Two liquids of similar nature are found in the body, known as the blood
and the lymph. These are closely related in function and together they
form the nutrient fluid referred to in the preceding chapter. The blood is
the more familiar of the two liquids, and the one which can best be
considered at this time.
*The Blood: where Found.*--The blood occupies and moves through a system of
closed tubes, known as the blood vessels. By means of these vessels the
blood is made to circulate through all parts of the body, but from them it
does not escape under normal conditions. Though provisions exist whereby
liquid materials may both enter and leave the blood stream, it is only
when the blood vessels are cut or broken that the blood, as blood, is able
to escape from its inclosures.
*Physical Properties of the Blood.*--Experiments such as those described at
the close of this chapter reveal the more important physical properties of
the blood. It may be shown to be heavier and denser than water; to have a
faint odor and a slightly salty taste; to have a bright red color when it
contains oxygen and a dark red color when oxygen is absent; and to
undergo, when exposed to certain conditions, a change called coagulation.
These properties are all accounted for through the different materials
that enter into the formation of the blood.
[Fig. 8]
Fig. 8--Blood corpuscles, highly magnified. _A._ Red corpuscles as they
appear in diluted blood. _B._ Arrangement of red corpuscles in rows
between which are white corpuscles, as may be seen in undiluted blood.
_C._ Red corpuscles much enlarged to show the form.
*Composition of the Blood.*--To the naked eye the blood appears as a thick
but simple liquid; but when examined with a compound microscope, it is
seen to be complex in nature, consisting of at least two distinct
portions. One of these is a clear, transparent liquid; while the other is
made up of many small, round bodies that float in the liquid. The liquid
portion of the blood is called the _plasma_; the small bodies are known as
_corpuscles_. Two varieties of corpuscles are described--the _red_
corpuscles and the _white_ corpuscles (Fig. 8). Other round particles,
smaller than the corpuscles, may also be seen under favorable conditions.
These latter are known as _blood platelets_.
*Red Corpuscles.*--The red corpuscles are classed as cells, although, as
found in the blood of man and the other mammals (Fig. 9), they have no
nuclei.(6) Each one consists of a little mass of protoplasm, called the
_stroma_, which contains a substance having a red color, known as
_hemoglobin_. The shape of the red corpuscle is that of a circular disk
with concave sides. It has a width of about 1/3200 of an inch (7.9
microns(7)) and a thickness of about 1/13000 of an inch (1.9 microns). The
red corpuscles are exceedingly numerous, there being as many as five
millions in a small drop (one cubic millimeter) of healthy blood. But the
number varies somewhat and is greatly diminished during certain forms of
disease.
[Fig. 9]
Fig. 9--Red corpuscles from various animals. Those from mammals are without
nuclei, while those from birds and cold-blooded animals have nuclei.
It is the _function_ of the red corpuscles to serve as _oxygen carriers_
for the cells. They take up oxygen at the lungs and release it at the
cells in the different tissues.(8) The performance of this function
depends upon the hemoglobin.
*Hemoglobin.*--This substance has the remarkable property of forming, under
certain conditions, a weak chemical union with oxygen and, when the
conditions are reversed, of separating from it. It forms about nine tenths
of the solid matter of the red corpuscles and to it is due the colors of
the blood. When united with the oxygen it forms a compound, called
_oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a
dark red color. These colors are the same as those of the blood as it
takes on and gives off oxygen. The stroma, which forms only about one
tenth of the solid matter of the corpuscles, serves as a contrivance for
holding the hemoglobin. The conditions which cause the hemoglobin to unite
with oxygen in the lungs and to separate from it in the tissues, will be
considered later (Chapter VIII).
*Disappearance and Origin of Red Corpuscles.*--The red corpuscles, being
cells without nuclei, are necessarily short-lived. It has been estimated
that during a period of one to two months, all the red corpuscles in the
body at a given time will have disappeared and their places taken by new
ones. The origin of new corpuscles, however, and the manner of ridding the
blood of old ones are problems that are not as yet fully solved. The
removal of the products of broken down corpuscles is supposed to take
place both in the liver and in the spleen.(9)
Regarding the origin of the red corpuscles, the evidence now seems
conclusive that large numbers of them are formed in the red marrow of the
bones. The red marrow is located in what is known as the spongy substance
of the bones (Chapter XIV) and consists, to a large extent, of cells
somewhat like the red corpuscles, but differing from them in having
nuclei. These appear to be constantly in a state of reproduction. The
blood, flowing through the minute cavities containing these cells, carries
those that have been loosened out into the blood stream. Nuclei appear in
the red corpuscles at the time of their formation, but these quickly
separate and, according to some authorities, form the blood platelets.
*White Corpuscles.*--The white corpuscles, or _leucocytes_, are cells of a
general spherical shape, each containing one, two, or more nuclei. They
are much less numerous than the red, there being on the average only one
white corpuscle to about every five hundred of the red ones. On the other
hand, the white corpuscles are larger than the red, one of the former
being equal in volume to about three of the latter.
[Fig. 10]
Fig. 10--*Escape of white corpuscles from a small blood vessel* (Hall). At
_A_ the conditions are normal, but at _B_ some excitation in the
surrounding tissue leads to a migration of corpuscles. 1, 2, and 3 show
different stages of the passage.
The white corpuscles are found, when studied under favorable conditions,
to possess the power of changing their shape and, by this means, of moving
from place to place. This property enables them to penetrate the walls of
capillaries and to pass with the lymph in between the cells of the
tissues. The white corpuscles are, therefore, not confined to the blood
vessels, as are the red corpuscles, but migrate through the intercellular
spaces (Fig. 10). If any part of the body becomes inflamed, the white
corpuscles collect there in large numbers; and, on breaking down, they
form most of the white portion of the sore, called the _pus_.
New white corpuscles are formed from old ones, by cell-division. Their
production may occur in almost any part of the body, but usually takes
place in the lymphatic glands (Chapter VI) and in the spleen, where
conditions for their development are especially favorable. In these places
they are found in great abundance and in various stages of development.
*Functions of White Corpuscles.*--The main use of the white corpuscles
appears to be that of a destroyer of disease germs. These consist of
minute organisms that find their way into the body and, by living upon the
tissues and fluids and by depositing toxins (poisons) in them, cause
different forms of disease. Besides destroying germs that may be present
in the blood, the white corpuscles also leave the blood and attack germs
that have invaded the cells. By forming a kind of wall around any foreign
substance, such as a splinter, that has penetrated the skin, they are able
to prevent the spread of germs through the body. In a similar manner they
also prevent the germs from boils, abscesses, and sore places in general
from getting to and infecting other parts of the body.(10) Another
function ascribed to the white corpuscles is that of aiding in the
coagulation of the blood (page 31); and still another, of aiding in the
healing of wounds.
*Plasma.*--The plasma is a complex liquid, being made up of water and of
substances dissolved in the water. The dissolved substances consist mainly
of foods for the cells and wastes from the cells.
1. _The foods_ represent the same classes of materials as are taken in the
daily fare, _i.e._, proteids, carbohydrates, fats, and salts (Chapter IX).
Three kinds of proteids are found in the plasma, called _serum albumin_,
_serum globulin_, and _fibrinogen_. These resemble, in a general way, the
white of raw egg, but differ from each other in the readiness with which
they coagulate. Fibrinogen coagulates more readily than the others and is
the only one that changes in the ordinary coagulation of the blood. The
others remain dissolved during this process, but are coagulated by
chemical agents and by heat. While all of the proteids probably serve as
food for the cells, the fibrinogen, in addition, is a necessary factor in
the coagulation of the blood (page 31).
The only representative of the carbohydrates in the plasma is _dextrose_.
This is a variety of sugar, being derived from starch and the different
sugars that are eaten. The _fat_ in the plasma is in minute quantities and
appears as fine droplets--the form in which it is found in milk. While
several mineral salts are present in small quantities in the plasma,
_sodium chloride_, or common salt, is the only one found in any
considerable amount. The mineral salts serve various purposes, one of
which is to cause the proteids to dissolve in the plasma.
2. _The wastes_ are formed at the cells, whence they are passed by the
lymph into the blood plasma. They are carried by the blood until removed
by the organs of excretion. The two waste products found in greatest
abundance in the plasma are carbon dioxide and urea.
The substances dissolved in the plasma form about 10 per cent of the whole
amount. The remaining 90 per cent is water. Practically all the
constituents of the plasma, except the wastes, enter the blood from the
digestive organs.
*Purposes of Water in the Blood.*--Not only is water the most abundant
constituent of the blood; it is, in some respects, the most important. It
is the liquefying portion of the blood, holding in solution the
constituents of the plasma and floating the corpuscles. Deprived of its
water, the blood becomes a solid substance. Through the movements of the
blood the water also serves the purpose of a transporting agent in the
body. The cells in all parts of the body require water and this is
supplied to them from the blood. Water is present in the corpuscles as
well as in the plasma and forms about 80 per cent of the entire volume of
the blood.
*Coagulation of the Blood.*--If the blood is exposed to some unnatural
condition, such as occurs when it escapes from the blood vessels, it
undergoes a peculiar change known as _coagulation_.(11) In this change the
corpuscles are collected into a solid mass, known as the _clot_, thereby
separating from a liquid called the _serum_. The serum, which is similar
in appearance to the blood plasma, differs from that liquid in one
important respect as explained below.
*Causes of Coagulation.*--Although coagulation affects all parts of the
blood, only one of its constituents is found in reality to coagulate. This
is the fibrinogen. The formation of the clot and the separation of the
serum is due almost entirely to the action of this substance. Fibrinogen
is for this reason called the _coagulable constituent of the blood_. In
the plasma the fibrinogen is in a liquid form; but during coagulation it
changes into a white, stringy solid, called _fibrin_. This appears in the
clot and is the cause of its formation. Forming as a network of
exceedingly fine and very delicate threads (Fig. 11) _throughout the mass
of blood_ that is coagulating, the fibrin first entangles the corpuscles
and then, by contracting, draws them into the solid mass or clot.(12) The
contracting of the fibrin also squeezes out the serum. This liquid
contains all the constituents of the plasma except the fibrinogen.
[Fig. 11]
Fig. 11--*Fibrin threads* (after Ranvier). These by contracting draw the
corpuscles together and form the clot.
*Fibrin Ferment and Calcium.*--Most difficult of all to answer have been
the questions: What causes the blood to coagulate outside of the blood
vessels and what prevents its coagulation inside of these vessels? The
best explanation offered as yet upon this point is as follows: Fibrinogen
does not of itself change into fibrin, but is made to undergo this change
by the presence of another substance, called _fibrin ferment_. This
substance is not a regular constituent of the blood, but is formed as
occasion requires. It is supposed to result from the breaking down of the
white corpuscles, and perhaps also from the blood platelets, when the
blood is exposed to unnatural conditions. The formation of the ferment
leads in turn to the changing of the fibrinogen into fibrin.
Another substance which is necessary to the process of coagulation is the
element calcium. If compounds of calcium are absent from the blood,
coagulation does not take place. These are, however, regular constituents
of healthy blood. Whether the presence of the calcium is necessary to the
formation of the ferment or to the action of the ferment upon the
fibrinogen is unknown.
*Purpose of Coagulation.*--The purpose of coagulation is to check the flow
of blood from wounds. The fact that the blood is contained in and kept
flowing continuously through a system of _connected_ vessels causes it to
escape rapidly from the body whenever openings in these vessels are made.
Clots form at such openings and close them up, stopping in this way the
flow that would otherwise go on indefinitely. Coagulation, however, does
not stop the flow of blood from the large vessels. From these the blood
runs with too great force for the clot to form within the wound.
*Time Required for Coagulation.*--The rate at which coagulation takes place
varies greatly under different conditions. It is influenced strongly by
temperature; heat hastens and cold retards the process. It may be
prevented entirely by lowering the temperature of the blood to near the
freezing point. The presence of a foreign substance increases the rapidity
of coagulation, and it has been observed that bleeding from small wounds
is more quickly checked by covering them with linen or cotton fibers. The
fibers in this case hasten the process of coagulation.
*Quantity of Blood.*--The quantity of blood is estimated to be about one
thirteenth of the entire weight of the body. This for the average
individual is an amount weighing nearly twelve pounds and having a volume
of nearly one and one half gallons. About 46 per cent by volume of this
amount is made up of corpuscles and 54 per cent of plasma. Of the plasma
about 10 per cent consists of solids and 90 per cent of water, as already
stated.
*Functions of the Blood.*--The blood is the great carrying, or
distributing, agent in the body. Through its movements (considered in the
next chapter) it carries food and oxygen to the cells and waste materials
from the cells. Much of the blood may, therefore, be regarded as _freight_
in the process of transportation. The blood also carries, or distributes,
heat. Taking up heat in the warm parts of the body, it gives it off at
places having a lower temperature. This enables all parts of the body to
keep at about the same temperature.
In addition to serving as a carrier, the blood has antiseptic properties,
i.e., it destroys disease germs. While this function is mainly due to the
white corpuscles, it is due in part to the plasma.(13) Through its
coagulation, the blood also closes leaks in the small blood vessels. The
blood is thus seen to be a liquid of several functions.
[Fig. 12]
Fig. 12--*A balanced change* in water. The level remains constant although
the water is continually changing; suggestive of the changes in the blood.
*Changes in the Blood.*--In performing its functions in the body the blood
must of necessity undergo rapid and continuous change. The red corpuscles,
whose changes have already been noted, appear to be the most enduring
constituents of the blood. The plasma is the portion that changes most
rapidly. Yet in spite of these changes the quantity and character of the
blood remain practically constant.(14) This is because there is a
_balancing_ of the forces that bring about the changes. The addition of
various materials to the blood just equals the withdrawal of the same
materials from the blood. Somewhat as a vessel of water (Fig. 12) having
an inflow and an outflow which are equal in amount may keep always at the
same level, the balancing of the intake and outgo of the blood keeps its
composition about the same from time to time.
*Hygiene of the Blood.*--The blood, being a changeable liquid, is easily
affected through our habits of living. Since it may be affected for ill as
well as for good, one should cultivate those habits that are beneficial
and avoid those that are harmful in their effects. Most of the hygiene of
the blood, however, is properly included in the hygiene of the organs that
act upon the blood--a fact which makes it unnecessary to treat this subject
fully at this time.
From a health standpoint, the most important constituents of the blood
are, perhaps, the corpuscles. These are usually sufficient in number and
vigor in the blood of those who take plenty of physical exercise, accustom
themselves to outdoor air and sunlight, sleep sufficiently, and avoid the
use of injurious drugs. On the other hand, they are deficient in quantity
and inferior in quality in the bodies of those who pursue an opposite
course. Impurities not infrequently find their way into the blood through
the digestive organs. One should eat wholesome, well-cooked food, drink
freely of _pure_ water, and limit the quantity of food _to what can be
properly digested_. The natural purifiers of the blood are the organs of
excretion. The skin is one of these and its power to throw off impurities
depends upon its being clean and active.
*Effect of Drugs.*--Certain drugs and medicines, including alcohol and
quinine,(15) have recently been shown to destroy the white corpuscles. The
effect of such substances, if introduced in considerable amount in the
body, is to render one less able to withstand attacks of disease. Many
patent medicines are widely advertised for purifying the blood. While
these may possibly do good in particular cases, the habit of doctoring
one's self with them is open to serious objection. Instead of taking drugs
and patent medicines for purifying the blood, one should study to live
more hygienically. We may safely rely upon wholesome food, pure water,
outdoor exercise and sunlight, plenty of sleep, and a clean skin for
keeping the blood in good condition. If these natural remedies fail, a
physician should be consulted.
*Summary.*--The blood is the carrying or transporting agent of the body. It
consists in part of constituents, such as the red corpuscles, that enable
it to carry different substances; and in part of the materials that are
being carried. The latter, which include food and oxygen for the cells and
wastes from the cells, may be classed as freight. Certain constituents in
the blood destroy disease germs, and other constituents, by coagulating,
close small leaks in the blood vessels. Although subject to rapid and
continuous change, the blood is able--by reason of the balancing of
materials added to and withdrawn from it--to remain about the same in
quantity and composition.
Exercises.--1. Compare blood and water with reference to weight, density,
color, odor, and complexity of composition.
2. Show by an outline the different constituents of the blood.
3. Compare the red and white corpuscles with reference to size, shape,
number, origin, and function.
4. Name some use or purpose for each constituent of the blood.
5. What constituents of the blood may be regarded as freight and what as
agents for carrying this freight?
6. After coagulation, what portions of the blood are found in the clot?
What portions are found in the serum?
7. What purposes are served by water in the blood?
8. Show how the blood, though constantly changing, is kept about the same
in quantity, density, and composition.
9. In the lungs the blood changes from a dark to a bright red color and in
the tissues it changes back to dark red. What is the cause of these
changes?
10. If the oxygen and hemoglobin formed a strong instead of a weak
chemical union, could the hemoglobin then act as an oxygen carrier? Why?
11. What habits of living favor the development of corpuscles in the
blood?
12. Why will keeping the skin clean and active improve the quality of
one's blood?
PRACTICAL WORK
*To demonstrate the Physical Properties of Blood* (Optional).--Since blood
is needed in considerable quantity in the following experiments, it is
best obtained from the butcher. To be sure of securing the blood in the
manner desired, take to the butcher three good-sized bottles bearing
labels as follows:
*1* Fill two thirds full. While the blood is cooling, stir rapidly with
the hand or a bunch of switches to remove the clot.
*2* Fill two thirds full and set aside without shaking or stirring.
*3* Fill two thirds full and thoroughly mix with the liquid in the bottle.
Label 3 must be pasted on a bottle, having a tight-fitting stopper, which
is filled one fifth full of a saturated solution of Epsom salts. The
purpose of the salts is to prevent coagulation until the blood is diluted
with water as in the experiments which follow.
*Experiments.*--1. Let some of the defibrinated blood (bottle 1) flow (not
fall) on the surface of water in a glass vessel. Does it remain on the
surface or sink to the bottom? What does the experiment show with
reference to the relative weight of blood and water?
2. Fill a large test tube or a small bottle one fourth full of the
defibrinated blood and thin it by adding an equal amount of water. Then
place the hand over the mouth and shake until the blood is thoroughly
mixed with the air. Compare with a portion of the blood not mixed with the
air, noting any difference in color. What substance in the air has acted
on the blood to change its color?
3. Fill three tumblers each two thirds full of water and set them in a
warm place. Pour into one of the tumblers, and thoroughly mix with the
water, two tablespoonfuls of the blood containing the Epsom salts. After
an interval of half an hour add blood to the second tumbler in the same
manner, and after another half hour add blood to the third. The water
dilutes the salts so that coagulation is no longer prevented. Jar the
vessel occasionally as coagulation proceeds; and if the clot is slow in
forming, add a trace of some salt of calcium (calcium chloride). After the
blood has been added to the last tumbler make a comparative study of all.
Note that coagulation begins in all parts of the liquid at the same time
and that, as the process goes on, the clot shrinks and is drawn toward the
center.
4. Place a clot from one of the tumblers in experiment 3 in a large vessel
of water. Thoroughly wash, adding fresh water, until a white, stringy
solid remains. This substance is fibrin.
5. Examine the coagulated blood obtained from the butcher (bottle 2).
Observe the dark central mass (the clot) surrounded by a clear liquid (the
serum). Sketch the vessel and its contents, showing and naming the parts
into which the blood separates by coagulation.
*To examine the Red Corpuscles.*--Blood for this purpose is easily obtained
from the finger. With a handkerchief, wrap one of the fingers of the left
hand from the knuckle down to the first joint. Bend this joint and give it
a sharp prick with the point of a sterilized 'needle just above the root
of the nail. Pressure applied to the under side of the finger will force
plenty of blood through a very small opening. (To prevent any possibility
of blood poisoning the needle should be sterilized. This may be done by
dipping it in alcohol or by holding it for an instant in a hot flame. It
is well also to wash the finger with soap and water, or with alcohol,
before the operation.) Place a small drop of the blood in the middle of a
glass slide, protect the same with a cover glass, and examine with a
compound microscope. At least two specimens should be examined, one of
which should be diluted with a little saliva or a physiological salt
solution.(16) In the diluted specimen the red corpuscles appear as
amber-colored, circular, disk-shaped bodies. In the undiluted specimen
they show a decided tendency to arrange themselves in rows, resembling
rows of coins. (Singly, the corpuscles do not appear red when highly
magnified.)
A few white corpuscles may generally be found among the red ones in the
undiluted specimen. These become separated by the formation of the red
corpuscles into rows. They are easily recognized by their larger size and
by their silvery appearance, due to the light shining through them.
*To examine White Corpuscles.*--Obtain from the butcher a small piece of
the neck sweetbread of a calf. Press it between the fingers to squeeze out
a whitish, semi-liquid substance. Dilute with physiological salt solution
on a glass slide and examine with a compound microscope. Numerous white
corpuscles of different kinds and sizes will be found. Make sketches.
*To prepare Models of Red Corpuscles.*--Several models of red corpuscles
should be prepared for the use of the class. Clay and putty may be pressed
into the form of red corpuscles and allowed to harden, and small models
may be cut out of blackboard crayon. Excellent models can be molded from
plaster of Paris as follows: Coat the inside of the lid of a baking powder
can with oil or vaseline and fill it even full of a thick mixture of
plaster of Paris and water. After the plaster has set, remove it from the
lid and with a pocket-knife round off the edges and hollow out the sides
until the general form of the corpuscle is obtained. The models may be
colored red if it is desired to match the color as well as the form of the
corpuscle.