Re: Lung ventilation rates

[Gautam Majumdar <gautam@majumdar.demon.co.uk>]

In article <199609231452.JAA08069@juliet.ucs.indiana.edu>, Jeffrey
Martz <martz@holly.ColoState.EDU> writes
>Gautam Majumdar wrote:
>
>    Yeah, but isn't this an abnormal condition?  The kideys are being 
>used as an emergency back up system, but they aren't normally controlling 
>the acid base balance to this extent, are they?  How could they keep 
>this increased level of activity if the problem is not corrected?  I think 
>that the CO2 is NORMALLY the the driving force of respiration tells you 
>something.  I note that evolution has not seen fit to provide some sort of 
>back up system to control hypoxia (altering the oxygen content of the air they
>inhale isn't something most organisms can do), presumably because it isn't as 
>common a problem.  

Yes, you are right. Hypercapnia does not occur under physiological 
conditions because even in very shallow breathing with normal lungs 
CO2 is easily passed across the alveolar membrane. And animals 
cannot induce hypercapnia voluntarily (except in experimental 
conditions by breathing air artificially enriched with CO2). Kidneys are 
not the emergency back up system for maintaining the acid base 
balance. They are the prime organs for this purpose. Metabolism 
produces a multitude of organic and inorganic acids. Lungs can get 
rid of only the volatile acids, such as H2CO3. All other acids are 
excreted by the kidneys. People live for decades with chronic 
hypercapnia - kidneys are pretty efficient for this purpose.

However, animals can and do alter the oxygen content of their inhaled 
air. Birds do that during high altitude flight and terrestrial animals, 
specially humans, do that during climbing or trekking up the 
mountains. And there are compensatory mechanisms for low oxygen 
content in the inhaled air. Kidneys, those magnificent organs, sense 
hypoxia and elaborate erythropoietin which stimulates bone marrow 
to produce extra red cells and haemoglobin (compensatory 
polycythaemia). The purpose is to increase the efficiency of the 
carrier system. Secondly, the level of 2,3-DPG in the red cells goes 
up. That reduces the oxygen affinity of haemoglobin, so more 
oxygen is released into the tissues. Normally, in humans, only 60% of 
the oxygen is released into the tissues and the rest remains tied up
with the haemoglobin. Under severe hypoxia this can go up to almost 
75%. Some animals, such as whales, can release 90% of the oxygen 
from their red cells.

Gautam Majumdar                 gautam@majumdar.demon.co.uk