What is the nature of oxygen? What is its purpose in the body and how does
it serve this purpose? How is the blood able to take it up at the lungs
and give it off at the cells? What becomes of it after being used? These
are questions touching the maintenance of life and they deserve careful
consideration.
*Nature of Oxygen.*--To understand the relation which oxygen sustains to
the body we must acquaint ourselves with certain of its chemical
properties. It is an element(44) of intense affinity, or combining power,
and is one of the most active of all chemical agents. It is able to
combine with most of the other elements to form chemical compounds. A
familiar example of its combining action is found in ordinary combustion,
or burning. On account of the part it plays in this process, oxygen is
called the _supporter of combustion_; but it supports combustion by the
simple method of uniting. The ashes that are left and the invisible gases
that escape into the atmosphere are the compounds formed by the uniting
process. It thus appears that oxygen, in common with the other elements,
may exist in either of two forms:
1. That in which it is in a _free_, or uncombined, condition--the form in
which it exists in the atmosphere.
2. That in which it is a part of compounds, such as the compounds formed
in combustion.
Oxygen manifests its activity to the best advantage when it is in a free
state, or, more accurately speaking, when it is passing from the free
state into one of combination. It is separated from its compounds and
brought again into a free state by overcoming with heat, or some other
force, the affinity which causes it to unite.
*How Oxygen unites.*--The chemist believes oxygen, as well as all other
substances, to be made up of exceedingly small particles, called _atoms_.
The atoms do not exist singly in either elements or compounds, but are
united with each other to form groups of atoms that are called
_molecules_. In an element the molecules are made up of one kind of atoms,
but in a compound the molecules are made up of as many kinds of atoms as
there are elements in the compound. Changes in the composition of
substances (called chemical changes) are due to rearrangements of the
atoms and the formation of new molecules. The atoms, therefore, are the
units of chemical combination. In the formation of new compounds they
unite, and in the breaking up of existing compounds they separate.
The uniting of oxygen is no exception to this general law. All of its
combinations are brought about by the uniting of its atoms. In the burning
of carbon, for example, the atoms of oxygen and the atoms of carbon unite,
forming molecules of the compound known as carbon dioxide. The chemical
formula of this compound, which is CO_2, shows the proportion in which the
atoms unite--one atom of carbon uniting with two atoms of oxygen in each of
the molecules. The affinity of oxygen for other elements, and the affinity
of other elements for oxygen, and for each other, resides in their atoms.
*Oxidation.*--The uniting of oxygen with other elements is termed
_oxidation_. This may take place slowly or rapidly, the two rates being
designated as _slow_ oxidation and _rapid_ oxidation. Examples of slow
oxidation are found in certain kinds of decay and in the rusting of iron.
Combustion is an example of rapid oxidation. Slow and rapid oxidation,
while differing widely in their effects upon surrounding objects, are
alike in that both produce heat and form compounds of oxygen. In slow
oxidation, however, the heat may come off so gradually that it is not
observed.
*Movement of Oxygen through the Body.*--Oxygen has been shown in the
preceding chapters to pass from the lungs into the blood and later to
leave the blood and, passing through the lymph, to enter the cells. That
oxygen does not become a permanent constituent of the cells is shown by
the constancy of the body weight. Nearly two pounds of oxygen per day are
known to enter the cells of the average-sized person. If this became a
permanent part of the cells, the body would increase in weight from day to
day. Since the body weight remains constant, or nearly so, we must
conclude that oxygen leaves the body about as fast as it enters. Oxygen
enters the body as a _free_ element. The form in which it leaves the body
will be understood when we realize the purpose which it serves and the
method by which it serves this purpose.
*Purpose of Oxygen in the Body.*--The question may be raised: Is it
possible for oxygen to serve a purpose in the body without remaining in
it? This, of course, depends upon what the purpose is. That it is possible
for oxygen to serve a purpose and at the same time pass on through the
place where it serves that purpose, is seen by studying the combustion in
an ordinary stove (Fig. 54). Oxygen enters at the draft and for the most
part passes out at the flue, but in passing through the stove it unites
with, or oxidizes, the fuel, causing the combustion which produces the
heat.
[Fig. 54]
Fig. 54--*Coal stove* illustrating rapid oxidation.
Now it is found that certain chemical processes, mainly oxidations, are
taking place in the body. These produce the heat for keeping it warm and
also supply other forms of energy,(45) including motion. It is the purpose
of oxygen to keep up these oxidations and, by so doing, to aid in
supplying the body with energy. It serves this purpose in much the same
way that it supports combustion, _i.e._, by uniting with, or oxidizing,
materials derived from foods that are present in the cells.
*Does Oxygen serve Other Purposes?*--It has been suggested that oxygen may
serve the purpose of oxidizing, or destroying, substances that are
injurious and of acting, in this way, as a purifying agent in the body. In
support of this view is the natural tendency of oxygen to unite with
substances and the well-known fact that oxygen is an important natural
agent in purifying water. It seems probable, therefore, that it may to a
slight extent serve this purpose in the body. It is probable also that
oxygen aids through its chemical activity in the formation of compounds
which are to become a part of the cells. Both of these uses, however, are
of minor importance when compared with _the main use of oxygen_, which _is
that of an aid in supplying energy to the body_.
*Oxygen and the Maintenance of Life.*--In the supplying of energy to the
body, one of the conditions necessary to the maintenance of life is
provided. Because oxygen is necessary to this process, and because death
quickly results when the supply of it is cut off, oxygen is frequently
called the supporter of life. This idea is misleading, for oxygen has no
more to do with the maintenance of life than have the food materials with
which it unites. Life appears to be more dependent upon oxygen than upon
food, simply because the supply of it in the body at any time is
exceedingly small. Being continually surrounded by an atmosphere
containing free oxygen, the body depends upon this as a constant source of
supply, and does not store it up. Food, on the other hand, is taken in
excess of the body's needs and stored in the various tissues, the supply
being sufficient to last for several days. When the supply of either
oxygen or food is exhausted in the body, life must cease.
*The Oxygen Movement a Necessity.*--Since _free_ oxygen is required for
keeping up the chemical changes in the cells, and since it ceases to be
free as soon as it goes into combination, its continuous movement through
the body is a necessity. The oxygen compounds must be removed as fast as
formed in order to make room for more free oxygen. This movement has
already been studied in connection with the blood and the organs of
respiration, but the consideration of certain details has been deferred
till now. By what means and in what form is the oxygen passed _to_ and
_from_ the cells?
*Passage of Oxygen through the Blood.*--In serving its purpose at the
cells, the oxygen passes twice through the blood--once as it goes toward
the cells and again as it passes from the cells to the exterior of the
body:
_Passage toward the Cells._--This is effected mainly through the hemoglobin
of the red corpuscles. At the lungs the oxygen and the hemoglobin form a
weak chemical compound that breaks up and liberates the oxygen when it
reaches the capillaries in the tissues. The separation of the oxygen from
the hemoglobin at the tissues appears to be due to two causes: first, to
the weakness of the chemical attraction between the atoms of oxygen and
the atoms that make up the hemoglobin molecule; and second, to a
difference in the so-called _oxygen pressure_ at the lungs and at the
tissues.(46)
The attraction of the oxygen and the hemoglobin is sufficient to cause
them to unite where the oxygen pressure is more than one half pound to the
square inch, but it is not sufficiently strong to cause them to unite or
to prevent their separation, if already united, where the oxygen pressure
is less than one half pound to the square inch. The oxygen pressure at the
lungs, which amounts to nearly three pounds to the square inch, easily
causes the oxygen and the hemoglobin to unite, while the almost complete
absence of any oxygen pressure at the tissues, permits their separation.
The blood in its circulation constantly flows from the place of high
oxygen pressure at the lungs to the place of low oxygen pressure at the
tissues and, in so doing, loads up with oxygen at one place and unloads it
at the other (Fig. 55).
_Passage from the Cells._--Since oxygen leaves the free state at the cells
and becomes a part of compounds, we are able to trace it from the body
only by following the course of these compounds. Three waste compounds of
importance are formed at the cells--carbon dioxide (CO2), water (H2O), and
urea (N2H4CO). The first is formed by the union of oxygen with carbon, the
second by its union with hydrogen, and the third by its union with
nitrogen, hydrogen, and carbon. These compounds are carried by the blood
to the organs of excretion, where they are removed from the body. The
water leaves the body chiefly as a liquid, the urea as a solid dissolved
in water, and the carbon dioxide as a gas. The passage of carbon dioxide
through the blood requires special consideration.
[Fig. 55]
Fig. 55--*Diagram illustrating movement, of oxygen and carbon dioxide
through the body* (S.D. Magers). Each moves from a place of relatively
high to a place of relatively low pressure. (See text.)
*Passage of Carbon Dioxide through the Blood.*--Part of the carbon dioxide
is dissolved in the plasma of the blood, and part of it is in weak
chemical combination with substances found in the plasma and in the
corpuscles. Its passage through the blood is accounted for in the same way
as the passage of the oxygen. Its ability to dissolve in liquids and to
enter into chemical combination varies as the _carbon dioxide
pressure_(47) This in turn varies with the amount of the carbon dioxide,
which is greatest at the cells (where it is formed), less in the blood,
and still less in the lungs. Because of these differences, the blood is
able to take it up at the cells and release it at the lungs (Fig. 55).
[Fig. 56]
Fig. 56--*Soap bubble* floating in a vessel of carbon dioxide, illustrating
the difference in weight between air and carbon dioxide gas.
*Properties of Carbon Dioxide.*--Carbon dioxide is a colorless gas with
little or no odor. It is classed as a heavy gas, being about one third
heavier than air(48) (Fig. 56). It does not support combustion, but on the
contrary is used to some extent to extinguish fires. It is formed by the
oxidation of carbon in the body, and by the combustion of carbon outside
of the body. It is also formed by the decay of animal and vegetable
matter. From these sources it is continually finding its way into the
atmosphere. Although not a poisonous gas, carbon dioxide may, if it
surround the body, shut out the supply of oxygen and cause death.(49)
*Final Disposition of Carbon Dioxide.*--It is readily seen that the union
of carbon and oxygen, which is continually removing oxygen from the air
and replacing it with carbon dioxide, tends to make the whole atmosphere
deficient in the one and to have an excess of the other. This tendency is
counteracted through the agency of vegetation. Green plants absorb the
carbon dioxide from the air, decompose it, build the carbon into compounds
(starch, etc.) that become a part of the plant, and return the free oxygen
to the air (Fig. 57). In doing this, they not only preserve the necessary
proportion of oxygen and carbon dioxide in the atmosphere, but also put
the carbon and oxygen in such a condition that they can again unite. The
force which enables the plant cells to decompose the carbon dioxide is
supplied by the sunlight (Chapter XII).
[Fig. 57]
Fig. 57--*Under surface* of a geranium leaf showing breathing pores, highly
magnified (O.H.).
*Summary.*--Oxygen, by uniting with materials at the cells, keeps up a
condition of chemical activity (oxidation) in the body. This supplies heat
and the other forms of bodily energy. Entering as a free element, oxygen
leaves the body as a part of the waste compounds which it helps to form.
The free oxygen is transported from the lungs to the cells by means of the
hemoglobin of the red corpuscles, while the combined oxygen in carbon
dioxide and other compounds from the cells is carried mainly by the
plasma. The limited supply of free oxygen in the body at any time makes
necessary its continuous introduction into the body.
*Exercises.*--1. Describe the properties of oxygen. How does it unite with
other elements? How does it support combustion?
2. State the purpose of oxygen in the body. What properties enable it to
fulfill this purpose?
3. What is the proof that oxygen does not remain permanently in the body?
How does the oxygen entering the body differ from the same oxygen as it
leaves the body?
4. What is the necessity for the _continuous_ introduction of oxygen into
the body, while food is introduced only at intervals?
5. How are the red corpuscles able to take up and give off oxygen? How is
the plasma able to take up and give off carbon dioxide?
6. If thirty cubic inches of air pass from the lungs at each expiration
and 4.5 per cent of this is carbon dioxide, calculate the number of cubic
feet of the gas expelled in twenty-four hours, estimating the number of
respirations at eighteen per minute.
7. What is the weight of this volume of carbon dioxide, if one cubic foot
weigh 1.79 ounces?
8. What portion of this weight is oxygen and what carbon, the ratio by
weight of carbon to oxygen in carbon dioxide being twelve to thirty-two?
9. What is the final disposition of carbon dioxide in the atmosphere?
PRACTICAL WORK
*To show the Difference between Free Oxygen and Oxygen in
Combination.*--Examine some crystals of potassium chlorate (KClO3). They
contain oxygen _in combination_ with potassium and chlorine. Place a few
of these in a small test tube and heat strongly in a gas or alcohol flame.
The crystals first melt, and the liquid which they form soon appears to
boil. If a splinter, having a spark on the end, is now inserted in the
tube, it is kindled into a flame. This shows the presence of _free_
oxygen, the heat having caused the potassium chlorate to decompose. The
difference between free and combined oxygen may also be shown by
decomposing other compounds of oxygen, such as water and mercuric oxide.
*Preparation and Properties of Oxygen.*--Intimately mix 3 grams (1/2
teaspoonful) of potassium chlorate with half its bulk of manganese
dioxide, and place the mixture in a large test tube. Close the test tube
with a tight-fitting stopper which bears a glass tube of sufficient length
and of the right shape to convey the escaping gas to a small trough or pan
partly filled with water, on the table. Fill four large-mouthed bottles
with water and, by covering with cardboard, invert each in the trough of
water. Arrange the test tube conveniently for heating, letting the end of
the glass tube terminate under the mouth of one of the bottles (Fig. 58).
Using an alcohol lamp or a Bunsen burner, heat over the greater portion of
the tube at first, but gradually concentrate the flame upon the mixture.
Do not heat too strongly, and when the gas is coming off rapidly, remove
the flame entirely, putting it back as the action slows down. After all
the bottles have been filled, remove the end of the glass tube from the
water, but leave the bottles of oxygen inverted in the trough until they
are to be used. On removing the bottles from the trough, keep the tops
covered with wet cardboard.
[Fig. 58]
Fig. 58--*Apparatus* for generating oxygen.
1. Examine a bottle of oxygen, noting its lack of color. Insert a small
burning splinter in the upper part of the bottle and observe the change in
the rate of burning. The air contains free oxygen, but it is diluted with
nitrogen. Compare this with the undiluted oxygen in the bottle as to
effect in causing the splinter to burn.
2. In a second bottle of oxygen insert a splinter without the flame, but
having a small spark on the end. As soon as the oxygen kindles the spark
into a flame, withdraw from the bottle and blow out the flame, but again
insert the spark. Repeat the experiment as long as the spark is kindled by
the oxygen into a flame. This experiment is usually performed as a test
for undiluted oxygen.
3. Make a hollow cavity in the end of a short piece of crayon. Fasten a
wire to the crayon, and fill the cavity with powdered sulphur. Ignite the
sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it
into a bottle of oxygen. Observe the change in the rate of burning, the
color of the flame, and the material formed in the bottle by the burning.
The gas remaining in the bottle is sulphur dioxide (SO2), formed by the
_uniting_ of the sulphur and the oxygen.
4. Bend a small loop on the end of a piece of picture wire. Heat the loop
in a flame and insert it in some powdered sulphur. Ignite the melted
sulphur which adheres, and insert it quickly in a bottle of oxygen.
Observe the dark, brittle material which is formed by the burning of the
iron. It is a compound of the iron with oxygen, similar to iron rust, and
formed by their uniting.
*Preparation and Properties of Carbon Dioxide.*--1. (_a_) Attach a piece of
carbon (charcoal) no larger than the end of the thumb to a piece of wire.
Ignite the charcoal in a hot flame and lower it into a vessel of oxygen.
Observe its combustion, letting it remain in the bottle until it ceases to
burn. Note that the burning has consumed a part of the carbon and has used
up the free oxygen. Has anything been formed in their stead?
(_b_) Remove the charcoal and add a little limewater. Cover the bottle
with a piece of cardboard, and bring the gas and the limewater in contact
by shaking. Note any change in the color of the limewater. If it turns
white, the presence of carbon dioxide is proved.
2. Burn a splinter in a large vessel of air, keeping the top covered. Add
limewater and shake. Note and account for the result.
3. Place several pieces of marble (limestone) in a jar holding at least
half a gallon. Barely cover the marble with water, and then add
hydrochloric acid until a gas is rapidly evolved. This gas is carbon
dioxide.
(_a_) Does it possess color?
(_b_) Insert a burning splinter to see if it supports combustion.
(_c_) Place a bottle of oxygen by the side of the vessel of carbon
dioxide. Light a splinter and extinguish the flame by lowering it into the
vessel of carbon dioxide. Withdraw immediately, and if a spark remains on
the splinter, thrust it into the bottle of oxygen. Then insert the
relighted splinter into the carbon dioxide. Repeat several times, kindling
the flame in one gas and extinguishing it in the other. Finally show that
the spark also may be extinguished by holding the splinter a little longer
in the carbon dioxide.
(_d_) Tip the jar containing the carbon dioxide over the mouth of a
tumbler, as in pouring water, though not far enough to spill the acid, and
then insert a burning splinter in the tumbler. Account for the result.
Inference as to the weight of carbon dioxide.
[Fig. 59]
Fig. 59--*Simple apparatus* for illustrating passage of oxygen through the
body.
(_e_) Review experiments (page 101) showing the presence of carbon dioxide
in the breath.
*To illustrate the General Movement of Oxygen through the Body.*--Into a
glass tube, six inches in length and open at both ends, place several
small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means
of a stopper, a smaller glass tube which is bent at right angles and which
is made to pass through a close-fitting stopper to the bottom of a small
bottle. Another small tube is fitted into a second hole in this stopper,
but terminating near the top of the bottle, and to this is connected a
rubber tube about eighteen inches in length. The arrangement is now such
that by sucking air from the top of the bottle, it is made to enter at the
distant end of the tube containing the charcoal. After filling the bottle
one third full of limewater, heat the tube containing the charcoal until
it begins to glow. Then suck the air through the apparatus (as in smoking,
without drawing it into the lungs), observing what happens both in the
tube and in the bottle. What are the proofs that the oxygen, in passing
through the tube, unites with the carbon, forms carbon dioxide, and
liberates energy? Compare the changes which the oxygen undergoes while
passing through the tube with the changes which it undergoes in passing
through the body.