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Hind Leys Biology
F211
Transport in animals 5.1
Transport systems
Objectives
 Explain the need for transport systems in multicellular animals in terms of size,
activity and surface-area-to-volume ratios.
 Explain the meaning of the terms single and double circulatory systems with
reference to the circulatory systems of fish and animals.
Large animal transport systems
All living cells need a supply of oxygen and nutrients to survive. They also need to remove
waste products to prevent their build up and potential toxicity. Very small animals do not
need a separate transport system as all their cells are surrounded by, or very close to
the environment in which they live. Diffusion of substances is enough to keep the cells
alive.
Once an animal has a complex anatomy with more than two layers of cells, diffusion alone
will be too slow.
There are three main factors that influence the need for a transport system;
 Size
 Surface-area-to-volume-ratio

Level of activity
Size
Organisms with several layers of cells can no longer rely on diffusion to supply oxygen
and nutrients as these will be used up by the outer layers, and will not reach the inner
layers.
Surface-area-to-volume-ratio
We have already seen that as an organism grows in size, its
surface-area-to-volume-ratio decreases. The ratio is also
affected by an organisms shape. A flatworm has a very thin,
flat body, which gives it a large surface-area-to-volume-ratio.
However, such a body form limits the overall size an organism
can reach.
Level of activity
Figure 1 Planarian flatworm
It can grow to 10 cm but is
flat and thin.
Animals need energy from food so that they can move around. Releasing energy from
food by respiration requires oxygen. If an animal is very active, its cells need a good
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Hind Leys Biology
F211
Transport in animals 5.1
supply of nutrients and oxygen. Those animals which need to maintain their body
temperature need even more energy.
Features of a good transport system
An effective transport system will include;
 A fluid/medium to carry nutrients and oxygen around- blood
 A pump to create pressure to push fluid around- heart
 Exchange surfaces to allow nutrients and oxygen to enter blood
 Tubes or vessels to carry blood
 Two circuits- one to pick up oxygen and another to deliver it to the tissues
Single and double circulatory systems
Fish have a single circulatory
system. Blood passes through the
heart once during each circulation
of blood around the body.
heart → gills → body → heart
Mammals have a circulation that
involves two separate circuits- a
double circulation. During each
circulation blood passes twice
through the heart. The heart is
effectively two pumps.
Figure 2 Comparison of circulatory systems in vertebrates
heart → body → heart → lungs → heart
Advantages of a double circulation
Efficient circulatory systems deliver oxygen and nutrients quickly to respiring tissues.
Blood can be made to flow more quickly by increasing the pressure created by the heart.
In the single circulatory system of fish;
 The blood pressure is reduced as blood passes through the tiny capillaries of
gills
 Blood therefore moves slowly to the rest of the body
 This limits the rate at which oxygen and nutrients are delivered to tissues
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Hind Leys Biology
F211
Transport in animals 5.1
This type of system is adequate and efficient since fish are not particularly active and
they do not need to maintain their body temperature.
In the mammalian double circulatory system;
 A higher pressure of blood can be used to pump blood from heart to body
 The systemic system carries blood at higher pressure and so more quickly
 A reduced pressure is used to pump blood from the heart to lungs; two
pressure system
Mammals are more active and need to maintain their body temperature. Therefore, they
have higher energy requirements and need to be able to deliver oxygen and nutrients to
respiring tissues quickly.
1. List the features of a good transport system.
2. Explain the need for a transport system by large mammals.
3. With reference to fish and mammals, compare single and double circulatory
systems.
4. What is the main advantage of the double circulation found in mammals.
5. Explain why the circulatory systems of fish and mammals are described as closed
systems.
This work can be reinforced using pages 62-64 of your textbook.
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