Area to Volume Ratio
Surface area: volume ratio crops up in many
exam questions. They can be questions relating to trees, plants,
fish or mammals. The question will be about the size/shape of
the particular organism or how its size/shape is adapted to its
usually adverse surroundings.
Exchange In Organisms
A small organism, like an amoeba, has a
large surface area: volume ratio and so it can take all the oxygen
it needs by diffusion across the body surface. However, a large
organism, like a mammal, has a much smaller surface area: volume
ratio, so it cannot get all the oxygen it needs in this way. (A
large surface area: volume ratio is preferable for carrying out
exchange of substances). Such large organisms need special respiratory
organs such as lungs for taking in oxygen.
- Alveoli in the lungs have a large surface
area: volume ratio meaning gas exchange in humans occurs at a
- The filaments used in gas exchange for
fish also have a large surface area: volume ratio as its surfaces
are covered in lamellae. This larger ratio means it is suitable
- The leaves of plants have a large ratio
meaning again exchange is carried out more effectively.
Heat and water loss
Heat/water loss is affected by surface area:
volume. In large organisms heat/water loss is less than in small
organisms. This is because the organism has longer pathways and
longer distances, probably more insulation so it is harder for
the heat to escape. Conversely, in smaller organisms heat/water
loss is greater than in large organisms. The organism has much
shorter pathways; all its internal organs are closer to the surface
and have less insulation.
Calculating the ratio
- Look at surface area and volume
- Check they are in the same units
- Divide the larger one by the smaller one=
- The answer: 1is the ratio, where the answer
is the figure for the larger volume
Large Mammals have difficulties
regulating body temperature in hot climates due to:
- Small Surface Area to Volume Ratio
- Less heat is lost to the environment
- Homeotherms Generate heat by metabolic
Blood vessels near the
surface of the skin help to regulate body temperature by:
- Cooling the body from the core of the
- More heat is lost due to Radiation
- More heat is lost due to Convection
- More heat is lost due to Conduction
- More heat is lost due to sweating
- Air flow over surface can be increased
The importance of a larger
body size and mass to mammals in colder climates are:
- They have a small surface area to volume
- They are homiothermic
- Lose less heat to the environment
- They have Fat for Insulation
- Lose less heat by Radiation/Conduction/Convection
Fish Gas Exchange
Structure of Respiratory Surfaces
- Gills provide a large Surface Area,
mainly given by the filaments and secondary lamellae.
- The gills are highly capillarised which
gives a good blood supply.
- Gills have a short diffusion distance;
this is provided by flattened cells in capillaries and epithelium
(surface of gill plates). This enables 02 to get into
the bloodstream faster.
- In the respiratory system of a fish there
is a countercurrent, this increases the efficiency
of gas exchange. The blood flows in the opposite direction
to water, this helps to maintain a diffusion gradient
right along the gill. A result of this more 02 can
diffuse from the water to the blood.
- Fish ventilate using unidirectional
respiration this is due to the density of water being
too great for the fish to breathe tidally as humans.
- The fish firstly expands its Buccal
Cavity creating a large surface area for the intake
- Pressure decreases in the buccal cavity lower than that of the external
atmospheric pressure and water enters down a pressure gradient.
- As the fish closes its mouth it
raises the floor of the buccal cavity, decreasing
volume, increasing pressure.
- Water is forced over the gills.
- At the same time the Opperculum cavity
bulges out, decreasing the pressure within the cavity
water is drained over the gills.
- Removal of carbon dioxide occurs as the
blood containing high concentrations of the waste gas goes to
the gills and diffuses out into the water down a diffusion gradient
(external water has lower concentrations of carbon dioxide than
levels in the blood sets up a diffusion gradient.)
Very small organisms such as those consisting
of a single cell, have no special tissues, organs or systems for
gaseous exchange. Mammals are large, multi cellular organisms
and they have a complex system for gaseous exchange. Mammals needs
such a system single celled organism does not.
Single celled organisms
- Large surface area to volume (ratio) for
- short diffusion pathway ( to all parts
- oxygen/ carbon dioxide diffuse in and
- Small surface area to volume
- long diffusion pathway
- waterproof/ gastight skinneed internal
gas exchange surface which is moist with a large s/a
Maintenance of Breathing:
At Rest (Nervous System)
- Medulla controls breathing. Impulses from
the inspiratory centre in the medulla cause contraction of breathing
- Stretch receptors are stimulated by increase
in size of thorax/lungs. Impulses are then sent to the expiratory
centre, which inhibits the inspiratory impulses.
- Chemoreceptors (in the medulla/aortic
bodies/carotid bodies) are sensitive to the rise in the carbon
dioxide level in blood. Impulses are then sent to the inspiratory
- This causes a more rapid rate of impulses
to breathing muscles. This produces larger amounts of carbon
dioxide so more oxygen is released; therefore a high rate of
respiration is maintained with more haemoglobin free to act as
a buffer.(A buffer is a substance, which can absorb hydrogen)
- Diaphragm contracts and flattens.
- Intercostal muscles contract, therefore
ribs move up and out.
- The volume of the thorax increases, decreasing
pressure below atmospheric pressure.
- Oxygen flows into large air passages i.e
Trachea => Bronchi => largest Bronchioles
- Final pathway oxygen diffuses into
alveoli along the concentration gradient. In the alveoli, oxygen
dissolves into a film of liquid, which then diffuses the short
distance into the blood capillaries.