Abstract
The O2 cascade concept draws together areas of respiratory physiology covered in the previous few chapters. In an examination setting, it allows the examiner to assess your knowledge of more than one topic within a single question.
The O2 cascade concept draws together areas of respiratory physiology covered in the previous few chapters. In an examination setting, it allows the examiner to assess your knowledge of more than one topic within a single question.
What is the oxygen cascade?
Aerobic metabolism is the body’s most efficient method of energy production. O2 tension (PO2) is high in the atmosphere (21.2 kPa) and low in the mitochondria (<5 kPa). The O2 cascade refers to the stepwise reduction in PO2 as O2 passes from the environment to the tissues (Figure 19.1).
Figure 19.1 The O2 cascade.
Explain each of the steps in the oxygen cascade
The steps along the O2 cascade are:
Atmosphere. PB at sea level is 101.325 kPa and FiO2 of dry air is 20.93%. Atmospheric PO2 is calculated as follows:
PO2=PB×FiO2⇒PO2=101.325×20.93%=21.2kPa
Trachea. By the time inspired air reaches the carina, it has become fully saturated with water vapour. Water has a saturated vapour pressure PSVP water of 6.3 kPa at 37°C. The PO2 of humidified air at the carina is calculated as follows:
PO2=PB–PSVPwater×FiO2⇒PO2=101.325–6.3×20.93%=19.9kPa
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