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Re: My apologies, but... physiology revisited



In article <199701122138.QAA09369@juliet.ucs.indiana.edu>, "Mickey P.
Rowe" <mrowe@indiana.edu> wrote

>More recently we were told (by Douglas Orr) that:
>
>  The generation of the majority of the body heat in endotherms is not
>  a result of aerobic metabolism (which also occurs in ectotherms) but
>  is, instead, a result of muscular contraction. 
>
>For an alternate view I'll quote Ruben's _Annual Review of Physiology_
>paper from 1995:
>
>  It has long been recognized that about 70% of heat production at
>  rest in humans is generated by the internal organs (i.e. liver,
>  kidneys, brain, heart, intestines), even though they comprise only
>  about 8% of body mass (1).
>
>  1. Aschoff J, Gunther B, Kramer K. 1971. _Energiehaushalt und
>     Temperaturregulation_ Munich: Urban and Schwarzenberg.

If I may clarify something here. When measuring Resting (or Basal) 
Metabolic Rate (RMR) all voluntary (skeletal) muscular activity are 
positively eliminated. As in this test the energy expenditure for absolute 
essential bodily functions is measured, the volunteer (less commonly 
a patient) is laid recumbent in the most relaxing position, thus 
eliminating muscle contraction to maintain posture. In most cases 
s/he would be asked to close the eyes to stop ocular muscles from 
working. Therefore, it is only expected that contribution of the skeletal 
muscles would be minimum. However, a lot of essential bodily 
functions are maintained by muscle contraction. Heart is nothing but a 
bag of contractile muscle, respiration is maintained by contraction of 
the diaphragm, in fasting state (RMR is always measured after 
overnight fast) intestine's only action is peristalsis - wave-like 
contraction of the circular muscles. Though these muscles are not 
skeletal and their action is not under voluntary control, the basic 
mechanism or energy utilisation is the same.

Muscular contraction is wasteful of energy. At the best of the times 
only 50% of the energy liberated goes to work and the rest is 
dissipated as heat. Maintaining posture (isometric contraction) is even 
more wasteful as no actual work is done. So all the energy liberated is 
dissipated as heat. RMR of a 75kg man is about 80 Watt/m^2/min 
(conversion factor for kcal is x0.0144 & for kJ is x0.06) which 
increases to 225 during slow walking (2 mph). Almost a 200% 
increase. Compared to that, intensive brain work (solving 
mathematical puzzles) increases RMR by 3% and ingestion of food 
(depending on food type) increases it by 6 - 10%. 

>It's true that skeletal muscle provides the majority of our heat
>production during exercise, but it's not at all clear to me why said
>heat is to be distinguished from "aerobic metabolism" since the
>biggest difference between mammals and lizards in this regard is that
>mammals are better at maintaining high activity levels while burning
>the requisite energy aerobically.

No - it cannot be differentiated from aerobic metabolism - because it 
_is_ aerobic metabolism. Both ectotherm and endotherm animals are 
ultimately dependent on aerobic metabolism. During intense activity 
anaerobic glycolysis, which is much faster, provides the necessary 
energy and the body builds up an _oxygen debt_. A sprinter may not 
breath at all during the 10 sec dash for the tape at 100 mts, all the 
necessary energy coming from anaerobic glycolysis. But there will be 
a long recovery period during which oxygen intake will be higher than 
the basal rate. In one experiment at the end of a 4 min _running on a 
spot_ the oxygen intake increased by 500%. It fell back to about 150% 
after 4 minutes of rest but the basal state was not reached for about 
90 minutes. During this period the accumulated lactic acid was 
reconverted to glucose in the liver (muscles cannot do that, they lack 
an essential  enzyme) - the so-called Cori cycle. The ability to continue
intensive work for prolonged period, for any animal, depends on the
maximum rate of oxygen transfer (here comes the width of the trachea -
the narrowest part of the respiratory tract, among other things) and the
efficiency of the Cori cycle  to lower the lactic acid level. Efficacy of Cori
cycle depends not only on the rate of the enzymatic conversion but also
on the rate of the blood flow through the liver and the liver cell's efficacy of
extracting lactic acid from the blood. In humans (and I would think in all
mammals) oxygen supply is the rate limiting factor. A herpetologist could
illuminate us about the extant ectotherms.

Dr Gautam Majumdar MBBS, MD, MRCP, FRCPath
gautam@majumdar.demon.co.uk