_Demonstration._--Structure of blood, fresh frog's blood and
human blood. Drawings.
_Demonstration._--Clotting of blood.
_Demonstration._--Use of models to demonstrate that the
heart is a force pump.
_Demonstration._--Capillary circulation in web of frog's
foot or tadpole's tail. Drawing.
_Home or laboratory exercise._--On relation of exercise on
rate of heart beat.
Function of the Blood.--The chief function of the digestive tract is to
change foods to such form that they can be absorbed through the walls of
the food tube and become part of the blood.[45]
Footnote 45: This change is due to the action of certain
enzymes upon the nutrients in various foods. But we also
find that peptones are changed back again to proteins when
once in the blood. This appears to be due to the
_reversible_ action of the enzymes acting upon them. (See
page 307.)
If we examine under the microscope a drop of blood taken from the frog or
man, we find it made up of a fluid called _plasma_ and two kinds of bodies,
the so-called _red corpuscles_ and _colorless corpuscles_, floating in this
plasma.
Composition of Plasma.--The plasma of blood is found to be largely (about
90 per cent) water. It also contains a considerable amount of protein, some
sugar, fat, and mineral material. It is, then, the medium which holds the
fluid food that has been absorbed from within the intestine. This food is
pumped to the body cells where, as work is performed, oxidation takes place
and heat is given off as a form of energy. The almost constant temperature
of the body is also due to the blood, which brings to the surface of the
body much of the heat given off by oxidation of food in the muscles and
other tissues. When the blood returns from the tissues where the food is
oxidized, the plasma brings back with it to the lungs part of the carbon
dioxide liberated where oxidation has taken place. Some waste products, to
be spoken of later, are also found in the plasma.
[Illustration: Human blood as seen under the high power of the compound
microscope; at the extreme right is a colorless corpuscle.]
The Red Blood Corpuscle; its Structure and Functions.--The red corpuscle in
the blood of the frog is a true cell of disklike form, containing a
nucleus. The red corpuscle of man is made in the red marrow of bones and in
its young stages has a nucleus. In its adult form, however, it lacks a
nucleus. Its form is that of a biconcave disk. So small and so numerous are
these corpuscles that over five million are found in a cubic centimeter of
normal blood. They make up almost one half the total volume of the blood.
The color, which is found to be a dirty yellow when separate corpuscles are
viewed under the microscope, is due to a protein material called
_haemoglobin_. Haemoglobin contains a large amount of iron. It has the power
of uniting very readily with oxygen whenever that gas is abundant, and,
after having absorbed it, of giving it up to the surrounding media, when
oxygen is there present in smaller amounts than in the corpuscle. This
function of carrying oxygen is the most important function of the red
corpuscle, although the red corpuscle also removes part of the carbon
dioxide from the tissues on their return to the lungs. The taking up of
oxygen is accompanied by a change in color of the mass of corpuscles from a
dull red to a bright scarlet.
Clotting of Blood.--If fresh beef blood is allowed to stand overnight,
it will be found to have separated into two parts, a dark red, almost solid
_clot_ and a thin, straw-colored liquid called _serum_. Serum is found to
be made up of about 90 per cent water, 8 per cent protein, 1 per cent
other organic foods, and 1 per cent mineral substances. In these respects
it very closely resembles the fluid food that is absorbed from the
intestines.
If another jar of fresh beef blood is poured into a pan and briskly whipped
with a bundle of little rods (or with an egg beater), a stringy substance
will be found to stick to the rods. This, if washed carefully, is seen to
be almost colorless. Tested with nitric acid and ammonia, it is found to
contain a protein substance which is called _fibrin_.
Blood plasma, then, is made up of a fluid portion of serum, and fibrin,
which, although in a fluid state in the blood vessels within the body,
coagulates when blood is removed from the blood vessels. This coagulation
aids in making a blood clot. A clot is simply a mass of fibrin threads with
a large number of corpuscles tangled within. The clotting of blood is of
great physiological importance, for otherwise we might bleed to death even
from a small wound.
Blood Plates.--In blood within the circulatory system of the body, the
fibrin is held in a fluid state called _fibrinogen_. An enzyme, acting upon
this fibrinogen, the soluble protein in the blood, causes it to change to
an insoluble form, the fibrin of the clot. This change seems to be due to
the action of minute bodies in the blood known as _blood plates_. Under
abnormal conditions these blood plates break down, releasing some
substances which eventually cause this enzyme to do its work.
[Illustration: A small artery (_A_) breaking up into capillaries (_c_)
which unite to form a vein (_V_). Note at (_P_) several colorless
corpuscles, which are fighting bacteria at that point.]
The Colorless Corpuscle; Structure and Functions.--A colorless corpuscle is
a cell irregular in outline, the shape of which is constantly changing.
These corpuscles are somewhat larger than the red corpuscles, but less
numerous, there being about one colorless corpuscle to every three hundred
red ones. They have the power of movement, for they are found not only
inside but outside the blood vessels, showing that they have worked their
way between the cells that form the walls of the blood tubes.
[Illustration: A colorless corpuscle catching and eating germs.]
A Russian zoologist, Metchnikoff, after studying a number of simple
animals, such as medusae and sponges, found that in such animals some of the
cells lining the inside of the food cavity take up or engulf minute bits of
food. Later, this food is changed into the protoplasm of the cell.
Metchnikoff believed that the colorless corpuscles of the blood have
somewhat the same function. This he later proved to be true. Like the
amoeba, they feed by engulfing their prey. This fact has a very important
bearing on the relation of colorless corpuscles to certain diseases caused
by bacteria within the body. If, for example, a cut becomes infected by
bacteria, inflammation may set in. Colorless corpuscles at once surround
the spot and attack the bacteria which cause the inflammation. If the
bacteria are few in number, they are quickly eaten by certain of the
colorless corpuscles, which are known as _phagocytes_. If bacteria are
present in great quantities, they may prevail and kill the phagocytes by
poisoning them. The dead bodies of the phagocytes thus killed are found in
the pus, or matter, which accumulates in infected wounds. In such an event,
we must come to the aid of nature by washing the wound with some
antiseptic, as weak carbolic acid or hydrogen peroxide.
Antibodies and their Uses.--In case of disease where, for example, fever is
caused by poison given off from bacteria we find the cells of the body
manufacture and pour into the blood a substance known as an _antibody_.
This substance does not of necessity kill the harmful germs or even stop
their growth. It does, however, unite with the toxin or poison given off by
the germs and renders it entirely harmless.
Function of Lymph.--The tissues and organs of the body are traversed by a
network of tubes which carry the blood. Inside these tubes is the blood
proper, consisting of a fluid plasma, the colorless corpuscles, and the red
corpuscles. Outside the blood tubes, in spaces between the cells which form
tissues, is found another fluid, which is in chemical composition very much
like plasma of the blood. This is the _lymph_. It is, in fact, fluid food
in which some colorless amoeboid corpuscles are found. Blood gives up its
food material to the lymph. This it does by passing it through the walls of
the capillaries. The food is in turn given up to the tissue cells, which
are bathed by the lymph.
[Illustration: The exchange between blood and the cells of the body.]
Some of the amoeboid corpuscles from the blood make their way between the
cells forming the walls of the capillaries. _Lymph, then, is practically
blood plasma plus some colorless corpuscles. It acts as the medium of
exchange between the blood proper and the cells in the tissues of the
body._ By means of the food supply thus brought, the cells of the body are
able to grow, the fluid food being changed to the protoplasm of the cells.
By means of the oxygen passed over by the lymph, oxidation may take place
within the cells. Lymph not only gives food to the cells of the body, but
also takes away carbon dioxide and other waste materials, which are
ultimately passed out of the body by means of the lungs, skin, and kidneys.
Internal Secretions.--In addition to all the functions given above, the
blood has recently been shown to carry the secretions of a number of glands
through which it passes, although these glands have no ducts to carry off
their secretions. These internal secretions seem _absolutely necessary_ for
the health of the body. Several glands, the thyroid, adrenal bodies, the
testes, and ovaries, as well as the pancreas, give off these remarkable
substances.
The Amount of Blood and its Distribution.--Blood forms, by weight, about
one sixteenth of the body. This would be about four quarts to a body weight
of 130 pounds. Normally, about one half of the blood of the body is found
in or near the organs lying in the body cavity below the diaphragm, about
one fourth in the muscles, and the rest in the head, heart, lungs, large
arteries, and veins.
Blood Temperature.--The temperature of blood in the human body is normally
about 98.6 deg. Fahrenheit when tested under the tongue by a thermometer,
although the temperature drops almost two degrees after we have gone to
sleep at night. It is highest about 5 P.M. and lowest about 4 A.M. In
fevers, the temperature of the body sometimes rises to 107 deg.; but unless
this temperature is soon reduced, death follows. Any considerable drop in
temperature below the normal also means death. Body heat results from the
oxidation of food, and the circulation of blood keeps the temperature
nearly uniform in all parts of the body.
Cold-blooded Animals.--In animals which are called cold-blooded, the blood
has no fixed temperature, but varies with the temperature of the medium in
which the animal lives. Frogs, in the summer, may sit for hours in water
with a temperature of almost 100 deg. In winter, they often endure freezing so
that the blood and lymph within the spaces under the loose skin are frozen
into ice crystals. This change in body temperature is evidently an
adaptation to the mode of life.
Circulation of the Blood in Man.--The blood is the carrying agent of the
body. Like a railroad or express company, it takes materials from one part
of the human organism to another. This it does by means of the organs of
circulation,--the heart and blood vessels. These blood vessels are called
_arteries_ where they carry blood away from the heart, _veins_ where they
bring blood back to the heart, and _capillaries_ where they connect the
larger blood vessels. The organs of circulation thus form a system of
connected tubes through which the blood flows.
The Heart; Position, Size, Protection.--The heart is a cone-shaped muscular
organ about the size of a man's fist. It is located immediately above the
diaphragm, and lies so that the muscular apex, which points downward, moves
while beating against the fifth and sixth ribs, just a little to the left
of the midline of the body. This fact gives rise to the notion that the
heart is on the left side of the body. The heart is surrounded by a loose
membranous bag called the _pericardium_, the inner lining of which secretes
a fluid in which the heart lies. When, for any reason, the pericardial
fluid is not secreted, inflammation arises in that region.
[Illustration: Diagram showing the front half of the heart cut away: _a_,
aorta; _l_, arteries to the lungs; _la_, left auricle; _lv_, left
ventricle; _m_, tricuspid valve open; _n_, bicuspid or mitral valve closed;
_p_ and _r_, veins from the lungs; _ra_, right auricle; _rv_, right
ventricle; _v_, vena cava. Arrows show direction of circulation.]
Internal Structure of Heart.--If we should cut open the heart of a mammal
down the midline, we could divide it into a right and a left side, _each of
which would have no internal connection with the other_. Each side is made
up of an upper thin-walled portion with a rather large internal cavity, the
_auricle_, which opens into a lower smaller portion with heavy muscular
walls, the _ventricle_. Communication between auricles and ventricles is
guarded by little flaps or _valves_. The auricles receive blood from the
veins. The ventricles pump the blood into the arteries.
The Heart in Action.--The heart is constructed on the same plan as a force
pump, the valves preventing the reflux of blood into the auricle when it is
forced out of the ventricle. Blood enters the auricles from the veins
because the muscles of that part of the heart relax; this allows the space
within the auricles to fill. Almost immediately the muscles of the
ventricles relax, thus allowing blood to pass into the chambers within the
ventricles. Then, after a short pause, during which time the muscles of the
heart are resting, a wave of muscular contraction begins in the auricles
and ends in the ventricles, with a sudden strong contraction which forces
the blood out into the arteries. Blood is kept on its course by the valves,
which act in the same manner as do the valves in a pump. The blood is thus
made to pass into the arteries upon the contraction of the ventricle walls.
[Illustration: The heart is a force pump; prove it from these diagrams.]
[Illustration:
I. Circulation in a fish. _G_, gills; _C_, capillaries of the body. Notice
the two-chambered heart.
II. The circulation in a frog. _L_, the lungs; _C_, the capillaries. Notice
the heart has three chambers. What is the condition of blood leaving the
ventricle to go to the cells of the body?
III. The circulation in man. _H_, head; _A_, arms; _L_, lungs; _S_,
stomach; _Li_, liver; _K_, kidney; _S.I._, small intestine; _L.I._, large
intestine; _Le_, legs; _1_, right auricle; _2_, right ventricle; _3_, left
ventricle; _4_, left auricle; _5_, dorsal aorta; _6_, vein to lungs. ]
The Course of the Blood in the Body.--Although the two sides of the heart
are separate and distinct from each other, yet every drop of blood that
passes through the right heart likewise passes later through the left
heart. There are two distinct systems of circulation in the body. The
_pulmonary circulation_ takes the blood through the right auricle and
ventricle, to the lungs, and passes it back to the left auricle. This is a
relatively short circulation, the blood receiving in the lungs its supply
of oxygen, and there giving up some of its carbon dioxide. The greater
circulation is known as the _systemic circulation_; in this system, the
blood leaves the left ventricle through the great dorsal _aorta_. A large
part of the blood passes directly to the muscles; some of it goes to the
nervous system, kidneys, skin, and other organs of the body. It gives up
its supply of food and oxygen in these tissues, receives the waste products
of oxidation while passing through the capillaries, and returns to the
right auricle through two large vessels known as the _venae cavae_. It
requires only from twenty to thirty seconds for the blood to make the
complete circulation from the ventricle back again to the starting point.
This means that the entire volume of blood in the human body passes three
or four thousand times a day through the various organs of the body.[46]
Footnote 46: See Hough and Sedgwick, _The Human Mechanism_,
page 136.
Portal Circulation.--Some of the blood, on its way back to the heart,
passes to the walls of the food tube and to its glands. From there it is
sent with its load of absorbed food to the liver. Here the vein which
carries the blood (called the portal vein) breaks up into capillaries
around the cells of the liver, when it gives up sugar to be stored as
glycogen. From the liver, blood passes directly to the right auricle. The
_portal circulation_, as it is called, is the only part of the circulation
where the blood passes through two sets of capillaries on its way from
auricle to auricle.
[Illustration: Capillary circulation in the web of a frog's foot, as seen
under the compound microscope. _a_, _b_, small veins; _c_, pigment cells in
the skin; _d_, capillaries in which the oval corpuscles are seen to follow
one another in single series.]
Circulation in the Web of a Frog's Foot.--If the web of the foot of a live
frog or the tail of a tadpole is examined under the compound microscope, a
network of blood vessels will be seen. In some of the larger vessels the
corpuscles are moving rapidly and in spurts; these are _arteries_. The
arteries lead into smaller vessels hardly greater in diameter than the
width of a single corpuscle. This network of _capillaries_ may be followed
into larger _veins_ in which the blood moves regularly. This illustrates
the condition in any tissue of man where the arteries break up into
capillaries, and these in turn unite to form veins.
Structure of the Arteries.--A distinct difference in structure exists
between the arteries and the veins in the human body. The arteries, because
of the greater strain received from the blood which is pumped from the
heart, have thicker muscular walls, and in addition are very elastic.
Cause of the Pulse.--The _pulse_, which can easily be detected by pressing
the large artery in the wrist or the small one in front of and above the
external ear, is caused by the gushing of blood through the arteries after
each pulsation of the heart. As the large arteries pass away from the
heart, the diameter of each individual artery becomes smaller. At the very
end of their course, these arteries are so small as to be almost
microscopic in size and are very numerous. There are so many that if they
were placed together, side by side, their united diameter would be much
greater than the diameter of the large artery (_aorta_) which passes blood
from the left side of the heart. This fact is of very great importance, for
the force of the blood as it gushes through the arteries becomes very much
less when it reaches the smaller vessels. This gushing movement is quite
lost when the capillaries are reached, first, because there is so much more
space for the blood to fill, and second, because there is considerable
friction caused by the very tiny diameter of the capillaries.
Capillaries.--The capillaries form a network of minute tubes everywhere in
the body, but especially near the surface and in the lungs. It is through
their walls that the food and oxygen pass to the tissues, and carbon
dioxide is given up to the plasma. They form the connection that completes
the system of circulation of blood in the body.
Function and Structure of the Veins.--If the arteries are supply pipes
which convey fluid food to the tissues, then the veins may be likened to
drain pipes which carry away waste material from the tissues. Extremely
numerous in the extremities and in the muscles and among other tissues of
the body, they, like the branches of a tree, become larger and unite with
each other as they approach the heart.
[Illustration: Valves in a vein. Notice the thin walls of the vein.]
If the wall of a vein is carefully examined, it will be found to be neither
so thick nor so tough as an artery wall. When empty, a vein collapses; the
wall of an artery holds its shape. If you hold your hand downward for a
little time and then examine it, you will find that the veins, which are
relatively much nearer the surface than are the arteries, appear to be very
much knotted. This appearance is due to the presence of tiny valves within.
These valves open in the direction of the blood current, but would close if
the direction of the blood flow should be reversed (as in case a deep cut
severed a vein). As the pressure of blood in the veins is much less than in
the arteries, the valves thus aid in keeping the flow of blood in the veins
toward the heart. The higher pressure in arteries and the suction in the
veins (caused by the enlargement of the chest cavity in breathing) are the
chief factors which cause a steady flow of blood through the veins in the
body.
Lymph Vessels.--The lymph is collected from the various tissues of the body
by means of a number of very thin-walled tubes, which are at first very
tiny, but after repeated connection with other tubes ultimately unite to
form large ducts. These lymph ducts are provided, like the veins, with
valves. The pressure of the blood within the blood vessels forces
continually more plasma into the lymph; thus a slow current is maintained.
On its course the lymph passes through many collections of gland cells, the
_lymph glands_. In these glands some impurities appear to be removed and
colorless corpuscles made. The lymph ultimately passes into a large tube,
the _thoracic duct_, which flows upward near the ventral side of the spinal
column, and empties into the large subclavian vein in the left side of the
neck. Another smaller lymph duct enters the right subclavian vein.
[Illustration: The lymph vessels; the dark spots are lymph glands: _lac_,
lacteals; _rc_, thoracic duct.]
The Lacteals.--We have already found that part of the digested food
(chiefly carbohydrates, proteins, salts, and water) is absorbed directly
into the blood through the walls of the villi and carried to the liver.
Fat, however, is passed into the spaces in the central part of the villi,
and from there into other spaces between the tissues, known as the
_lacteals_. The lacteals carry the fats into the blood by way of the
thoracic duct. The lacteals and lymph vessels have in part the same course.
It will be thus seen that lymph at different parts of its course would
have a very different composition.
The Nervous Control of the Heart and Blood Vessels.--Although the muscles
of the heart contract and relax without our being able to stop them or
force them to go faster, yet in cases of sudden fright, or after a sudden
blow, the heart may stop beating for a short interval. This shows that the
heart is under the control of the nervous system. Two sets of nerve fibers,
both of which are connected with the central nervous system, pass to the
heart. One set of fibers accelerates, the other slows or inhibits, the
heart beat. The arteries and veins are also under the control of the
sympathetic nervous system. This allows of a change in the diameter of the
blood vessels. Thus, blushing is due to a sudden rush of blood to the
surface of the body caused by an expansion of the blood vessels at the
surface. The blood vessels of the body are always full of blood. This
results from an automatic regulation of the diameter of the blood tubes by
a part of the nervous system called the _vasomotor nerves_. These nerves
act upon the muscles in the walls of the blood vessels. In this way, each
vessel adapts itself to the amount of blood in it at a given time. After a
hearty meal, a large supply of blood is needed in the walls of the stomach
and intestines. At this time, the arteries going to this region are dilated
so as to receive an extra supply. When the brain performs hard work, blood
is supplied in the same manner to that region. Hence, one should not study
or do mental work immediately after a hearty meal, for blood will be drawn
away to the brain, leaving the digestive tract with an insufficient supply.
Indigestion may follow as a result.
The Effect of Exercise on the Circulation.--It is a fact familiar to all
that the heart beats more violently and quickly when we are doing hard work
than when we are resting. Count your own pulse when sitting quietly, and
then again after some brisk exercise in the gymnasium. Exercise in
moderation is of undoubted value, because it sends the increased amount of
blood to such parts of the body where increased oxidation has been taking
place as the result of the exercise. The best forms of exercise are those
which give as many muscles as possible work--walking, out-of-door sports,
any exercise that is not violent. Exercise should not be attempted
immediately after eating, as this causes a withdrawal of blood from the
digestive tract to the muscles of the body. Neither should exercise be
continued after becoming tired, as poisons are then formed in the muscles,
which cause the feeling we call _fatigue_. Remember that extra work given
to the heart by extreme exercise may injure it, causing possible trouble
with the valves.
[Illustration: Stopping flow of blood from an artery by applying a tight
bandage (ligature) between the cut and the heart.]
Treatment of Cuts and Bruises.--Blood which oozes slowly from a cut will
usually stop flowing by the natural means of the formation of a clot. A cut
or bruise should, however, be washed in a weak solution of carbolic acid or
some other antiseptic in order to prevent bacteria from obtaining a
foothold on the exposed flesh. If blood, issuing from a wound, gushes in
distinct pulsations, then we know that an artery has been severed. To
prevent the flow of blood, a tight bandage known as a _tourniquet_ must be
tied between the cut and the heart. A handkerchief with a knot placed over
the artery may stop bleeding if the cut is on one of the limbs. If this
does not serve, then insert a stick in the handkerchief and twist it so as
to make the pressure around the limb still greater. Thus we may close the
artery until the doctor is called, who may sew up the injured blood vessel.
The Effect of Alcohol upon the Blood.--It has recently been discovered that
alcohol has an extremely injurious effect upon the colorless corpuscles of
the blood, lowering their ability to fight disease germs to a marked
degree. This is well seen in a comparison of deaths from certain infectious
diseases in drinkers and abstainers, the percentage of mortality being much
greater in the former.