Title: Water - Fountain of Life
Full Citation
Permanent Link: http://ufdc.ufl.edu/WL00004717/00001
 Material Information
Title: Water - Fountain of Life
Physical Description: Book
Language: English
Publisher: Life Science Library
Spatial Coverage: North America -- United States of America -- Florida
Abstract: Jake Varn Collection - Water - Fountain of Life
General Note: Box 28, Folder 13 ( Water - 1966 ), Item 6
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
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Bibliographic ID: WL00004717
Volume ID: VID00001
Source Institution: Levin College of Law, University of Florida
Holding Location: Levin College of Law, University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Full Text



"THE WATERS OF LIFE" is more than a poetic phrase. Life actually arose
in water, to start the long line of evolution that links primitive animals
and plants, which are virtually nothing but water, to man, who is two
thirds water. Before birth, much of man's life is spent in water, in the
sheltering membranous sac of his mother's womb, and water flows
through his body till the day he dies. Man can live several weeks with-
out food; one Indian fakir survived 81 days with no nourishment what-
ever. But without water, the longest any human being can expect to live
is 10 days. Some bacteria flourish without oxygen, but neither they nor
any other form of life can grow without water. It defies barriers to pene-
trate the living cells of plants and animals, and it overcomes gravity to
climb the highest trees, bringing nutrients to their topmost branches.
The earth's water was there when the planet was formed. It lay in the
oceans before the present atmosphere was created, and it was this cir-
cumstance, according to one theory, that led to the development of life.
Today the oxygen in the atmosphere absorbs much of the sun's ultra-
violet radiation. But eons ago this energetic portion of the solar spec-
trum beat down uninterruptedly on the great primeval seas. Mixed with
the water in those days were substantial quantities of ammonia, meth-
ane and carbon dioxide-providing all the chemical elements necessary
to give rise to living molecules. The powerful ultraviolet radiation may
have stimulated the arrangement and rearrangement of these elements
into one pattern after another until finally, purely by chance, com-
pounds that could duplicate themselves were formed. If such random
syntheses, stimulated by the sun and supported by the water, continued
over hundreds of millions of years, life could have evolved. (The theory
is supported by recent experiments which reproduced on a laboratory
scale the presumed conditions of the ancient seas-and yielded complex
chemicals that are the precursors of living protein.)
Life's watery beginning continues to be reflected in all living processes,
plant and animal. The simplest single-celled organisms are surrounded
by and permeated with water. It moves in and out of their walls, bring-
ing food and oxygen along and taking wastes away. The principle is the
same but the processes are more complicated in higher forms of life.
With a few exceptions, plants make their own food from water and air.
To survive, they must act like pipelines, taking water out of the soil, de-
livering it to cells for use, and emptying whatever is left over into the
air. The water, absorbed through the fine root hairs underground, travels
upward through bundles of long, microscopic tubes penetrating the stem
and branches, and passes back to the atmosphere as transpiration
through tiny leaf pores called stomata (which also serve as entrance
and exit ports for the carbon dioxide and oxygen essential to photo-
synthesis and growth). One square inch of leaf may contain as many as
300,000 stomata, most of which are on the underside, and they release
an astonishing amount of water. Although transpiration varies with con-
ditions of temperature, humidity, light, wind, and soil moisture, it usually
totals several hundred times the dry weight of the plant itself during a

single growing season. During its lifetime, a crop of corn, for example,
may release water sufficient to cover its entire field to a depth of 11
inches. And in one warm day a single birch tree can dispose of 60 to 80
gallons of water.
The mechanics of this remarkably capacious water-handling system
are still not completely understood. The movement of water in certain
plants-very tall trees, for example-poses one of the most intriguing
puzzles of biology. What is known, however, testifies again to the dis-
tinctive characteristics of water.
Groundwater enters the root hairs of a plant by a special kind of dif-
fusion called osmosis, a fundamental process that goes on in nearly all
living tissues.
Through this process water molecules are able to cross living mem-
branes which apparently will not admit water in the form of drops of
liquid. This seeming paradox can be demonstrated with a piece of cello-
phane, which is a synthetic membrane quite similar to natural ones.
Cellophane is watertight in the sense that a drop of water placed on its
surface will not drip through; even when it is examined with an ordinary
light microscope no pores can be seen. Yet water molecules do diffuse
through cellophane despite its apparently smooth and continuous struc-
ture; as most housewives are aware, a slice of bread wrapped in cello-
phane will dry out-not so fast, of course, as if it had lain unwrapped,
yet faster than if it had been wrapped in such impermeable material as
aluminum foil.

Seeping through a membrane
A plant's membrane also seems smooth and continuous; a microscope
reveals no pores. Yet somehow water crosses to enter the plant. This
puzzling circumstance is resolved by a closer examination of the mem-
brane. There are pores after all, but they are too small to be seen with
an ordinary microscope. Like all substances, a membrane is composed
of molecules. And the molecules, no matter how tightly they are packed
together, have spaces between them. The spaces are large enough to offer
easy passage to water molecules but are far too small to let water pene-
trate in packages as large as a drop. Thus, a drop may pass through the
barrier-but only a few molecules of it at a time.
Diffusion, the force that plays a major part in pushing molecules
through the pores of a plant membrane, arises from the random move-
ments of the molecules, their continuous jiggling back and forth. They
bounce against one another and fly apart, tending always to spread from
a region where they are closely packed together to regions of lesser con-
centration. This action is the same one that diffuses dissolved molecules
throughout a liquid--and is the reason a lump of sugar eventually
sweetens a cup of coffee whether it is stirred or not.
The rate at which molecules slip through a plant's intermolecular
pores depends on the size of both the molecules and the pores. Small
molecules like those of water travel through the pores of living mem-

branes at a fairly rapid rate. The larger molecules of substances that
may be dissolved in water, such as minerals, travel through much more
slowly. It is the difference in the rate of transmission that makes the
membrane a kind of sieve; it can hold minerals on one side while letting
water pass through.
This sieving action can build up a substantial pressure. The reason
is that there are relatively more water molecules outside the plant than
inside, where minerals are present in the liquid. The water molecules
therefore flow from the region where they are proportionately more nu-
merous, or more concentrated, to the region where their concentration
is less. This osmotic pressure is great enough to raise a column of water
as high as 66 feet through the trunk of a tree.

The mystery of the climbing water
Osmotic pressure is only one of the forces that drives water from roots
through stem to the leaves of a plant. Aiding it is capillarity-the attrac-
tion between the molecules of water and those of other compounds
that pulls liquid water upward through the plant's microscopic tubes.
Capillarity alone can raise water several feet in most plants. In addi-
tion, atmospheric pressure may push water upward to fill low pressure
areas within a plant.
But none of these mechanisms can account for the enormous heights
to which water is actually delivered in trees. In the Pacific Northwest,
for instance, there are Douglas firs which tower 400 feet; since their
roots may sink 50 feet into the ground, some of the water that reaches
the treetop must be lifted a total of 450 feet. The pressure required for
this task is more than 30 times greater than the pressure of the atmos-
phere at sea level.
No one knows for certain how this astonishing feat is accomplished.
The best current theory explains it in terms of another outstanding
characteristic of water: tensile strength, or resistance to being pulled
apart. Hydrogen bonds link the molecules in liquid water so tightly that
a column of water is as strong in some ways as a tough chain. Labora-
tory experiments have demonstrated that very pure water, enclosed in
a slender, airtight tube, can withstand a pull of 5,000 pounds per square
inch-a tensile strength close to that of some metals. The tensile strength
of plant sap is less; it amounts to some 3,000 pounds per square inch.
Still this is enough to enable a slender column of sap, enclosed in one
of the fine fibrous tubes of a tree, to be lifted several thousand feet-
much higher than the height of any tree.
But water's great tensile strength does not become a factor in the
process of climbing a tree unless some kind of force is exerted on the
column. This is supplied by transpiration. When water transpires from
the leaves, it lowers the concentration in the cell walls. The cell walls
replace this lost moisture by pulling in water molecules from the liquid
in the shoot. This transfer of water molecules puts the tree's columns
of water under tension, as if they were being grabbed at the top and





AN ARTIFICIAL ROOT, this glass tube
attached to a cellophane sac pumps water
upward as a plant does. The sac contains a
sugar solution, and the cellophane lets water
but not sugar pass through it. Because the
concentration of water is greater outside
the sac, water enters, by the process of
osmosis. Thus the solution rises in the tube.

65 per cent water. The liquid permeates all
human tissue, fills cellular gaps and bony
hollows, and flows through 60,000 miles of
arteries and veins. Water in the cells makes up
41 per cent of the body's weight, the blood
plasma provides 4 per cent and the fluid
occupying empty cavities, such as
the intestines or eyeballs, comprises 5 per cent.
This distribution is not static: water knows
no anatomical boundaries and passes
constantly through membranes from one
compartment of the body to another.

hauled upward. Since these columns are continuous from leaves to
roots, the pulling at the top raises water from the bottom, a molecule at
a time.
There seem to be obvious holes in the theory. What happens if the
slender column of water breaks? This must disrupt the whole continu-
ous water-raising process. It is difficult to see how such breaks can be
avoided as trees sway in a high wind; certainly they cannot be avoided
when whole branches are lopped off. Yet neither wind nor cutting
wounds seem seriously to affect the rise of water. In certain respects,
the process remains an enigma.

A multipurpose fluid
No such mystery surrounds the circulation of water in animals. They
have mechanical pumps-hearts-to assist capillarity and osmosis in
supplying water to body parts. With this added complexity goes far
greater complexity of water use. In animals as in plants, water serves
as the common carrier, transporting food and waste, oxygen and carbon
dioxide, and it plays a critical role in the digestion of food. In animals
it also serves to lubricate the joints, lest they creak, and the soft tissues,
lest they stick. Its cooling action keeps the heat of metabolism from
becoming unbearable. Further, all of these demands require involved
systems for controlling the water. Its quantity and concentration must
be precisely regulated-either too much or too little may quickly cause
The human body gets its water from several sources. Only about 47
per cent is supplied in the most obvious way, by drinking. As much as
14 per cent of the daily requirement is manufactured by the body itself
as a by-product of the chemical process of cellular respiration. Another
39 per cent or so comes from what we think of as "solid" food. Most
foodstuffs-the living cells of vegetables and animals raised to be eaten
-contain at least as much water as the cells of human beings; only fats,
such as butter, are largely without water content. A tomato, for exam-
ple, is 94 per cent water. Meats are 50 to 70 per cent water, and bread
contains about 35 per cent.
Water is involved in the most fundamental process of the human
body. Acting on food with the help of chemical accelerators called en-
zymes, it breaks up the great molecules of carbohydrates into simple
molecular groups-chiefly the sugar glucose-that are small enough to
be absorbed through cell membranes. There these molecules can combine
with oxygen, also transported into the cell in water solution, and be-
come metabolized: the food is oxidized, or "burned," to produce energy
for the body. The products of this combustion are organic compounds
-such as starch, which can be stored, heat, which must be distributed






at once, and carbon dioxide, which is carried to the lungs. Another prod-
uct of this metabolic "burning" is additional water, which remains in
the body and joins the other water taken in from outside.
The heat of the body's metabolic fires is far greater than might be
supposed. When called on it can perform astonishing feats-such as en-
abling the primitive Indians of Tierra del Fuego to survive, without
clothes and unshivering, in their bitter, near-Arctic climate.
Water helps control this heat. It absorbs large quantities of metabolic
heat with comparatively little increase in temperature; its rapid circu-
lation throughout the body via the bloodstream enables it to carry ex-
cess heat to the surface of the body for quick release to the surrounding
air. In this sense it works exactly like the coolant in any liquid-cooled
engine. And a related property, water's high latent heat of vaporization,
further helps to protect the body from high temperatures. Water in the
form of perspiration disposes of almost three times as much heat in the
process of evaporation as the same weight of alcohol. In addition, water
protects the internal chemical processes of the body from violent fluc-
tuations in pressure, acidity and chemical composition.

The body's water-control center
While water is itself the great stabilizer of bodily processes, its own
balance must be regulated-not only in quantity but also in the concen-
tration of dissolved materials. The balance of water within the body
needs to be very precise; a variation of no more than 1 or 2 per cent from
the normal immediately makes itself felt as thirst or pain. The master
control center is the hypothalamus, a small section in the center of the
brain just above the spinal cord. The hypothalamus governs processes
that must be responded to automatically without any delay for conscious
decisions: heart action, sleeping and waking, appetite, sex, digestion and
thirst. It maintains water balance by secreting a hormone that regulates
the kidneys and also stimulates nerves at the back of the throat. True
thirst is sensed mainly there-even a man dying of lack of water might
feel no thirst if his throat could be kept moist.
Either too much or too little water can be disastrous. When a man
loses only 5 per cent of his normal body water, his skin will shrink, his
mouth and tongue will go dry, and he may experience hallucinations;
a loss of 15 per cent is usually fatal. Too much water causes nausea and
weakness, and enforced drinking-the water "cure" inflicted by some
savages on their enemies-leads successively to mental confusion, dis-
orientation, tremors, convulsions, coma and death.
Besides controlling the total amount of water, the body must care-
fully meter the materials dissolved in it. Too great a loss of salt through
heavy perspiration, for example, brings on heat cramps. The muscle cells

react to the loss of salt by contracting into hard and painful knots. Too
much salt-from drinking seawater, for example-causes a tortured
death as the cells become dried out and shriveled.
Water leaves the body by several routes. About 15 per cent is exhaled
in the breath, and perspiration evaporates another 20 per cent, al-
though this last figure may jump to 33 per cent in hot weather. The
rest is released by direct excretion. While these proportions may vary,
the combined rate of disposal is always held within narrow limits. Excre-
tion rarely falls below the minimum of about four pints per day, no
matter how dehydrated one may be. To survive, a human being must
always balance this limited loss by acquiring close to his average five
or six pints of water every day, neither much more nor much less. He
cannot live in water, but neither can he live far from it.

Adapting to arid life
Some animals and many plants are not so restricted. They can flour-
ish where water seems totally absent. They do require water, of course,
but unusual adaptations enable them to get along with small amounts,
to acquire it from hidden sources, and to survive long periods of total
drought. Desert animals conserve water by foraging at night, when
lower temperatures make water losses far less than they would be dur-
ing the day. They keep out of the sun as much as they can, often living
underground. And many of them, like the armadillo and the desert-
dwelling lizards, have developed hides that are virtually impervious to
water loss by perspiration. The pocket mouse and the kangaroo rat,
small rodents inhabiting the deserts of the southwestern United States,
may drink no free fluid as adults. These animals manufacture the water
they need by combining the oxygen in the air with hydrogen from the
dry seeds they eat. A diet of succulent plants supplies the water require-
ments of the desert tortoise of the Southwest. In a pair of bladderlike
sacs located along the inner surface of its upper shell, it can store as
much as a pint of liquid for use in time of drought. Several small species
of land snails inhabiting the Death Valley area of eastern California and
southern Nevada-one of the world's most arid regions-feed only in
the brief season when tender green vegetation is available. During the
long dry periods that intervene, they hide in the coolness of deep crev-
ices or beneath rocks, and prevent evaporation of their moisture supply
by building a wall of mucus across the opening of their shells.
Still more varied are the habits of plants. Those which must
cope with occasional droughts can simply shut off or drastically reduce
transpiration. The guard cells of the leaf stomata are designed so that
they hold the pores open only when the cells are turgid with water;
when the plants wilt, as they do when water becomes deficient, the
pores close.
Plants of the driest deserts carry this process further; their stomata
normally open only at night. Plants like ocotillo accomplish the same
end by dropping their leaves during time of drought and growing new

foliage following a rain. Cacti, whose leaves have been reduced to
spines, have a thick, waxy cuticle which prevents the escape of water.
The barrel cactus is well known for its ability to absorb and conserve
large amounts of water, and many tales are told of how it has saved the
lives of travelers dying of thirst in America's Western deserts. After a
rain, its thousands of roots take in moisture from the soil. This moisture
is then carried up into the plant, whose cylindrical body swells to
barrel-like proportions as it makes room for its water supply and shrinks
again as the supply is used up. Contrary to popular belief, however,
drinking water cannot be obtained from the barrel cactus simply by
cutting off its top and draining off its liquid contents. The inside is a
white pulpy mass which must be crushed in order to extract its bitter
but potable juice.
Another group of desert plants, the ephemerals, has adapted to take
prompt advantage of infrequent desert rain. The seeds of these plants
are remarkably long-lived, yet quick to germinate in the presence of the
slightest moisture. Ephemerals mature with amazing rapidity, complet-
ing their entire life cycle within a few weeks. These are the plants
which spread a carpet of color across the desert after a single spring
shower, then almost as quickly disappear.

Death in the desert
For man, the desert remains a hostile environment, as inhospitable
as space or the open sea. His inability to limit his body's consumption
of water or to reduce the amount it loses-his total dependence on a
regular intake of water-never confronts him more clearly than when
he ventures into arid land. In 1965 an Egyptian desert patrol on routine
duty came across the bodies of five travelers lying lifeless beneath the
searing sun. The tragedy was unique only in the detailed record the
dead had left behind. From notebooks and from photographs-one
taken only hours before their death-it was possible to reconstruct a
fairly accurate record of what had happened to them.
They were Germans living in Egypt, who had set out from Cairo on
the first Saturday in June in two Volkswagens, a sedan and a station
wagon, for a short visit to the Roman temple ruins at the Siwa oasis
300 miles across the Libyan desert. They had kept to the coast road
until they reached El Alamein in the afternoon, and had then turned
south across the trackless desert. Some time on Sunday, first the sedan,
then the wagon, broke down in the sand. At that time they had two
gallons of water and five large cans of mango juice between them.
If they had used every possible trick to reduce perspiration-had
kept themselves covered, had improvised some shade and had remained
quietly by the cars-they might have survived until found. But foolishly,
perhaps from delirium in the 140" F. heat, they changed into bathing
suits and set off to find help. In less than two days they were dead, their
bodies totally drained of moisture. By ignoring the fragile link of water
that sustains all life, they had made disaster inevitable.

The Indispensable


An adequate water supply is literally a matter of life or
death, not only for human beings but for every form of ani-
mal and plant life, from the lowliest amoeba to the tallest
redwood tree. A man would soon die if he lost as little as
12 per cent of his body's water, and almost every organism
is heavily dependent on water for better than 50 per cent
of its body weight. Water dissolves and distributes such
necessities of life as carbon dioxide, oxygen and salts. In
the human body water is essential for blood circulation,
waste removal and even muscle movements: without it, a
man could not so much as bat an eyelash.
Every organism must constantly replenish water lost
through excretion and evaporation, and each has evolved
an effective means of satisfying its need. This endless thirst
is a legacy of the sea, in which all life began. Biochemists
believe that the concentration of salt in human protoplasm
-0.9 per cent-is the same as that of the sea three billion
years ago, when the first living organisms took to the land.
Thus, in a figurative sense, man still carries within him
the primordial waters from which his ancestors emerged.

The tangled plants of the forest, their distinctive teams absorb more water than the plants retain.
leaves glistening in the rain, are equipped by na- Some is stored for later use and the rest is elim-
ture to maintain a delicate balance between too inated by the leaves-either transpired as vapor
much and too little water. Their efficient root sys- or forced out in the form of dewlike droplets.


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f ~r~3srW ~' ~~i:

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r ~L' C

~Y*C*/ Y.~~LLL~I

Drawing Moisture

from Sea and Soil

Plants, like cities, often find their
need for water far greater than the
readily available supply. And like
cities, they tend to develop increas-
ingly elaborate supply systems as
water becomes harder to find. For
some plants, like the aquatic algae
at left below, the system is quite
simple. They absorb water-which
makes up 95 per cent of their sub-
stance-by direct contact, and would
quickly perish if removed from their

water-filled environment. The semi-
aquatic marsh plant (center) merely
extends its body an inch or two into
the damp ground for all of its water.
By contrast, the land plant has
evolved a root system that can ab-
sorb moisture from as far as 30 feet
underground. A single rye plant may
have 14 billion root hairs, with a root
network totaling 380 miles. In a sin-
gle growing season, a plant may soak
up 20 times its dry weight in water.




The single-celled algae Chlamydomonas, which
float by the millions in green, scumlike clusters,
need no roots since water is filtered directly
through their cell walls. Food is manufactured
by a photosynthetic process in the chloroplast,
then stored in the cytoplasm. The eyespot dic-
tates movements by the flagella toward light.

The Marchantia, a mosslike liverwort without
roots, leaves or stem, grows on water-saturat-
ed soil. The pores of the marsh plant admit air
to tiny chambers containing water drawn up
by rootlike rhizoids. Chlorophyll cells then pro-
duce food photosynthetically, while the waxy
cutin surface prevents excessive evaporation.




When water reaches a plant's leaves, the chlo-
rophyll in the cells causes the water to react
with carbon dioxide to produce glucose, or sug-
ar-the plant's food. Excess water is transpired
through the porelike openings, called stomata.

The stem system carries water and mineral
nutrients up from the roots through the xylem,
and food, down from the leaves through the
phloem. The spongy pith tissues are storage
areas which can absorb both food and water.




The vast root network of the columbine deliv-
ers water to the intricate stem system, which
transports it slowly upward into the leaves,
supplying the plant's cells along the way. In the
leaves, water is transformed into liquid food
and is distributed by a separate network of the
stem system all the way back to the roots.

A root tip, one of the hundreds protruding be-
neath many plants, is equipped with billions
of hairs that increase its absorbing surface by
2,000 per cent. The probing root cap is pro-
tected by a hard surface layer of dead cells.


The amount of water in the human body, av-
eraging 65 per cent, varies considerably from
person to person and even from one part of the
body to another (right). A lean man may have
as much as 70 per cent of his weight in the
form of body water, while a woman, because of
her larger proportion of water-poor fatty tissues.
may be only 52 per cent water. The lowering
of the water content in the blood is what trig-
gers the hypothalamus, the brain's thirst cen-
ter, to send out its familiar demand for a drink.






FROG 78%







Animals, unlike plants, must maintain fairly rigid
percentages of water in their bodies in order to
live. Most, however, have adapted their physi-
ology to match the water levels of their environ-

A Universal

Craving for Water

ment: the desert-dwelling kangaroo rat gets
along on an absolute minimum of water, and
the pea weevil needs very little-but the jelly-
fish must remain immersed. Between these ex-

The average man has approximately
50 quarts (about 100 pounds) of water
in his body and every day he must re-
place about two and a half quarts of
it. Drinking returns about one and a
half quarts and the water content of
food brings in another quart; an extra
half-pint is produced by the metabo-
lizing of "dry" food. Like all mam-
mals, man uses water throughout his
body, from 2 per cent in tooth enamel
to 83 per cent in his blood. He may
live without food for more than two
months but would probably die with-

tremes are some surprises. Herring have about
the same water proportion as mammals. The
earthworm actually contains more water than
either the aquatic lobster or amphibious frog.

out water in less than a week. Some
mammals, however, are gifted with
unusual ability to go without water.
The donkey can survive in a desert
for four days; in the process it may
lose 30 per cent of its body weight
in water-doubling the amount that
would fatally dehydrate a human. But
while man replenishes his body wa-
ter in short doses, both by drinking
and by eating watery foods (below),
the donkey is one of the world's fast-
est, most avid drinkers. It can down
five gallons of water in two minutes.



Fruit and vegetables, which fill much of man's
water needs, tend to increase their percentages
of water as they ripen. Thus an apple seed,
only 10 per cent water, will eventually produce

fruit that is 80 per cent water. A ripe pineap- food that man can eat-baked sunflower seeds
pie or tomato is virtually saturated with water, -is 5 per cent water. The wettest is the aptly
while corn kernels are more moist than the in- named watermelon, which ripens into a sum-
edible cob on which they grow. Even the driest mer thirst-quencher that is 97 per cent water.




s ,


Most of the body's water loss is through kidney
waste fluid, or urine, which is roughly 95 per
cent water. But ordinary breathing and perspira-
tion dispose of about a quart of water per day.

The Body's

Busiest Substance

There is no stagnant water in the
body. All the water molecules present
in any part of the body at any given
moment are somewhere else seconds
later, and have been replaced by new
molecules. Much of this water is re-
circulated and used over and over
again, but close to two and a half
quarts a day-an amount equal to
the daily intake-is permanently re-
moved, or excreted, in various ways.
There is a small but steady outflow
through the tear glands, which pro-
duce a salty secretion that lubricates
and cleans the eyes. The sweat glands
use up about a pint of water each day
in cooling the skin's surface by evap-
oration. The normal breathing proc-
ess draws off another pint or so as ex-
haled air carries moisture out of the

lungs. But nowhere does the body's
water perform a more vital function
than in the kidneys, where it serves
as the medium which purges wastes
from the bloodstream. Fifteen times
an hour, all the blood in the body
passes through the two kidneys. A
total of about 2,000 quarts of blood
is "washed" each day; from this
amount two quarts of waste are re-
moved as urine. The rest is absorbed
back into the bloodstream. The kid-
neys are so efficient that even if one is
incapacitated the other can continue
cleaning the entire blood supply by
itself. If both kidneys fail, however, a
condition known as uremia results:
salts and other wastes pollute the
blood. A man cannot live more than
three weeks with uncleansed blood.

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The tea. 6t taerimnl gtadb protect the eyebals by coating them
wtrae wenVy Am that not onty washes away dust ad othsr for-
g pertles. bio that ea oe ubnctes the surface for the b Wing
yOvtM. Tbiy exertoy ducts carry the liquid to the upper lids.
w~th aweep it ove th eyes 26 times every minute. Eventuawy
the tas are ducted down into the nose. where they evaporate.

The body's two million swee glands. which wor unnoticed at
normal tempatures, step up thetr activity whoever the body
becomes oerheated. ObeVyg a ignal flthed from the brain, the
glandaexcreteweat-which is 99 per ent water. with factions
of salt and ure. The iheatthe blooed.brougt alote to the skb by
capiHaries, evaporates the sweet-a process which cools the body.

i .




The kidney is more than a mere filter. It is also a conserve of both water
and minerals, and the watchdog of the body's chemical balance. Every
one of the million or more nephrons fright) in each kidney serves as a
complete blood-filtration system Most of the blood, stripped of vital
proteins and red blood cells as it enters the nephron. is quickly reclaimed
Some 85 per cent has returned to the bloodstream by the time the re-
mainder reaches the U-shaped tube. called Henle's loop. where urine is
concentrated. Only 05 per cent of the onginal fluid is finally excreted.



Henle's loop is the key unit of the nephron
Here the proportions of waste and water in
urine are regulated according to the body's
constantly fluctuating requirements Through
intricate exchanges of water (blue arrows) and
salt (white arrows)-mncluding a recirculation of
salt from the nght to the left arm of the loop
-the concentration of urine is determined

Flushing Out

the Body's Salt

All organisms keep the amount of salt
in their bodies in balance, using wa-
ter to remove excessive amounts of
the substance. (No animal can toler-
ate a body-salt concentration of more
than 0.9 per cent.) Human kidneys,
incapable of concentrating more than
2.2 per cent salt in urine, cannot cope
with seawater, which is 3.5 per cent
salt; if a man drank enough seawater,
his body would dehydrate itself try-
ing to flush out the excess. The horse,
with even less efficient kidneys, can-
not drink water from certain brackish
streams that is perfectly fit for hu-
man consumption. But some animals
can cope with heavy doses of salt.
Two of these, the camel and kanga-
roo rat, live in the desert, where wa-
ter is scarce. The camel, even though

it can survive a 40 per cent loss of its
body weight in water, needs every
possible drop to cool its body under
the broiling sun. The nocturnal kan-
garoo rat, which never drinks at all,
metabolizes about two ounces of wa-
ter every five weeks from its diet of
dry seeds. It loses only a drop or two
in ridding itself of unwanted salt.
The whale, a mammalian sea dweller,
has kidneys so efficient that it can
drink seawater without harm. Most
marine birds and reptiles, however,
are equipped with special glands de-
signed exclusively for salt removal.
Using these glands, a thirsty gull can
drink up to one tenth of its weight in
seawater-which would be equivalent
to two gallons in man-and excrete
all excess salt in about three hours.


The amount of salt an animal can concentrate afford to use much of it for flushing; as a conse-
in its urine is directly related to its need for wa- quence, the amount of their urine is slight but
ter. Creatures such as the camel and kangaroo the concentration of salt in it is high. The kidneys
rat, which live where water is scarce, cannot of the horse, an animal which consumes large

quantities of water and excretes freely, produce
a very low percentage of salt. The whale, while
it can drink seawater, gets most of its salt and
water from the marine life on which it feeds.

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