Peripheral Gas Exchange

During pulmonary gas exchange oxygen from inhaled air is diffused into the alveoli in the lungs and the wast product carbon dioxide from the body diffuses out through the alveoli to be exhaled back into the air. With the help of the cardiovascular system the freshly inhaled O2 rich blood is transported to the tissues of the body.  At this point the final stage of respiration occurs as the much needed O2 is absorbed by the tissues and the waste CO₂ that the tissues have created is diffuses back into the blood and is transported back to the lungs to be exhaled.

This exchange of gases at tissue level is called peripheral gas exchange.

Peripheral gas exchange is also known as ‘internal respiration’, as it involves the respiratory processes that occur within the tissues of the body rather than the lungs.  This can be seen in the adjacent image.

The capillaries of the cardiovascular system deliver the O₂ rich blood to the tissues of the body. 

The O₂ then diffuses across the thin capillary walls from the high concentrations in the blood to the low concentrations in the tissues. 

At the same time the CO₂ that has been produced as a waste product by the tissues diffuses through the thin capillary walls into the blood where it is transported back to the alveoli.

The process of internal respiration and the volume of gas exchange depends on several factors, these are:

1.  The amount of O₂ within the blood – the less O₂ in the blood then the less diffusion of O₂will occur
2.  The amount of CO₂ in the tissues – the less CO₂ in the tissues then the less diffusion of CO₂ will  occur
3.  The amount of blood flow – with less blood in the tissues less gas will be exchanged
4.  The local conditions at the tissue – when tissues are producing a lot of H+ ions (becoming acidic) and are getting warm from doing lots of work, the rate of diffusion of O₂ and CO₂ increases.  This ‘localised’ response to the state of the tissue allows gas exchange in working tissues to be dramatically higher than tissues that are doing very little.

Once the de-oxygenated blood from internal respiration reaches the alveoli the whole four stage process of respiration starts again:

1. Pulmonary ventilation
2. Pulmonary gas exchange
3. Respiratory gas transport
4. Peripheral gas exchange

This cycle of respiration works continuously in order to ensure the supply of O₂ to the tissues of the body and the removal of CO₂ is sufficient.  Increases and decreases in respiratory (breathing) rate will occur automatically to meet the changing demands that we place on the body.