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Hind Leys Biology
F211
Transport in animals 5.5
Blood vessels
Objectives
 Explain the meaning of the terms closed and open circulatory systems with
reference to the circulatory systems of insects and fish.
 Describe the structures and functions of arteries, veins and capillaries.
Open circulatory systems
The blood of mammals never gets out into the body cavity- it is always confined to the
blood vessels. However, many animals have an open circulatory system e.g. insects.
In an open circulatory system the blood is not always in blood vessels.
In insects the blood fluid circulates through
the body cavity, so the tissues and cells of
the animal are bathed directly in blood.
To aid circulation of the blood, insects have a
muscular pumping organ like the heart.
This long, muscular tube lies just beneath the
dorsal surface of the insect. Blood from body
enters the heart through pores called ostia.
The heart then pumps the blood towards the
head by peristalsis. At the head end of the
heart the blood simply pores out into the body
cavity.
Large, more active insects, such as locusts,
have open-ended tubes attached to the heart.
These direct blood towards active parts of
the body, such as the leg and wing muscles.
Figure 1 Circulation of a locust
Characteristics of Open System
 The blood flows at a very low velocity and at low pressure due to the
absence of smooth muscles.
 There is direct exchange of materials between the cells and the blood
because of the direct contact between them.
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Hind Leys Biology
F211
Transport in animals 5.5
 The respiratory pigment, when present, is dissolved in the plasma of the
blood and there are no red corpuscles.
An open system works for insects as they are small. The blood does not have to travel
far. Also, they do ot rely on blood to transport oxygen and carbon dioxide. They use a
separate system for this.
Larger organisms rely on blood to transport oxygen and carbon dioxide. In an open
circulatory system the blood remains at a low pressure and flows slowly which is not
sufficient to supply the needs of the muscles in a large, active animal.
Closed circulatory systems
In larger animals the blood always stays inside vessels. The
tissues and cells are bathed in a separate fluid, tissue fluid.
This enables the hart to pump the blood at higher pressure, so
that it travels more quickly and can deliver oxygen and
nutrients, and remove waste products more quickly.
Figure 2 Closed circulation of a fish
Fish have a closed single circulation, meaning that blood remains inside vessles that
carry it in a single pathway around the body. There must be exchange surfaces at the
gills and at the body tissues.
Characteristics of Closed System
 The speed of circulation is more rapid due to the presence of muscular and
contractile blood vessels.
 The supply and removal of materials to and from the tissues by the blood is
enhanced, thereby increasing the efficiency of circulation.
 The volume of blood flowing through a tissue or organ is regulated by the
contraction and relaxation of the muscles of the blood vessels.
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Hind Leys Biology
F211
Transport in animals 5.5
Blood vessels
Each type of blood vessel is adapted to its particular role in relation to its distance from
the heart. All types of blood vessel have an inner layer make of a single layer of cells
called the endothelium. This provides a smooth inner surface to reduce friction with the
flowing blood.
Artery
b)
Vein
Figure 3a Structure of
arteries, veins and capillaries
and b) Light microscope of a
section through an artery and a
vein (x36)
a)
Arteries
Arteries carry oxygenated (except for the pulmonary artery) blood away from the
heart. The blood is at high pressure so the artery wall is structured to withstand that
pressure.
Arteries have three layers:
 tunica intima - layer of flattened epithelium (endothelium) and supporting
connective tissue. The endothelium is folded and can unfold when the artery
stretches.
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Hind Leys Biology
F211
Transport in animals 5.5
 tunica media - smooth muscle and elastic fibres. The elastic fibres in the
tunica media allow the artery to expand when blood is pumped through it and
then return to its normal size by elastic recoil. Felt as pulse. Recoil
maintains pressure, forcing the blood along the vessel. The smooth muscle
can contract and constrict, narrowing the lumen.
 tunica adventitia (or externa) - connective tissue
With distance from the heart the arteries get progressively smaller. Smaller arteries
have a greater proportion of smooth muscle in the tunica media then do larger ones.
They have less capacity for elastic recoil but are able to undergo vasoconstriction
causing the re-routing of blood
Veins
Veins carry deoxygenated blood back to the heart (except the pulmonary arteries).
Veins have same layers as arteries, but since blood is at low pressure the walls do not
need to be thick.
 thinner walls and thinner layers of collagen, smooth muscle and elastic
tissue.
 larger lumen to ease flow of blood.
 have valves to prevent backflow
 larger veins found between panels of muscle in the limbs - contraction of
muscles provides pressure to drive blood along
Capillaries
Capillaries have very thin walls to allow exchange of materials between the blood and the
tissues via the tissue fluid.
 wall is single layer of flattened squamous epithelium/endothelium to reduce
diffusion path.
 Narrow lumen (0.7µm)- same diameter as an RBC to ensure cells are
squeezed along slowly and in close contact with capillary wall.
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Hind Leys Biology
F211
Transport in animals 5.5
1. Compare open and closed circulatory systems with reference to locusts and fish.
2. Discuss why an open circulatory system is not as efficient as a closed system.
3. Complete the table to compare the structure of arteries, veins and capillaries.
Outer layer;
collagen
fibres
Middle
layer;
elastic fibres
and smooth
muscle fibres
Artery
Present. Lots to
Arteriole
Present. Less as
Capillary
Absent to ensure
Vein
Present. Little as
give strength.
lower strength req
short diffusion path
pressure is lower.
Thick Lots of
Less elastic tissue
Absent to ensure
Present. Little
elastic tissue to
as lower pressure.
short diffusion
elastic tissue as
cope with
More muscle tissue
path.
pressure low. Little
fluctuating
to control blood
muscle as
pressure. Thick
flow by
vasodilation/constri
muscle.
vasoconstriction.
ction not need to
divert blood
Endothelium;
pavement
epithelium
Lumen
Present. Very
Present
Present. Permeable
smooth to reduce
and with pores to
friction
allow exchange.
Narrow compared
Narrow
to veins.
Present as arteries
Very narrow so
Large so less
RBCs squeezed
resistance to blood
against capillary
flow.
wall and close
contact. Gaps in
wall allow WBCs
out.
Blood;
direction,
state,
pressure,
pulse, speed
Oxygenated, apart
Oxy and deoxy.
Deoxy apart from
from pulmonary
Joins arteries to
pulmonary vein. To
artery. Away from
veins. Falling
heart. Low
heart. High
pressure. No pulse,
pressure. No pulse.
pressure. Moves in
flow slowing.
Slow flow.
Absent
Present to ensure
pulses, rapidly.
Valves
Absent except in
Absent
arteries leaving
blood flows in right
heart
direction
Table 1 Structure related to function of the different types of blood vessel
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Hind Leys Biology
F211
Transport in animals 5.5
4. Would you expect more elastic tissue in the wall of the aorta or the pulmonary
artery. Explain your answer.
5. How would you expect blood to emerge if you cut an artery and why?
6. Use Figure 4 to answer the following questions.
a) Describe and explain the changes in
the rate of blood flow as blood passes
from the aorta to the vena cava.
b) Explain the fluctuating blood pressure
in region A.
c) Explain how the rate of blood flow in
the capillaries increases the rate of
exchange.
Figure 4 Flow of blood to and from the heart
7. Table 2 shows the mean wall thickness of different blood vessels. Suggest the
letter that is most likely to refer to;
a) the aorta C b) a capillary B c) a veinE d) an arteriole D e) the renal arteryA
Blood
Mean wall
vessel
Thickness/nm
A
1.000
B
0.001
C
2.000
D
0.500
E
0.030
Table 2
This work can be reinforced using pages 73-77 of your textbook.
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