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Internal Respiration

Distal Respiratory Tree

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.)

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.

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.

This may be summarised in simple diagrams of "concentration gradients", such as:

Concentration of Oxygen in the blood/airways

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 ...

Concentration of Carbon Dioxide in the blood/airways

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.

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.


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.

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)

 

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

High Concentration of Oxygen (O2) in the blood
(due to exchange of gases across the alveolar-capillary membrane).

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).

Oxygen (O2) is released from the erythrocytes in the blood into the tissues of the body.

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]

 

* 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].

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