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Transport
in mammals
Joanna
George-Johnson, Contributor
All
cells require nutrients and oxygen.
Waste also needs to be removed from
the cells. A small organism can easily
do this through diffusion. However,
larger or very active organisms need
a more sophisticated transport system
with a pump to ensure that the supply
meets the demand of all cells, even
those deep within the body. Diffusion
is too slow and unspecific to be used
for these larger organisms.
In
mammals, the pump is the heart. Substances
are carried in a transport medium
of the blood. The blood travels within
vessels, with substances being released
out of, or into the blood as it flows
through capillaries.
The
heart has two main functions:
1)
To carry nutrients and oxygen to all
the cells of the body.
2)
To remove carbon dioxide from the
body.
The
heart pumps the blood along a series
of tubes that are collectively called
blood vessels.
There
are three types of vessels: arteries,
capillaries and veins.
Arteries
- Carry
blood AWAY from the heart to every
tissue in the body.
- They
carry blood at very high pressure.
- The
blood flow is rapid.
- They
mainly carry blood rich with oxygen
(except for the pulmonary artery).
- They
have thick, elastic walls to withstand
the high pressure of blood from
the heart.
- Each
time the heart beats, it fires blood
into the arteries at high pressure,
so they need to be tough so they
don't burst.
- The
blood flows in pulses through the
arteries.
- They
are normally found deep within the
body.
Section
through an artery
Capillaries
- The
site of the exchange of materials
between the blood and tissues.
- Capillaries
are tiny, thin-walled vessels they
are made up of a single layer of
endothelial cells around a very
small lumen.
- Molecules
can easily move into and out of
the capillaries by diffusion. This
allows food, gas and waste molecules
to be taken to and from every cell
in the body.
- Links
arteries to veins.
- The
blood flow is smooth and slow.
- Blood
becomes deoxygenated as it passes
through.
A
section of a capillary with living cells
surrounding it.
Veins
- Take
blood from every tissue in the body
back TOWARDS the heart.
- The
blood returning from the body is
at a much lower pressure than that
leaving the heart, therefore, veins
do not need to be as strong as arteries.
- Veins
have a cross-sectional structure
that is very similar to arteries.
- One
of the obvious differences is that
they have a much wider lumen and
thinner walls.
- The
other main difference is that veins
have valves inside them, while arteries
do not.
- The
valves occur occasionally along
their length and ensure that blood
can only travel in one direction.
- They
mainly carry deoxygenated blood
(except for the pulmonary veins).
Section
through a vein
Section
of a vein showing the valves
Questions
1.
Why do large organisms need transport
systems?
2.
How does the structure of an artery
relate to its role as a blood vessel?
3.
Compare the structure of an artery
with that of a vein.
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Convent
of Mercy (Alpha Academy) athletes
pose for the camera inside the
bleachers during the fourth
and final day of the GraceKennedy/ISSA
Boys' and Girls' Athletic Championships
at the National Stadium in St
Andrew on Saturday, March 15.
- Anthony Minott/Freelance Photographer
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Joanna
George-Johnson teaches at Ardenne
High School.
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