Respiratory Physiology

Amanda,

Reynold’s values in question 2003b(13)/1999b(3)/1998b(8) seem to be the wrong way round. While obviously the are variously quoted cut-offs the direction is that laminar flow is more likely with Re 2000. Cheers, Damien…

Damien

June 6, 2010 at 10:11 pm

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- Hi Damien, looks like you are becoming a regular! The changes have been made accordingly. BTW..if you want to contribute with answers to some of the most recent SAQs, please feel free! 😉
  
  primarysaqs
  
  June 7, 2010 at 11:22 am
  
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In 1997a(7) you claim low ventilation @ bases and high @ apices when in fact is the other way around. The low V/Q @ bases and high @ apices is due to relatively bigger difference in perfusion in relation to ventilation. However both V and Q are high @ bases and low @ apices.

Tom

March 20, 2011 at 10:31 am

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Hi Amanda, thanks a lot for the site, I am embarking on my first go at the primary and it is an awesome resource! And by the way, congratulations! (I am a Newcastle local…)

I am a bit confused about this answer for 2006a(10)02b(10)95b(9) List the physiological factors which increase respiratory rate and include a brief explanation of the mechanisms by which each achieves this effect.

You state near the beginning that RR and pCO2 are directly proportionate, I think this should read are inversely proportional, as low RR equates to high pCO2? I agree that an increase in pCO2 will lead to an increase in RR but is this the same as saying they are directly proportional?

Anyway, thanks again for a great site.

cheers

Michael Hicks

October 12, 2011 at 1:49 pm

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- Hi Michael
  
  Thanks for the warm wishes – good luck with the exam! Absolutely, you are correct – the answer should read “inversely proportional”.
  
  Regards
  
  Amanda
  
  primarysaqs
  
  October 12, 2011 at 6:06 pm
  
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Thanks Amanda!

Another query, “2011a(13)/2006b(10)/2000a(5): Describe the determinants of work of breathing in an adult human at rest.”

I like your diagram, I am bit confused about the proportions of work done to overcome elastic and non-elastic forces though. On your (and other) diagrams, it looks like there is more work being done to overcome elastic forces (green area), yet your answer says 50% each, and I can’t find it quantified elsewhere…

cheers

Michael Hicks

October 23, 2011 at 3:57 pm

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- Hi Michael
  
  I am sorry, but I am unable to provide the exact reference for the text of my answer (these were only meant to be my study notes!) as it’s been a good few years since I sat – but the vast majority of information for my respiratory physiology answers were obtained from West’s Respiratory Physiology, Power & Kam’s Physiology for the Anaesthetist and Brandis’ Physiology Viva book.
  
  I hope this helps!
  
  AD
  
  primarysaqs
  
  October 23, 2011 at 6:44 pm
  
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For 2007b(10), you have a graph of pCO2/Minute Ventilation.
I believe the curve is incorrect and pCO2 should be depicted as decreasing with increased MV.

Angus

November 4, 2011 at 12:19 am

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- I think that you have gotten confused as to the meaning of the graph. The independent variable is pCO2, on the x axis – this is what is being controlled and changed. The dependent variable is minute ventilation on the y axis – this is the response that occurs at any given change of pCO2. As you change pCO2 the graph depicts that there is an increase in the minute ventilation – this occurs because of multiple factors which increase respiratory drive, which I am sure you already know!
  
  It is true that an increase in minute ventilation would then result in a decrease in pCO2 (which is part of the negative feedback loop and the reason why the respiratory centre is stimulated) – in this case the independent variable (x axis) would be minute ventilation, and we would be looking at how pCO2 changes with this (the y axis) – a different graph.
  
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  primarysaqs
  
  November 11, 2011 at 2:00 pm
  
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2000a(2)/1996a(1): List the normal values for mixed venous blood gases and explain briefly the factors determining O2 partial pressure in venous blood:

You give the O2 flux equation and then mention:
↓Hb will ↓O2 content of arterial blood without altering O2 tension, leading to ↓O2 content

I think that last statement “Leading to ↓O2 content” is wrong.
A drop in your Hb will ↓O2 content for a given O2 tension. (Ie the Y axis numbers change but the shape of curve should be same), but if tension is plotted against saturation it should not change. Therefore no changes in PvO2.

By the way your site is wonderful. Things like this is why the internet is so great.

Mike

November 17, 2011 at 7:23 pm

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- Hi
  
  “A drop in your Hb will ↓O2 content for a given O2 tension. (Ie the Y axis numbers change but the shape of curve should be same), but if tension is plotted against saturation it should not change. Therefore no changes in PvO2.”
  
  I agree with the first part of this statement, the shape of the O2/Hb dissociation curve is independent of the Hb concentration, although consequences of anaemia may alter pH, 2,3DPG etc. but we can ignore for this for simplicity. But you need to remember that O2 content is primarily determined by [Hb]xSaO2 therefore a drop in [Hb] results in a decreased O2 content in arterial blood. With unchanged oxygen consumption in the tissues this results in a lower oxygen content in mixed venous blood. To work out SvO2 you need to rewrite the oxygen content equation for mixed venous blood as SvO2 = O2 Content / [Hb]. As both Content and [Hb] are both decreased in this scenario, SvO2 must also be decreased. If the shape of the O2/Hb dissociation curve is unchanged then PvO2 must also be decreased.
  
  The key thing here is that O2 tension determines SaO2 but the resultant PvO2 is actually determined by how much oxygen content is left after passing through the tissues. The tail wagging the dog as it were.
  
  Hope this helps
  
  AD
  
  primarysaqs
  
  November 17, 2011 at 8:38 pm
  
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Amanda,
Thanks for the site, it’s been an amazing resource, I’ve got a bit lost on your derivation of the shunt equation as the final result is the wrong way round.
Thanks

Gemma

April 1, 2012 at 9:48 am

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- Hi Gemma – You might need to bone up on your algebra…the way I have written out the shunt equation may look different to what is in West but is mathematically identical. You will find that you will get a negative number divided by another negative number on the right hand side of the final equation but mathematically this will give you the same result as negative numbers divide out to a positive number. If you want the equation to look like it does in West’s simply multiply the right hand side of the final equation by -1/-1 (i.e. 1) and you will get the more familiar equation.
  
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  primarysaqs
  
  April 1, 2012 at 10:39 pm
  
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2005 A (12) With regards to the CO2 production and related Respiratory Quotients: with CHO metabolism there is a RQ of 1.0 i.e. relatively more CO2 produced compared with FFA’s. I think the arrows have just been mixed up. Thanks for the site. Awesome.

Eddie

February 8, 2014 at 10:02 am

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- Question corrected. Thanks for the pick up!
  
  primarysaqs
  
  March 4, 2014 at 2:56 pm
  
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Mark,medical student from kenya…love the page…thanks!!

mark

July 9, 2015 at 7:09 pm

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Hey Amanda

In 2004a11

It says 2,3 DPG = Diphosphoglycerol, I think it should be 2,3-Diphosphoglycerate.

Thanks, this site is great!

lindenma

August 7, 2017 at 11:34 am

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