Animal Gas Exchange
Gas exchange in animals involves the intake of oxygen and its transfer to cells where it is used for
respiration and the expulsion of carbon dioxide produced as a waste product.
Insect gas exchange systems involve external openings controlled by spiracles leading to a
branching system of tubes called tracheoles which lead directly to the internal cells.
Fish possess internal gills that are divided into filaments creating a large gas exchange surface.
Mammals have paired lungs leading from the nasal cavity. The lungs are divided into a larger
number of smaller sac like structures called alveoli.
Insects are adapted to take in oxygen through
spiracles that lead to a network of trachea leading
to cells of the body. Many insects force air to move
in and out of the tracheal system by small
movements of the abdominal segments. The
simple pumping system and diffusion can only
work efficiently over short distances limiting the
size of insects. Carbon Dioxide diffuses through
body tissues very easily, and it is likely that carbon
dioxide diffuses out directly through the
exoskeleton.
Fish living in an aquatic environment rely on the
movement of water over a complex system of gills. Using a combination of water current flow and
active mouth and operculum movement fish are able to maintain a flow of oxygen carrying water
over the gills. Water leaving the operculum carries carbon dioxide.
Mammals take in oxygen through their nasal and mouth openings. Air carrying 21% oxygen is
drawn into the lungs by the raising of the rib cage and lowering of the diaphragm which creates a
negative pressure in the chest cavity resulting in outside air pressure forcing air into the lungs.
Efficient gas exchange requires an organism to maintain a moist diffusion surface. Insects are able
to survive in a range of environments including very dry conditions through the presence of a
waterproof exoskeleton that reduces water loss and spiracles that are able to open and close as
required to allow the movement of gases but reduce excess water loss. By maintaining moisture
within the tracheal system oxygen from air is able to dissolve and reach cells in solution.
Fish living in an aquatic environment do not have water loss problems. Water does however have
a higher buoyancy than air, this separates the gill filaments to ensure efficient gas exchange.
Terrestrial mammals take in oxygen via a nasal passage. As air is moved down the passage fine
airs filter out dust etc, the air is progressively warmed and moistened as it moves towards the
lungs so that when it reaches the alveoli it has a high humidity level. The lungs are positioned
within the chest cavity where chest and diaphragm movement allows inflation of the lungs but also
protects them from the external environment including cold and desiccation protection. This helps
maintain moist gas exchange surfaces.
For efficient gas exchange to be maintained the distance over which gases diffuse must be kept to
a minimum.
In insects the branching network of tracheoles lead directly to the body cells. Where the tracheoles
come in contact with a cell, oxygen moving into the cell and carbon dioxide leaving only diffuses
the thickness of the cell membrane and the thin membrane that makes up the end of the tracheole.
Fish must obtain their oxygen from water that has a lower concentration of oxygen than air and
must have an efficient exchange system. Fish gills are divided into multiple filaments. A complex
network of fine capillaries run near the surface of the gills ensuring that the distance oxygen must
diffuse is kept to a minimum. Water carrying oxygen is in direct contact with the gill filaments and
gill plates.
To further improve the efficiency of
diffusion fish have a counter current
gas exchange system with blood in
the capillaries flowing in the
opposite direction to the flow of
water over the gills. This ensures
the optimal diffusion gradient and
therefore ensures efficient
exchange.
In mammals oxygen is carried to
the alveoli. Because the lungs are
divided into many alveoli the
membrane surrounding them can be kept very thin. A fine network of capillaries run directly over
the surface of the alveoli ensuring that diffusion distances are kept to a minimum. The alveoli
would tend to collapse under normal conditions but the secretion of a surfactant keeps the alveoli
flexible and evenly inflated.
As gases must diffuse across the exchange surfaces a large surface area is required to facilitate
the gas movement required for body function.
The insect tacheole system branches out from the spiracles to contact a group of body cells. This
branching pattern results in a very large surface area being produced. This system does however
have limitations, in that the chitin that the tracheoles are constructed of is heavy. A large insect
would be too heavy to function which is another factor limiting the size to which insects can grow.
Fish ensure a large surface area by having multiple gills on each side of the body. Each gill is
divided into hundreds of filaments from which gill plates project. This structural arrangement
ensures that a large surface area is exposed to the water that flows over them. The combination of
large surface area and water flow allows fish to extract sufficient oxygen for body activity, despite
water having a lower oxygen content than air.
Mammals obtain a large surface area by dividing their
breathing system into two halves, leading to the left and right
lungs. The air tube from the nose, the trachea is split into two
bronchi that branch out into multiple bronchioles. The
bronchioles act as a passageway for incoming oxygen and
outgoing carbon dioxide. The bronchioles terminate at the
alveoli that are small sac like endings. The interior of the lungs
is very much like a sponge. This structure provides the large
surface area needed for rapid diffusion. A high oxygen level is
essential for respiration to occur. The heat generated by this
process helps maintain a constant core temperature of 370C. In
addition the high rate of diffusion provides oxygen needed for
active movement. Removal of waste carbon dioxide is also
essential to maintain efficient body function.