[Hans Blom, 961024f]
(Bill Powers (961022.1100 MDT))
It is a truism in physiology -- and I tend to see psychology as the
"higher levels" of physiology -- that all functions of the body
have as their single purpose homeostasis, i.e. the quality of life
of the individual cells.
I would say that all functions of the body have as their purpose
whatever reference level the associated control systems (if any)
have for the variables they control. I don't understand why you want
to introduce a sweeping and untestable generalization like that.
To familiarize you with physiology ;-). In fact, physiology hardly
talks about reference levels, although it does talk a lot about
"normal" levels. In Guyton's familiar textbook, for instance, he
immediately introduces the notions of homeostasis, negative feedback,
and even loop gain in his introduction, but NOT reference levels.
Why? One can speculate. One reason might be that the values of
reference levels would turn out to have ridiculously unintuitive
values if the loop gain is small, which is often the case. Another
reason might be that -- and that is expressed explicitly -- that
there are so enourmously many interacting control systems, that it is
impossible to isolate a certain reference level; they are
continuously modulated by a great many ongoing variations in other
control loops.
Rather than arrive at precise quantitative statements, a physiologist
would say somelike more qualitative like "every organ or vascular bed
autoregulates the blood flowing through it according to its metabolic
requirements" and will also indicate how he thinks this is (mainly)
done: "through varying the diameters of the fourth order arterioles
[small arteries] that supply blood to the organ", and more precisely
"by constriction or relaxation of the smooth musculature in the
arteriolar walls". This is essentially Ohm's law for fluids: raise
the resistance and the blood flow decreases, and with it the oxygen
volume carried to the tissues.
But at another place the same author might say that when one organ
requires an inordinate amount of oxygen, the metabolism of other
tissues will be downregulated. It is difficult to think clearly after
a copious meal, for instance. It all hangs together.
So think of the above not as _my_ statement but as the way a
physiologist would look at it.
I can't think of any justification for saying that cells control for
"quality of life." I don't think they can perceived such a thing,
nor that they control a variable that indicates "quality of life."
Aren't you being a bit anthropomorphic? Is an erythrocyte really
concerned with "quality of life?"
I (the totally of all my cells) can talk about "the quality of life".
A cell cannot talk. Yet they can -- and do -- let "me" know what they
need. If the cells lack sugars, they force "me" to go hunting for
food. If they are almost starving, I will feel pretty awful. And that
awful feeling can be expressed with words like "quality of life".
Through a great many levels, of course. So yes, an erythrocyte is
really concerned with the quality of life. If there is a lack of
erythrocytes, the body will do all it can to replenish them. If that
succeeds, I'll feel a whole lot better.
Since O2 and C02 are normally inversely related, it makes sense that
only one of them would be regulated; to try to regulate both
independently would lead to conflict.
This is not true. The respiratory quotient RQ (the expired volume of
CO2 divided by the consumed O2 volume) can range between about 0.7
and 1.0, depending on diet and level of exercise. Both O2 and CO2
need to be regulated; the cells require correct partial pressures of
both, regardless of how we eat and what we do. And they have all
sorts of safety mechanisms to ensure their health: if you overexert
yourself, the body will soon force you to slow down or give up,
regardless of what you might consciously want. So who is the boss?
You want to lift that heavy weight above your head and keep it there.
The body prevents you from doing so after a while; it starts to hurt.
Who wins?
Under unusual conditions (high altitude, artificially-maintained low
CO2 ambient concentrations) the increased losses of CO2 can slow
breathing to the point of oxygen starvation, at least before the
whole system readjusts to the new conditions.
That is right but something else altogether. The body is much more
sensitive to incorrect CO2 levels than to incorrect O2 levels. It is
indeed possible, in extreme cases, to die (happily and without
noticing it) of lack of oxygen if CO2 levels remain correct, e.g.
when you climb the Mount Everest. Not so with CO2. A slightly too
high pCO2 will cause strenuous respiration. Why is this so? That is
not altogether clear.
The biggest problem in indentifying physiological control systems is
that the measures which are most convenient to obtain (like partial
pressures) are not necessarily directly related to the variables
that are under control.
Yes, that is a real problem: what are the variables that are _really_
under control, i.e. the most intrinsic reference levels. Maybe even
physiology doesn't get there, except maybe cell physiology.
In evaluating a possible control system, one of the points you have
to keep in mind is that the control system may be sensing the
controlled variable at one place, while you (even if you have
guessed right as to what it is) may be measuring it in another place
and disturbing it in still another place.
Physiologists are usually acutely aware of this. They know that the
blood pressure is regulated; its range is limited. They have also
discovered some of the baroreceptors: one in the aortic arch, and one
in the brain. The former is thought to prevent too high pressures
that might damage the arteries. The latter is thought to ensure a
sufficiently high perfusion pressure for the brain. One mostly
regulates DOWN, the other mostly UP; together they seem to "discover"
a best compromise when all is well. Physiologists usually think in
terms of equilibria between systems rather than conflicts.
Natural buffering does make the control task easier, to be sure.
Mainly it reduces the effect of transient disturbances, in effect
putting a low-pass filter in series with them. Buffering, however,
is of little use in the long term; energy stores in the bloodstream,
whether from oxygen or glucose, must in the long run be replaced at
the same rate at which they are depleted.
Sure, but only on average. Without oxygen buffering, for instance, we
would not be able to remain under water for several minutes, with all
the advantages that this might bring (think of seals and whales).
Buffering is extremely important: it allows a whole slew of behaviors
that would not be possible otherwise.
It's possible that many traditional studies have under-measured loop
gain simply because they didn't wait long enough to see the effect
of a protracted constant disturbance. This would be particularly
true if the experimenter thought of a "disturbance" as a brief
event.
I don't think that physiologists usually make this error. In many
cases, of course, a protracted constant disturbance (e.g. lack of
oxygen) just is not possible. But even if a brief event must be
employed, they are aware that the response to it must be measured for
a far longer time. And if the system is not too non-linear, both
tests will provide about the same information. A more basic problem
is, however, that there seem to be many redundancies: if one system
fails, we might not notice it at all -- or only in extreme cases --
because a different system takes over. We have TWO lungs, although
normally one would be enough. TWO kidneys. Large redundancies in
(some) brain tissues. Many parallel nerve and muscle fibers. Etc.
MDs seem to agree that, on a complete medical examination of every
individual, people would have an average of five "problems". Without
ever noticing. We adapt to our limitations, often without realizing
that those limitations are there, except maybe intellectually.
That's what makes investigation of a single isolated subsystem so
difficult.
Greetings,
Hans