Internal Respiration

The processes of breathing or 'respiration' can be described in two parts: External Respiration and Internal Respiration.

Internal respiration is the processes by which gases in the air that has already been drawn into the lungs by external respiration are exchanged with gases in the blood (that has returned to the lungs after passing through tissues around the body) so that carbon dioxide (CO₂) is removed from the blood and replaced with oxygen (O₂).

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 concerns 'gasesous exchange' 'down a gradient'. That is, gases that are at a high concentration are reduced to a lower concentration and vice-versa.

This can be represented in diagrams of 'concentration gradients':

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 which is a narrow blood vessel whose boundaries are only one cell thick (capillary shown in dark pink), next to the alveolar-capillary membrane of an alveolus (shown in pale brown).

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 alveolus and the blood occurs by diffusion of the gases through the very fine structures of the boundary (or 'wall') of the capillary and the alveolar-membrane separating the capillary from the alveolus. This diffusion 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 very efficient, hence also very fast.

* The average total surface area of the alveolar-capillary membrane is approx. 50-100m², which is similar to the size of two tennis courts.

Summary of oxygen flow between tissue-types in the lungs-blood-body tissues: