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Page 9
It is essential in so complicated a structure as the body that some
apparatus should exist to provide for the interchange of material. The
innumerable cell units of the body must have material to provide
energy, and useless material which results from their activity must be
removed. A household might be almost as much embarrassed by the
accumulation of garbage and ashes as by the absence of food and coal.
The food, which is taken into the alimentary canal and converted by
the digestive fluids into material more directly adapted to the uses
of cells, must be conveyed to them. A supply of oxygen is essential
for the life of the cells, and the supply which is given by
respiration must be carried from the lungs to every cell of the body.
All this is effected by the circulation of the blood, which takes
place in the system of branching closed tubes in which the blood
remains (Fig. 11). Certain of these tubes, the arteries, have strong
and elastic walls and serve to convey and distribute the blood to the
different organs and tissues. From the ultimate branches of the
arteries the blood passes into a close network of tubes, the
capillaries, which in enormous numbers are distributed in the tissues
and have walls so thin that they allow fluid and gaseous interchange
between their contents and the fluid around them to take place. The
blood from the capillaries is then collected into a series of tubes,
the veins, by which it is returned to the heart. This circulation is
maintained by means of a pumping organ or heart, which receives the
blood from the veins and by the contraction of its powerful walls
forces this into the arteries, the direction of flow being determined
as in a pump, by a system of valves. The waste products of cell life
pass from the cells into the fluid about them, and are in part
directly returned into the blood, but for the greater part pass into
it indirectly through another set of vessels, the lymphatics. These
are thin-walled tubes which originate in the tissues, and in which
there is a constant flow towards the heart, maintained by the constant
but varying pressure of the tissue around them, the direction of flow
being maintained by numerous valves. The colorless fluid within these
vessels is termed "lymph." At intervals along these tubes are small
structures termed the lymph nodes, which essentially are filters, and
strain out from the fluid substances which might work great injury if
they passed into the blood. Between the capillary vessels and the
lymphatics is the tissue fluid, in which all the exchange takes place.
It is constantly added to by the blood, and returns fluid to the blood
and lymph; it gives material to the cells and receives material from
them.
[Illustration: FIG. 11.--A DIAGRAMMATIC VIEW OF THE BLOOD VESSELS. An
artery (_a_) opens into a system of capillaries, (_c_) and
after passing through these collects into a vein (_b_). Notice
that the capillaries connect with other vascular territories at
numerous points (_d_). If the artery (_a_) became closed the
capillaries which it supplies could be filled by blood coming from
other sources.]
In addition to the strength and elasticity of the wall of the
arteries, which enables them to resist the pressure of the blood, they
have the power of varying their calibre by the contraction or
expansion of their muscular walls. Many of the organs of the body
function discontinuously, periods of activity alternating with
comparative repose; during the period of activity a greater blood
supply is demanded, and is furnished by relaxation of the muscle
fibres which allows the calibre to increase, and with this the blood
flow becomes greater in amount. Each part of the body regulates its
supply of blood, the regulation being effected by means of nerves
which control the tension of the muscle fibres. The circulation may be
compared with an irrigation system in which the water supply of each
particular field is regulated not by the engineer, but by an automatic
device connected with the growing crop and responding to its demands.
[Illustration: FIG. 12.--THE VARIOUS CELLS IN THE BLOOD. (_a_) The red
blood cells, single and forming a roll by adhering to one another;
(_b_) different forms of the white blood cells; those marked "1" are
the most numerous and are phagocytic for bacteria.]
The blood consists of a fluid, the blood plasma, in which numerous
cells are contained. The most numerous of these are small cup-shaped
cells which contain a substance called _h�moglobin_, to which the
red color of the blood is due. There are five million of these cells
in a cubic millimeter (a millimeter is .03937 of an inch), giving a
total number for the average adult of twenty-five trillion. The
surface area of all these, each being one thirty-three hundredth of an
inch in diameter, is about thirty-three hundred square yards. The
h�moglobin which they contain combines in the lungs with the oxygen in
the inspired air, and they give up this indispensable substance to the
cells everywhere in the body. There are also eight thousand leucocytes
or colorless cells in a cubic millimeter of blood, this giving a total
number of four billion in the average adult, and these vary in
character and in relative numbers (Fig. 12). The most numerous of
these are round and slightly larger than the red cells; they have a
nucleus of peculiar shape and contain granules of a definite
character. These cells serve an important part in infectious diseases
in devouring and destroying parasites. They have power of active
independent motion and somewhat resemble certain of the free living
unicellular organisms. The blood plasma, when taken from the vessels,
clots or passes from a fluid into a gelatinous or semi-solid
condition, which is due to the formation within it of a network of
fine threads termed fibrin. It is by means of the clotting of the
blood that the escape of blood from ruptured vessels is arrested.
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