Hi
I just came across this very interesting article on Rec.Aviation.Soaring
and though that other people would be interested in reading it. It was
written by [EMAIL PROTECTED] (Hartley Falbaum). I have reformatted
and corrected a couple of typos, but otherwise it's as written.
Enjoy.
--
Robert Hart [EMAIL PROTECTED]
+61 (0)438 385 533 http://www.hart.wattle.id.au
=========================================================
Well--here it is FWIW: Sorry its long but it does not lend itself well
to a one paragraph answer.
This is the short explanation of renal function and water management for
glider pilots. First a few explanations and basic facts.
Water exists in the human body in two basic "compartments". Inside the
cells ( intracellular fluid "ICF") and outside the cells (extracellular
fluid
"ECF"). Visualize this as small, close together, islands in a vast
swamp. This fluid is not just water, but ions and small protein
molecules. The
major players are Sodium, Potassium, Bicarbonate, and Ammonium. These
regulate cell function and acid-base balance, among other things. They
(water and solutes)are transported back and forth across cell membranes
by active "pumps", which are part of the cell, and osmotic pressure
gradients, which transfers water.
We gain water by drinking, and by metabolism of foods, as well as
extraction of water from the foods (think watermelon, celery, oranges,
etc). We lose water by breathing out water vapour, by direct
transpiration from skin, sweating, and urine production. The amounts
vary with temperature, air moisture content, ambient pressure, and exercise.
Breathing, at sea level pressure, and standard atmosphere temperature
and moisture, and at rest loses us about 375 to 500 cc per day. Much
more at
high altitude and on oxygen.Transpiration, about 250 to 350 cc per day,
while sweating and urination can be quite variable, as we all know. The
US Army Surgeons Handbook suggests that total losses can approach 15
litres per day in combat troops--flack vests, equipment, heat, exertion
and the like. Sweat contains salt, but not as concentrated as in the
plasma, or ECF/ICF.
The body produces, just by metabolising, a certain amount of wastes per
day which the kidneys must remove. This requires about 400 cc of urine
per day, assuming normal kidney function.
So here's how it works---blood flows through capillaries in the kidney,
surrounding the glomerulus. This is a filter, allowing out water and the
solutes mentioned above. Then the filtrate passes to the renal tubule (a
little tube) and the capillaries follow. The tubule is an active structure
and pumps water back into the capps but leaves some for the tubule.
Under the influence of the pituitary gland, the water is regulated by
Vasopressin (AntiDiuretic Hormone) and the Sodium by Angiotensin. ADH is
a osmotic regulator, and Angiotension is a blood pressure regulator.
Acid base balance is controlled by pumping Bicarbonate and Ammonium (
and also by the lungs, adjusting CO2). Once the tubules have done their
job, the remainder is pumped by pressure and peristalsis (muscular
contraction waves in a tubular
structure) down the ureter into the bladder.
The bladder is a muscular-walled sack which can contract involuntarily,
and this contraction can be inhibited or facilitated by conscious
control. The sphincter (think of a O-ring which can change diameter and
thickness) controls the flow, and is under voluntary control and well as
involuntary
control. (This is the secret of toilet training.). When the bladder
begins to get full, it signals the spinal cord to produce the
"micturition" (peeing) reflex, and you get the signal. This starts at
about 150 cc and comes and goes until 4-500 cc, when the bladder is
full. The reason it comes and goes is that it is a pressure signal, not
a volume signal. The bladder, being a muscle, can relax, pressure will
decrease and it becomes larger in capacity. (this is the other secret of
toilet training). Eventually, it cannot be ignored, as you all know! You
relax the sphincter, command bladder contraction facilitation and
abdominal wall contraction--and, whew! relief!
If we don't provide a sufficient flow rate to the tubules, they will
concentrate the wastes as best they can, until the pumps can no longer
overcome the osmotic pressure gradient, then they give up! If we don't
empty the bladder, we eventually cannot overcome the pressure gradient
and the tubules and the glomeruli give up. If the wastes are too
concentrated too long, we form Calcium or Oxalate stones.
So what do we need to do? We must provide mild excesses of water and
electrolytes to the system and let it take what it needs. How do we know
we have done that--? Well, we know that we need to excrete at least
400-500 cc of urine per 24hr. We know that urine should not be overly
concentrated. We know that when we sleep, our blood pressure, heart rate
and respiratory rate is decresed, thereby requiring less urine output.
Therefore we need to excrete more urine during the day. Plan 35-50 cc
per hour, and moderate concentration--light to medium yellow. If we do
that, we are well hydrated and our urinary system is functioning well-no
danger. Acid-base balance is maintained (another source of cognitive
impairment).If we drink enough to maintain those parameters, we should
be fine. Water is good if we are not doing muscular effort (as we are
not losing electrolytes much), Gatorade or Powerade if we are doing
heavy muscular effort (assemble 3 ASW20B's in a row)--but be careful.
The glucose can cause an insulin spike, and drop your blood sugar.
In the final analysis, it's really simple. Drink enough to maintain
about a 40-50cc/ hr urine output. For those who say "I don't pee much", I
say--measure--maybe it's enough after all-maybe not. To those who
measure their flights by "pee bags", well-enjoy, but you may be
overdoing it. (I
have seen a FULL 1 gallon ziplock bag come out of a glider after landing!)
I hope this helps
Hartley Falbaum
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