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Structure of blood vessels:
Arteries: carry blood away from the heart and into arterioles
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Blood vessels and
their function

Artery Structure related to function:
The function of arteries is to transport blood rapidly under high pressure from the heart to the tissues
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They also control the
flow of blood between the two
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lumen of the arteriole
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structure (from the outside inwards):
control the volume of blood passing through them
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es from both within and outside
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The elastic Vein structure related to function:
of blood
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It then reVeins transport blood slowly, under low pressure, rom
coils when the heart relaxes (diastole) in the same way as the capillaries in tissues to the heart
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This action helps to maintain
stretching and recoiling
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tion cannot control the flow of blood to the tissues
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Overall thickness of the wall is great: this also resits the
Elastic layer is relatively thin: the low pressure of blood
Lumen: not a layer but the central cavity of the blood
vessels bursting under pressure
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There are no valves: (except in arteries leaving the heart) sure is too low to create a recoil action
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It therefore
thick wall because the pressure within the veins is too
tends not to flow backwards
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Also allows them to
be flattened easily, aiding the flow of blood within them
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There are valves at intervals throughout: because the
pressure is so low, blood could potentially flow backwards
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The valves
ensure that this pressure directs the blood towards the
heart only
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Tissue fluids supplies all of
Capillary structure related to function:
these substances to the tissues
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Tissue fluid is thereterials such as oxygen, CO2 and glucose between
fore the means by which materials are exchanged between
the blood and the cells of the body
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blood in capillaries is much slower to allow more
It is the immediate environment of cells and is, in effect where
time for exchange
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Walls consist mainly of lining layer: extremely thin,
so the distance over which diffusion occurs is short
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Numerous and highly branched: providing a large
surface area for exchange
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Brings them even
loser to the cells to which they supply oxygen = reduces diffusion distance
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Return of tissue fluid to the circulatory system:
Once tissue fluid has exchanged metabolic materials
with the cells it bathes, it is returned to the circulatory
system
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This return occurs as follows:

1
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Tissue fluid is formed from blood plasma, and the composition 2
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As a result tissue fluid provides a mostly constant environment for the cells it surrounds
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Formation of tissue fluid:

As a result, by the time the blood has reached
the venous end of the capillary network its hydrostatic pressure is usually lower than that of
the tissue fluid outside it
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Pumping by the heart
creates hydrostatic pressure at the arterial end of the capillar- 4
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This pressure causes tissue fluid to move out of the blood
plasma
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hydrostatic pressure of the tissue fluid outside the capil- 5
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2
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In addition, the plasma has lost water and still
contains proteins
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As a result, water leaves the tissue by osmosis
down a water potential gradient
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blood within the capillaries
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Therefore the final journey of metabolic materials is made in a liquid soluHowever, the combined effect of all these forces is to create
tion that bathes the tissues (tissue fluid)
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This pressure is only enough to force
small molecules out of the capillaries, leaving all cells and
proteins in the blood because these are too large to cross the
membranes
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Not all the tissue fluid can return to the capillaries the
remainder is carried back via the lymphatic system
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Initially they resemble capillaries but they gradually
merge into larger vessels that form a network
throughout the body
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The contents of the lymphatic system (lymph) are not
moved by the pumping of the heart
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Blood vessels and
their function



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