Gas Exchange in Animals
All living things respire – release
energy from food.
• To respire oxygen has to come in across a
membrane, be carried to the cells and
carbon dioxide released.
• Gas exchange is movement of gases
across a membrane.
• Breathing is the physical movements that
help GE.
• Animals need a respiratory surface. All
have 3 main requirements:
– Must be moist, gas has to be dissolved in
water to pass over membrane
– Large to maximise gas exchange
– Thin to allow gas to diffuse across
Different Gas Exchange
Surfaces
Diffusion across a membrane
• Protozoa, hydra and jellyfish
– It’s cells are bathed in water and as it is only 1
or 2 cells thick the diffusion is efficient and
requires no energy.
– Oxygen passes straight into cells where
concentration is lower.
Diffusion is an effective means of
transporting gases only when the
distance is
< 1 mm
Open circulatory systems greatly
increase the efficiency of
transport of substances within a
body compared to diffusion
Insects – trachea systems
• Throughout the insect is a system of
hollow tubes (trachea) that reach every
cell. They open to the outside through
spiracles (small holes) in the exoskeleton
along the side of the body. Oxygen
diffuses in through the trachea. They are
held open by chiton bands.
• Some insects also use air sacs, some
pump bodies.
Closed circulatory systems are
even more efficient than open
circulatory systems
Fig. 42.3
Fish - gills
• The respiratory system is external to body.
• It is made of thin filaments supported by
bony structures. Each filament is a
delicate plate of capillaries.
• Fish continuously pump water through
respiratory surface.
• They use the counter current
exchange system.
Fish gills
Gill capillaries are the sites of gas exchange
with the environment
Counter-current exchange helps maximize the
efficiency of gas exchange
Terrestrial vertebrates - lungs
• Inside the body cavity so needs an
efficient transport system to connect to
gas outside body.
Human Gas Exchange
• Air enters through nose passes to
pharynx, then larynx to trachea. Branches
down the two bronchi then to bronchioles
and finally alveoli. These are covered by
capillaries and O2 diffuse in and CO2 out
over the thin surface.
Ventilation of lungs
• Inhalation: breathing in, diaphragm
lowered, rib muscles contract, making
partial vacuum which pulls air in.
• Exhalation: breathing out. Everything
relaxes and forces air out.