Internal Respiration
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The processes of breathing or "respiration" are
commonly described in two parts: External Respiration
and Internal Respiration.
This page is about internal respiration,
which is the processes by which the gases
in the air that has already been drawn into
the lungs by external respiration are
exchanged with gases in the blood/tissues
so that carbon dioxide (CO2)
is removed from the blood and replaced with
oxygen (O2).
(External
respiration concerns the processes by which external
air is drawn into the body
in order to supply the body with oxygen gas, and
the (used) air is expelled from the lungs
in order to remove carbon dioxide from to body. It
is
described
on the previous
page.)
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The processes of internal respiration
take place in the distal respiratory tree, which is
described at the bottom of the page about the tracheobroncial
tree.
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Internal respiration is based on "gasesous
exchange" "down a gradient".
That is, gases that are at a high concentration are reduced
to a low concentration, and vice-versa.
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This may be summarised in simple diagrams
of "concentration gradients", such as: |
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Concentration
of Oxygen in the blood/airways |
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When the concentration of oxygen in the airways of
the lungs (including the alveolar sacs) is high relative
to the concentration of oxygen in the blood (passing
through the capillaries that cover the alveoli), the
concentration of oxygen in the lungs decreases while
the concentration of oxygen in the blood increases.
This is sometimes described as oxygen "travelling down
the concentration gradient", from the (high) concentration
in the lungs, to the (low) concentration in the blood. |
While the oxygen travels from the lungs
into the blood ... |
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Concentration
of Carbon Dioxide in the blood/airways
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When the concentration of carbon dioxide in
the blood (passing
through the capillaries that cover the alveoli) is high
relative to the concentration of carbon dioxide in the
airways of the lungs (including the alveolar sacs), then
concentration of carbon dioxide in the blood decreases while
the concentration of carbon dioxide in the lungs increases.
This is sometimes described as carbon dioxide "travelling
down the concentration gradient", from the (high)
concentration in the blood to the (low) concentration
in the lungs.
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This movement of gas particles occurs through
the alveolar-capillary membrane is shown in the following
diagram.
This is a simple representation of blood flowing through
a capillary next to the alveolar-capillary membrane of an
alveolus (for more about the rest of these structures see
the distal
respiratory tree). |
The blood corpuscles that carry carbon dioxide and/or
oxygen in the blood deliver carbon dioxide to the alveolus
because the concentration of carbon dioxide is higher
in the incoming blood than in the alveolus filled with
freshly inhaled air.
As the carbon dixoide leaves the blood corpuscles
they are "re-filled" with oxygen supplied
by the oxygen in the alveolus because the concentration
of oxygen
is higher in the freshly inhaled air in the alveolus
than in the incoming blood. |
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The exchange of gases between the alveoli and the blood occurs
by diffusion of the gases through the tissues and is driven
by the tendency for equalisation of pressures of the gases
on each side of the alveolar-capillary membrane, as well
as the tendency for fluids to diffuse from high- to lower-
concentrations (when free to do so). The extremely large*
total surface area of alveoli in the lungs makes this process
extremely efficient, and therefore also very fast.
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Summary of
oxygen flow between tissue-types in the lungs-blood-body
tissues:
(Some people find Flow-Charts simpler to remember than
text or diagrams)
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High Concentration of Oxygen (O2)
in the Lungs
and
Low Concentration of Oxygen (O2)
in the Blood,
85% of the O2 is carried by
erythrocytes
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High Concentration of Oxygen (O2)
in the blood
(due to exchange of gases across the alveolar-capillary
membrane).
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Oxygen (O2) concentration
in the blood is high compared with the concentration
of oxygen in
tissues throughout the body (tissues through
which blood flows via capillaries located
through tissues all over the body).
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Oxygen (O2) is released from the
erythrocytes in the blood into the tissues
of the body.
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Concentration of oxygen (O2) in
the blood is reduced.
So that by the time that blood returns to the lungs
...
... the concentration of oxygen (O2)
in the blood is lower than the
concentration of oxygen (O2)
in the lungs 
[Return
to top of Flow Chart]
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* The average total surface area of the alveolar-capillary
membrane is approx. 50-100m2, which is similar
to the size of two tennis courts. |
Next: See the page about [to
be added soon]. |
