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### Course: Health and medicine>Unit 4

Lesson 2: Gas exchange

# Alveolar gas equation - part 2

Find out how to calculate exactly how much oxygen is deep down inside your lungs! Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.

## Want to join the conversation?

• Around , Rishi talks about the Respiratory Quotient and how it is related to a person's diet. Instead of using what the person eats to compute the RQ, could a doctor measure the RQ to figure out what a person is eating?
• While the RQ can be directly measured using very cumbersome equipment (i.e. it's very rarely done in practice), doing so would provide almost no information about someone's diet since only the most extreme of diets result in a RQ that differs measurably from a value of 0.8. In the event the value was very different than 0.8, it would only tell you that the person's diet was extreme, but wouldn't necessarily inform you in what way.
• I am curious about using the alveolar gas equation at high altitude; but I got some funny numbers. For example, if I take the pressure of say Long's Peak (a popular hike near here in Denver) then it's around 450 mm Hg. So PAO2 = 0.20(450 - 47) - (40/0.8) = 30.6 mm Hg. But this just doesn't seem right -- it's way too low. I tried to think about it about and I am wondering if perhaps the vapor pressure of water changes at altitude and is no longer 47? Or if perhaps the PaCO2 is not 40? Or what is going on?
• The PaCO2 will be lower than 40mmHg because respiratory rate is going to increase at that altitude until acclimatization occurs. The pressure of water vapor will not change. If the altitude of Long's Peak is around 14,000 feet, PaO2 is going to fall dramatically. An unacclimatized person will show signs of hypoxia at around 11-20 thousand feet. Of course its different if a person lives in Denver and climbs the peak; they are already acclimatized to a higher altitude and will have compensatory mechanism in place to deal with low FiO2. I live at sea level and if I were to climb to the tip I would certainly be hypoxic.
• What is Respiratory Quotient?!
(1 vote)
• The ratio of the volume of carbon dioxide evolved to that of oxygen consumed by an organism, tissue, or cell in a given time.
• At , is the amount of CO2 the body produces only dependant on diet?
(1 vote)
• Co2 is produced by the body as a byproduct of metabolism, or in simpler terms CO2 is waste. It is removed from the body via respiration, or the entire breathing process, more specifically exhalation.
The production of CO2 is dependent on the activity of the body. For example, during exercise, there is high activity within your cells, and as we saw above, CO2 is a byproduct of cellular activity, waste. The more activity, the more waste.
With that said, THEORETICALLY, diet may have some effect if you eat a HUGE meal, but the effect would not be notable. Doing things like exercise would have a more marked effect on CO2 production.
• Could you help edit this equation to demonstrate what happens with diffusion hypoxia as a result of turning off a patients nitrous oxide flow? i.e. the massive diffusion of n2o back into the alveolar space?
(1 vote)
• When the speaker refers to R/Q, is this the same as V/Q ratio. Ventilation/ Perfusion (Q) ratio?
(1 vote)
• Why is the RQ for a person lower if they do not eat sugars? Is this based on which stage in cellular respiration that the fuel enters? For example, proteins or lipids bypassing glycolysis and jumping straight into TCA.

The second question related to that is why would a diet composed of fats or protein be more efficient than sugar? Shouldn't the body have to consume more fats or protein to produce the same amount of ATP? Glucose should yield more ATP as a more energy dense resource and should therefore be consumed at a slower rate to maintain ATP levels in the body. In that case wouldn't the body produce the same amount of CO2 regardless of the fuel source because it needs a certain amount of ATP?
(1 vote)
• Leaving off at explains PaO2 fine. It would be interesting to address PvO2, or venous oxygen content, to understand if it would work in the formula. The PAO2 equation is the basis for many related respiratory equations and I ask if this equation can be used with venous blood gas results.
(1 vote)
• What exactly is the formula for gas equation?
(1 vote)
• For the alveolar equation, why aren't we also considering the O2 that is leaving the pulmonary capillaries and re entering the alveli after being gone through the systemic circuit?
(1 vote)
• This video describes just part of the formula. In the last video of "Advanced Respiratory System Physiology", oxygen in the reverse direction is considered.
(1 vote)