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Why need a transport system?
Single-celled organisms, such as bacteria and amoeba
(below), can obtain nutrients and excrete waste simply by
diffusion.
nutrients
waste products
Multi-cellular organisms, such as insects, fish and mammals,
require a more specialized transport system. Why is this?
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Surface area to volume ratio
In larger organisms,
diffusion of substances
would occur far too
slowly to enable them
to survive: the rate of
diffusion increases
with the square of the
distance it has to
travel.
This is not just because of its size, however; more important
is an organism’s surface area to volume ratio.
Single-celled organisms have a very large surface area to
volume ratio, because the diffusion path is so short.
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Surface area and volume
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Components of circulatory systems
Multi-cellular animals overcome the limitations of diffusion by
having a specialized circulatory system. This comprises:

a heart

a fluid in which substances are transported

vessels through which the fluid can flow.
The two types of
circulatory system are
open (e.g. molluscs,
arthropods) and
closed (e.g.
vertebrates, a few
invertebrates).
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Open circulatory systems
An open circulatory system consists of a heart that pumps a
fluid called hemolymph through short vessels and into a
large cavity called the hemocoel.
In the hemocoel, the
hemolymph directly bathes
organs and tissues, enabling
the diffusion of substances.
heart
hemocoel
When the heart relaxes, the
hemolymph blood is sucked
back in via pores called ostia.
Hemolymph moves around the hemocoel due to the
movement of the organism.
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Closed circulatory systems
In a closed circulatory system, blood is fully enclosed
within blood vessels at all times.
From the heart, blood is
pumped through a series
of progressively smaller
vessels. In the smallest
vessels, capillaries,
substances diffuse in and
out of the blood and into
cells.
heart
capillaries
Blood then returns to the heart via a series of progressively
larger vessels.
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Closed circulatory systems
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The mammalian circulatory system
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Vertebrate circulatory system
• Adaptations in closed system
number of heart chambers differs
2
low
pressure
to body
3
4
low O2
to
body
high
pressure
& high O2
to body
What’s the adaptive value of a 4 chamber heart?
4 chamber heart is double pump = separates oxygen-rich &
oxygen-poor
blood; maintains high pressure
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Evolution of 4-chambered heart
• Selective forces
increase body size
• protection from predation
• bigger body = bigger stomach for herbivores
endothermy
• can colonize more habitats
flight
• decrease predation & increase prey capture
• Effect of higher metabolic rate
greater need for energy, fuels, O2, waste
removal
• endothermic animals need 10x energy
• need to deliver 10x fuel & O2 to cells
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Vertebrate cardiovascular system
• Chambered heart
atrium = receive blood
ventricle = pump blood out
• Blood vessels
arteries = carry blood away from heart
• arterioles
veins = return blood to heart
• venules
capillaries = thin wall, exchange / diffusion
• capillary beds = networks of capillaries
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Blood vessels
arteries
veins
artery
venules
arterioles
arterioles
capillaries
venules
veins
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Arteries: Built for high pressure pump
• Arteries
thicker walls
• provide strength for high pressure pumping
of blood
narrower diameter
elasticity
• elastic recoil helps
maintain blood
pressure even
when heart relaxes
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Veins: Built for low pressure flow
• Veins
Blood flows
thinner-walled
wider diameter
• blood travels back to heart
at low velocity & pressure
• lower pressure
toward heart
Open valve
– distant from heart
– blood must flow by skeletal muscle contractions
when we move
» squeeze blood through veins
Closed valve
valves
• in larger veins one-way valves
allow blood to flow only toward heart
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Capillaries: Built for exchange
• Capillaries
very thin walls
• lack 2 outer wall layers
• only endothelium
– enhances exchange
across capillary
diffusion
• exchange between
blood & cells
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Controlling blood flow to tissues
• Blood flow in capillaries controlled by
pre-capillary sphincters
• supply varies as blood is needed
• after a meal, blood supply to digestive tract increases
• during strenuous exercise, blood is diverted from digestive tract to skeletal
muscles
capillaries in brain, heart, kidneys & liver usually filled to capacity
sphincters
open
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sphincters
closed
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Exchange across capillary walls
Lymphatic
Fluid & solutes flows
out of capillaries to
tissues due to blood
pressure
capillary
Interstitial fluid flows
back into capillaries
due to osmosis
 plasma proteins  osmotic
pressure in capillary
• “bulk flow”
BP > OP
BP < OP
Interstitial
fluid
Blood
85% fluid returns
to capillaries
flow
Capillary
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Arteriole
15% fluid returns
via lymph
Venule
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Blood
• Plasma: liquid matrix of blood in which cells are suspended
(90% water)
• Erythrocytes (RBCs): transport O2 via hemoglobin
• Leukocytes (WBCs): defense and immunity
• Platelets: clotting
• Stem cells: pluripotent cells in the red marrow of bones
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Circulation: true or false?
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