surface area / volume
ratio = 24 / 8 = 3.0 cm-1
A 3 cm cube has a volume
of 27 cm3 (3 x 3 x 3), a total surface are of 6 x 3 x 3 = 54 cm2
So the surface area / volume
ratio = 54 / 27 = 2.0 cm-1
A 4 cm cube has a
volume of 64 cm3 (4 x 4 x 4), a total surface area of
6 x 4 x 4 = 96 cm2
So the surface area /
volume ratio = 96 / 54 =
: 1 ratio)
You can see clearly that
the smaller (thinner etc.) the 'system' or parts of the 'system'
the greater the surface to volume ratio - potentially increasing
the rate of transfer of substances.
Good examples of this are the
millions of tiny air sacs (alveoli) in the lungs and the thin
multi-layered sections of gills in fishes - both of which are to
do with animal respiration.
Another good example is the fine and
numerous villi in the intestine where their large surface
area is very efficient for absorbing nutrients from absorbed food.
The villi can be
envisaged as tall thin rectangular blocks in shape to
maximise surface area.
An extra calculation
based on a volume of 8 units. to make the point about
A 2 x 2 x 2 block
has a surface to volume ratio of 3 : 1 (see above).
A 1 x 2 x 4 block
has a surface area to volume ratio of 3.5 : 1 (see
0.1 x 0.1 x 800 block has
a surface area of (2 x 0.12) + (4 x 0.1 x 800) =
0.02 + 320 = 320.02 = ~320 (3 sf)
This gives a surface to
volume ratio of 320 / 8 =
: 1, much higher than the blocks above,
over 10 x higher in fact.
Just think about the very
fine capillaries in the blood system too.
In any surface area : volume
calculations, make sure all measurements and calculations are
quoted with the same length units!
The implications of these
calculations for transfer of substances
This is the mathematics behind
why for small cells
in single or multicellular organisms, the transfer of nutrients,
oxygen and waste products, diffusion rates are high - substances can
be moved quickly in and out of cells.
As the volume of a cell
increases, the distance from the outer cell membrane through the
cytoplasm to the centre of the cell increases.
This slows down the rate of
exchange of substances in or out of the cell from or to the
Cells larger than 1 mm in
diameter may not be viable because the rate of diffusion is too
slow to supply nutrients and oxygen sustain the cell's
Multicellular organisms, with
many layers of cells, tend to have a smaller surface to volume ratio
and therefore need specialised organ systems with large surface
areas for the efficient transfer of substances and also thermal
energy to avoid heating.
organisms have many layers of cells, this increases the time
needed for nutrients and oxygen to diffuse in and reach the
Therefore the cells of the
outer layers would tend to use up the resources first and
faster, depriving inner cells life-supporting resources.
Therefore adaptations have
evolved to enable complex multicellular organisms to overcome
Examples of surface : volume
ratio in various organisms
(based on the same length units)
Single cell bacterium 6 x 106
single celled amoeba6 x 104 : 1; fly 6 x 102
dog 6 : 1; whale 0.06 : 1
You can see there is quite a
contrast between microscopic single celled and large
This mathematical 'extra'
was 'adapted' from the page on
of plants and animals
and is also appropriate to various points in
Diffusion, osmosis and active transport
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Introduction to exchange surfaces
in cells and organs
Living organisms must be to exchange
substances with their surroundings in order to survive - grow, mature and
The size (volume) of an organism or a
specific organ, and its surface area, greatly affects how efficient this
exchange process is. The rate of transfer is often governed by the
surface area : volume ratio.
Diffusion is used by cells to take in
useful substances and remove waste products.
Why do we need exchange surfaces?
Exchange or transfer of substance usually involves diffusion through a
membrane (permeable, partially permeable), water movement by osmosis and also active transport e.g.
the transfer needs of organisms include:
(i) Useful nutrient substances e.g. food molecules
from digestion like amino acids and sugars, mineral ions, water taken up by
cells by osmosis,
(ii) Removal of waste products e.g. carbon dioxide from respiration, urea
(poisonous) from breakdown of proteins in animals - diffuses
from cells into blood plasma and transferred to be absorbed by the kidneys prior to
(iii) Gas exchange usually involves taking
oxygen into cells for aerobic respiration and passing out carbon dioxide to the environment.
Know and understand that many organ systems are specialised for exchanging
The ease with which an organism
can exchange substances with the environment depends on the organisms
surface area to volume ratio AND you can extend this idea to an organ
itself e.g. the lungs.
In single-celled microorganisms
gases and dissolved substances can often diffuse directly into and out of the cell
through the cell membrane.
This is very efficient because a
single cell has a large surface area to volume ratio membrane -
large surface area relative to the volume of the cell.
Therefore the single-celled organism
has no trouble in exchanging sufficient materials with its environment.
Know that the size and complexity of an organism increases the
difficulty of exchanging materials.
One reason for this increased
difficulty in exchanging materials is that the distance from the exchange
surface is getting further away from where the nutrients and oxygen are
needed and the waste to be removed.
Know that gas and solute exchange surfaces in humans and
other multi-cellular organisms are adapted to maximise effectiveness -
they don't have the obvious surface/volume ratio single-celled organisms
Multicellular organisms have a
smaller surface area to volume ratios compared to a single celled
This surface area is NOT sufficient to provide efficient rates
of diffusion of substances in and out of the organism without
significant adaptation through evolution - some examples are
described and explained on this page.
It is essential that the
transfer processes of moving sugars, amino acids, oxygen etc. into cells and
the removal of waste products, can happen as efficiently as possible.
Therefore exchange surfaces have
evolved to maximise the rate of transfer of wanted substances into, and unwanted chemicals out of, multicellular organisms.
To increase and maximise the efficiency of
transfer the exchange system needs to have/be ...
large surface area to increase
diffusion rate eg alveoli in lungs, villi in intestine,
permeable cell membranes are usually quite
thin to provide a short diffusion distance (part of thin layers of
cell tissue, so diffusion
distance and times
are short over a wide area),
exchange surface - gases can dissolve into and diffuse through.
Animals have lots of thin blood vessels to
bring in essential nutrient molecules and ions for life and carry waste molecules away e.g. the
thin bronchiole tubes in the lungs,
Thin capillaries which
have a particularly large surface to volume ratio - this allows fast
diffusion in either direction,
Animals need an efficient gaseous exchange
ventilation system to take in air for oxygen and give out air
including waste carbon dioxide,
in the lungs the tiny pockets
called alveoli greatly increase the gas exchange surface area :
Substance exchange problems for
The larger a multicellular
organism, the more difficult it is to exchange substances.
Cells deep in the body are some
distance to the surrounding environment - air or water.
Larger organisms have low
surface to volume ratio reducing exchange efficiency.
Therefore, through evolution,
instead of exchange through an outer membrane ('skin') multicellular
organisms have developed specialised exchange organs
including an equally specialised exchange surface.
BUT, specialised organs are not
enough on their own to serve a relatively large body, you also need
specialised transport systems to convey substances to and
from the body cells e.g. to provide nutrients or remove waste
In animals the
transport system is the circulatory system - blood
and also gaseous exchange
in lungs, the lengthy digestive system and the excretory
system - and all systems must work in harmony with each
The human circulatory system - heart, lungs, blood
In plants, transport is
effected through the xylem and phloem vessels.
Transport and gas exchange in plants,
transpiration, absorption of nutrients etc.
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3. Gas exchange in
the lungs by diffusion
- plus comments on COPD and ventilators
Examples of exchange systems are now
described in detail with diagrams
The lungs are the means of
transferring oxygen from air to the blood stream (blood plasma) and to
remove the waste gas carbon dioxide.
Know and understand that in humans:
surface area of the lungs is greatly increased by
the alveoli - millions of tiny air sacs of the end of the tiny bronchiole tubes in the lungs
where the gas exchange by diffusion takes place, but some basic stuff
before we reach the alveoli.
The diagram on the right shows the connection between
the mouth, windpipe (trachea), bronchus, bronchiole, lungs with their
alveolus and alveoli sub-structures.
Know and understand that the lungs are in the upper
part of the body (thorax), protected by the ribcage and separated from the
lower part of the body (abdomen) by the diaphragm.
should be able to recognise these structures of the lungs in the diagram on the right
- the lungs are in the thorax.
The ribcage physically protects
the lungs and heart from being easily crushed and damaged.
In between the ribs are the
intercostal muscles which help to move air in an out of the lungs
To increase the efficiency of
gas exchange in the lungs the bronchus divides in two (the bronchi), so each
lung gets a good supply of air.
Each bronchus divides and divides into many
bronchioles with a tiny sac at the end of each one - the alveoli -
tiny sacs that
considerably increases the area for oxygen and carbon dioxide gas exchange.
The 'ventilation' pathway for air
(including oxygen) is:
inhaled air ==> trachea ==>
bronchus ==> bronchiole ==> alveoli ==> individual alveolus (air
sac) in the lungs
The mouth and nasal passages
filter, warm and moisten the inhaled air, before finally reaching
Know and understand that the breathing system
takes air into and out of the body so that oxygen from the air can diffuse
into the bloodstream for respiration, and waste carbon dioxide from
respiration, can diffuse out of the bloodstream
into the air.
This gas exchange happens in the
lungs which has millions of tiny air sacs called alveoli at the ends of the
finest bronchiole tubes - a large surface area for gas exchange.
Surrounding the alveoli are many small arteries (fine
capillaries) bringing a good supply of 'dark red' deoxygenated blood to the lungs
- the thin walls of the fine capillaries of the small arteries
mean a short distance to enable faster diffusion rates for the
gases and they form a large surface area for gas exchange.
exchange occurs on the specialised moist thin membrane surfaces of the alveoli and the fine blood
vessels - the moisture in the membranes is good for dissolving gases and
increases the rate of gaseous diffusion.
When the blood from the rest of
the body arrives at the alveoli in the lungs it contains a
relatively high concentration of carbon dioxide and low
concentration of oxygen.
This maximises the diffusion
concentration gradients for the gas exchange i.e. the blood to
absorb fresh oxygen from the alveoli and the expulsion of carbon
dioxide from the blood in breathing out.
Direction of diffusion
gradients - from high to low concentration:
When air enters the alveoli
it has a greater concentration than the deoxygenated blood.
The steep concentration
gradient produces very efficient diffusion of oxygen into the
air in lungs ==> alveoli ==> blood, favours
oxygen transfer by diffusion through the alveoli membranes
Deoxygenated blood has a
greater concentration of carbon dioxide than the external air,
so it will diffuse out of the blood.
CO2 blood ==> alveoli
==> air in lungs, favours
carbon dioxide transfer by diffusion through alveoli membranes
Therefore the oxygen diffuses out
of the air into the blood capillaries of the alveoli (from high to
low concentration) and carbon dioxide diffuses out in the opposite
direction from the blood to the air in the lungs (again, from high
to low concentration).
So, oxygen, from breathing in, is transferred from the air in the
alveoli into the fine veins which carry the 'bright red' oxygenated blood away to
where it is needed in the rest of the body. Simultaneously carbon dioxide
diffuses in the opposite direction, from the deoxygenated blood into the alveoli
and breathed out.
The alveoli are well designed by
evolution to perform this gas exchange efficiently - refer to repeated
(from left to right with increasing detail)
Alveoli are very efficient
exchange surfaces and the
adaptations to increase the rate of transfer
of gas molecules are:
(i) The alveoli have a huge surface area because
of their tiny spherical sac like structure,
have a larger surface area : volume ratio than larger
spheres. For a given radius:
surface area /
volume = 3 / r. For more on this see
(ii) The sac walls are
very thin, only one cell thick, to reduce diffusion
distance and hence reduce diffusion time - giving a faster rate
of gas exchange,
(iii) The cell membrane lining is moist to
dissolve gases which can diffuse down their concentration gradients across
the exchange surface.
(iv) The alveoli have an
excellent blood supply from numerous tiny blood vessels - vein and artery
capillaries. Each alveolus is surrounded by blood capillaries that ensure
efficient transfer and the gas exchange can function down the
steepest concentration gradients.
BREATHING: Know and understand that to make air move into the
lungs the ribcage moves out and up and the diaphragm becomes flatter.
changes are reversed to make air move out of the lungs.
Know the movement of air
into and out of the lungs is known as ventilation - the mechanism of
should be able to describe the mechanism by which ventilation takes place,
including the relaxation and contraction of muscles leading to changes in
pressure in the thorax.
The lungs consist of soft sponge-like tissue protected
by the rib cage.
The diaphragm is a muscle located
underneath the ribcage. It moves up when it relaxes and down when it
breathe in, the
intercostal muscles contract, expanding the rib cage, and the diaphragm also
contracts making it flatter, both of which increase the volume of the
This has the effect of
decreasing the pressure in the lungs and allowing fresh air to be easily drawn in,
the air will flow in to the lungs naturally, due to the pressure difference between the
air in the lungs (lower pressure) and the 'outside' air (higher pressure).
Overall the air is drawn in down through the trachea
which splits in two tubes (bronchi, 2 bronchus) and further splitting of
the airways into the smaller tubes of the bronchioles with the tiny air
sacs called alveoli at their ends, where the gas exchange takes place.
breathing out, the
intercostal muscles relax (ribcage contracts), the diaphragm relaxes and
moves up, so the combined effect is to decrease the volume of the lungs and increase the air pressure and
waste air is expelled from the lungs.
of lung volume
Lung volume is the quantity of air you can breathe
in for a single breath and varies from person to person e.g.
children will have a smaller lung volume than
adults, taller people tend to have larger lung volumes and rib
Lung volume can be measured with a spirometer
machine, which, after you breathe in to full capacity, you
breathe out through a tube connected to the machine which measures
the volume of expelled air.
Your effective lung volume and surface area for gas
exchange can be reduced by disease e.g. emphysema or cancer, both
can be caused by inhalation of dust (e.g. miners) and smokers (tar
obstructive pulmonary disease (COPD)
Chronic obstructive pulmonary disease
(COPD) is the name for a group of lung conditions that cause breathing
They include emphysema – damage to
the air sacs in the lungs and chronic bronchitis – long-term
inflammation of the airways
COPD is a common condition that mainly affects middle-aged or older
adults who smoke.
The problems are often caused by
long-term exposure to irritants (particles in tobacco smoke, any
kind of fine dust e.g. mineral or coal, atmospheric particles from
vehicle exhaust) which damage and destroy the walls of the alveoli.
This means the gas exchange in
the lungs (O2 <=> CO2) is not as efficient as
it needs to be.
For COPD sufferers, the breathing problems tend to get gradually worse
over time and can limit your normal activities (readily become 'out of
breath'), although medication treatment can help keep the condition
Pandemic footnote in June 2020
As I'm adding this section on
COPD, we are in the middle of the Covid-19 coronavirus pandemic.
In the more serious cases, people
are suffering from breathing problems due to the virus causing
inflammation in the lungs - and this is where artificial ventilation
systems using oxygen are used to save lives.
to aid breathing
Ventilators move air
into and out of a persons lungs, where they cannot work unaided. This may be because some injury or medical
condition or undergoing an operation, which prevents them from breathing
This used to be done by a large
'capsule' called an 'iron lung' which encased the whole body of the patient
except for the head.
When the pump temporarily stops,
the ribcage relaxes, contracting the lungs and expelling the air.
This is a much more convenient
method with a wide range of applications, and, it doesn't interfere with the
body's blood supply, but there can be problems if the alveoli (may burst)
can't cope with the artificially increased air supply.
The human circulatory system - heart, lungs, blood,
blood vessels, causes/treatment of cardiovascular disease
Possible practical work
You can use
sensors, eg spirometers, to measure air flow and lung volume
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Gas exchange and the structure of
Fish have a single circulatory system in which
deoxygenated blood from the fish's body is pumped to the heart, which then
pumps it through the gills to absorb oxygen from the water and round through
the rest of the body in one continuous loop - just one circuit in operation
(unlike the double circulatory system of mammals).
Fish have very thin gills covered in protective
muscular flaps. Water is continuously through the mouth and forced over the
gill surfaces and ejected out through the flaps.
Gills are the gas exchange system in fishes and
the structure provides a large surface area for oxygen to be absorbed
into the blood stream and waste carbon dioxide passed out.
Water, containing dissolved oxygen, enters the fish
through its mouth and passes out through the gills facilitating gas exchange.
In the gills, oxygen diffuses from the water to the
blood, simultaneously, carbon dioxide diffuses from the blood into the
To make the gas exchange process as efficient as
possible, the surface area of the gills is greatly increased by the presence
of lots of thin plates called gill filaments - diagram on the right.
The surface area is increased even more by lots of
tiny thin tissues called lamellae (plural of lamella).
The lamellae of lots of blood capillaries, increasing
the contact area to speed up the diffusion of gases - oxygen or carbon
The lamellae also have a thin layer of surface cells
to minimise the gas diffusion distance and shorten diffusion times.
The blood flows through the lamellae in one direction
and water flows over them in the other direction and this produces a
continuous high concentration gradient between the blood and water.
The concentration of oxygen in the water is always
higher than its concentration in the blood so maintaining a good supply
of oxygen to the blood by diffusion from the water.
I presume the concentration of carbon dioxide is
higher in the blood than in the water, so the waste gas is continually
diffusing out of the blood?
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function of villi in the exchange
surface of the small
Digestion and enzymes - section on
human digestive system, metabolism and synthesis
The small intestine is about 7 m long, and is where dissolved digested
food particles are absorbed from the digestive system into the
bloodstream to supply the cells with the necessary nutrients.
The long length and large surface gives plenty
of time for the soluble food molecules to absorbed into the
bloodstream as the food moves slowly along. It takes at least 6-8
hours to travel through the small intestine.
The transfer through the partially permeable
membrane might be by 'natural' diffusion down a diffusion gradient
or by active transport against a diffusion gradient.
The partially permeable membrane regulates the
transfer of substances.
The efficiency of the process is considerably
increased by the structure of the small intestine - adaptations:
(i) a single layer of surface cells - short
diffusion time and distance - fast diffusion through the permeable
(ii) long length - increase contact time for
breakdown and absorption of food molecules.
(iii) a large surface area for absorption -
result of many small projections called villi which have microvilli
to increase the surface area even more,
(iv) and a good blood supply from numerous
capillaries that transport the nutrients away efficiently and
maintain the concentration gradient in the direction of absorption.
all of which speed up the process, so read on
for the transport details below in text and on diagram below.
Know and understand that the villi
in the small intestine provide a large surface area with an extensive
network of thin blood capillaries to absorb the products of
digestion by diffusion and active transport.
The tissue lining in the small
intestine is covered with millions of protuberances called villi, which poke
up from the intestine surface into the partially or wholly digested food
The villi consist of a single
layer of cells (thin) on the very large surface area of the intestine.
Both factors considerably speeds
up the food molecule absorption process.
Each villus (of the millions of
villi) has single layer of surface cells and each villus contains a
multitude of fine blood capillaries into which the small digested food
molecules can rapidly diffuse into and be absorbed into the body.
These molecules include amino acids, sugars like
glucose, glycerol, fatty acids and important ions of sodium, iron and
A good blood supply is needed to
efficiently carry the digested food away to where they are
The food molecules can
diffuse into the bloodstream down a normal concentration
but sometimes active transport is required.
For example ...
After a meal has been digested, the
concentration of food molecules in the blood can be higher than in
the intestine. In this situation, molecules are conveyed into the
blood by active transport e.g.
When there is a higher concentration of
glucose in the intestine than in the bloodstream, glucose molecules
will naturally diffuse into the blood stream down the
diffusion gradient (concentration gradient from higher to lower
However, if there is a lower concentration of
glucose in the intestine, your body still needs glucose for
respiration, therefore active transport must be deployed.
This uses energy in such a way as to transfer glucose molecules from
the intestine against the natural concentration (diffusion)
For more on active
Diffusion, osmosis and active transport
Enzymes - structure, functions, optimum conditions,
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Exchanges surface structure
adaptations in other animals
Tadpoles and aquatic worms
For their gas exchange, tadpoles and aquatic
worms absorb air through their skin and external gills.
The gills are feather-like projections that
produce a large surface area for gas exchange with the water - so
oxygen can be absorbed for cellular respiration and waste carbon
With adult amphibians the gas exchange takes
place mainly through their skin and lungs.
Insects do not have a transport system and
gases cannot be directly exchanged with respiring cell tissue and
the external air.
Insects have tiny holes called spiracles
all along the side of its body.
The spiracles open out into tiny tubes called
trachea - with a moist surface, through which insects pump air
in and out.
The trachea are stiffened to prevent the
minute 'tubes' collapsing.
The trachea have many minute branches called
tracheoles which connect to cells - this increases surface area and
shortens gas diffusion distance and time.
At the end of the trachea is a tiny drop of
water that connects it to the cells.
So, gases can diffuse through the trachea and
water into the cells - providing the cells with oxygen for
The spiracles can close to prevent evaporation
and keep the exchange surfaces moist.
The slug absorbs air through its skin and has
a moderately large surface area to volume ratio.
The gas exchange surface is moist to dissolve
The membrane thin for a short diffusion time.
exchange in plant leaves
For plants see
in plants notes