Basic Science Clinic by Steve Morgan & Sophie Connolly
If you can’t explain it simply, you do not understand it well enough.
Welcome to Basic Science Clinic Raw Science episode 6. The next step on the oxygen cascade relates to the composition of alveolar gas, how and why it differs from that in the upper respiratory tract and conducting airways. This composition is determined by the components of the alveolar gas equation. We will examine the AGE in more detail in the next podcast, but for now we can take it to be PAO2 = PiO2 – PaCO2/RQ. In this conceptual model the PiO2 describes the gas entering the alveolus and the second half, the minus PaCO2/RQ, is the net gas leaving the alveolus as oxygen is exchanged with CO2 across the alveolar capillary membrane. The PAO2 is therefore the net alveolar oxygen partial pressure reflecting the interaction of these two processes. The composition of PiO2 we ascertained in the last podcast where humidification and warming of inspiratory gas at 1 atm leaves us with ~150 mmHg of oxygen partial pressure at the carina. Before we analyse the gas in the alveolus we are going to examine how it gets there and the factors that affect pulmonary ventilation and respiratory gas flow.
Remember deranged physiology at each transition point on the oxygen cascade may limit the efficacy of oxygen transfer and hence reduce the amount of oxygen delivered to the mitochondria. It is important to understand the ways in which these steps can be disrupted and then systematically consider them all in your assessment of undifferentiated hypoxia. Step 1 is calculating the PiO2, which is FiO2 multiplied by Patm – PH2O. Therefore reduced FiO2, for example when oxygen is consumed in a house fire, or reduced barometric pressure, for example on the peak of mount Everest, are both potential causes in reduced oxygen partial pressure at step 1 and hence are causes of downstream tissue hypoxia.
For step 2 a comprehensive understanding of the complex of interrelated factors that affect respiratory gas flow and the provision of oxygen replete inspired gas to the alveolus is crucial core knowledge for a budding critical care physician. To bear the responsibility of mechanically ventilating a patient’s potentially injured lung, it is incumbent on us to be fortified by a high fidelity conceptual model.
In this pod we will cover:
Fluids and Flow
How can you predict the type of flow in a fluid system?
How do you define viscosity?
What about the specifics of gas flow in the airways?
What is ventilation?
So how does the respiratory apparatus generate a pressure differential?
Raw Science Factoids
The total length of the airways running through the two lungs is 1,500 miles or 2,400 kilometers.
The 300-500 million alveoli produce a combined surface area of 50-100 m2, a size roughly equivalent to a tennis court.
The relatively high oxygen content of air means we would only have to breathe once per minute to meet the body’s demand for oxygen at rest, the bulk of ventilatory work is for the elimination of carbon dioxide.
For feedback, corrections and suggestions find us on twitter @falconzao and @sophmcon or post on ICN.
Thanks for listening. Next up we’ll examine the oppositional forces of respiratory gas flow and the work of breathing.