In all living organisms, except the most minute, only a minimum number
of cells can come into immediate contact with the general world, whence
is to be drawn the food supply for the whole organism. Hence those
cells--and they are by far the most numerous--which do not lie on the
food-absorbing surface, must gain their nutriment by some indirect
means. Further, each living cell produces waste products whose
accumulation would speedily prove injurious to the cell, hence they must
be constantly removed from its immediate neighbourhood and indeed from
the organism as a whole. In this instance again, only a few cells can
lie on a surface whence such materials can be directly discharged to the
exterior. Hence the main number of the cells of the organism must depend
upon some mechanism by which the waste products can be carried away from
them to that group of cells whose duty it is to modify them, or
discharge them from the body. These two ends are attained by the aid of
a circulating fluid, a fluid which is constantly flowing past every cell
of the body. From it the cells extract the food materials they require
for their sustenance, and into it they discharge the waste materials
resulting from their activity. This circulating medium is the blood.
Whilst undoubtedly the two functions of this circulating fluid above
given are the more prominent, there are yet others of great importance.
For instance, it is known that many tissues as a result of their activity
produce certain chemical substances which are of essential importance to
the life of other tissue cells. These substances--_internal secretions_
as they are termed--are carried to the second tissue by the blood stream.
Again, many instances are known in which two distant tissues communicate
with one another by means of chemical messengers, bodies termed
_hormones_ ([Greek: ormaein], to stir up), which are produced by one
group of cells, and sent to the other group to excite them to activity.
Here, also, the path by which such messengers travel is the blood stream.
A further and most important manner in which the circulating fluid is
utilized in the life of an animal is seen in the way in which it is
employed in protecting the body should it be invaded by micro-organisms.
Hence it is clear that the blood is of the most vital importance to the
healthy life of the body. But the fact that it is present as a
circulating medium exposes the animal to a great danger, viz. that it
may be lost should any vessel carrying it become ruptured. This is
constantly liable to happen, but to minimize as far as possible any such
loss, the blood is endowed with the peculiar property of _clotting_,
i.e. of setting to a solid or stiff jelly by means of which the orifices
of the torn vessels become plugged and the bleeding stayed.
The performance of these essential functions depends upon the
maintenance of a continuous flow past all tissue cells, and this is
attained by the circulatory mechanism, consisting of a central pump, the
heart, and a system of ramifying tubes, the arteries, through which the
blood is forced from the heart to every tissue (see VASCULAR SYSTEM). A
second set of tubes, the veins, collects the blood and returns it to the
heart. In many invertebrates the circulating fluid is actually poured
into the tissue spaces from the open terminals of the arteries. From
these spaces it is in turn drained away by the veins. Such a system is
termed a _haemolymph system_ and the circulating fluid the haemolymph.
Here the essential point gained is that the fluid is brought into direct
contact with the tissue cells. In all vertebrates, the ends of the
arteries are united to the commencements of the veins by a plexus of
extremely minute tubes, the capillaries, consequently the blood is
always retained within closed tubes and never comes into contact with
the tissue cells. It is while passing through the capillaries that the
blood performs its work; here the blood stream is at its slowest and is
brought nearest to the tissue cell, only being separated from it by the
extremely thin wall of the capillary and by an equally thin layer of
fluid. Through this narrow barrier the interchanges between cell and
blood take place.
The advantage gained in the vertebrate animal by retaining the blood in
a closed system of tubes lies in the great diminution of resistance to
the flow of blood, and the consequent great increase in rate of flow
past the tissue cells. Hence any food stuffs which can travel quickly
through the capillary wall to the tissue cell outside can be supplied in
proportionately greater quantity within a given time, without requiring
any very great increase in the concentration of that substance in the
blood. Conversely, any highly diffusible substance may be withdrawn
from the tissues by the blood at a similarly increased pace. These
conditions are more peculiarly of importance for the supply of oxygen
and the removal of carbonic acid-especially for the former, because the
amount of it which can be carried by the blood is small. But as the rate
at which a tissue lives, _i.e_. its activity, depends upon the rate of
its chemical reactions, and as these are fundamentally oxidative, the
more rapidly oxygen is carried to a tissue the more rapidly it can live,
and the greater the amount of work it can perform within a given time.
The rate of supply is of much less importance in the case of the other
food substances because they are far more soluble in water, so that the
supply in sufficient quantity can easily be met by a relatively slow
blood flow. Hence we find that the gradual evolution of the animal
kingdom goes hand in hand with the gradual development of a greater
oxygen-carrying capacity of the blood and an increase in the rate of its
flow.
In the groundwork of a tissue are a number of spaces--the _tissue
spaces_. They are filled with fluid and intercommunicate freely, finally
connecting with a number of fine tubes, the lymphatics, through which
excess of fluid or any solid particles present are drained away. The
contained fluid acts as an intermediary between the blood and the cell;
from it, the cell takes its various food stuffs, these having in the
first instance been derived from the blood, and into it the cell
discharges its waste products. On the course of the lymphatics a number
of typical structures, the lymphatic glands, are placed, and the lymph
has to pass through these structures where any deleterious products are
retained, and the fluid thus purified is drained away by further
lymphatics and finally returned to the blood. Thus there is a second
stream of fluid from the tissues, but one vastly slower than that of the
blood. The flow is too slow for it to act as the vehicle for the removal
of those waste products (carbonic acid, &c.) which must of necessity be
removed quickly. These must be removed by the blood. The same is true
for the main number of other waste products, which, however, being of
small molecular size are readily absorbed into the blood stream.
But in addition to fluid, the tissue spaces may at times be found to
contain solid matter in the form of particles, which may represent the
debris of destroyed cells, or which are, as is quite commonly the case,
micro-organisms. Apparently such material cannot be removed from a
tissue by absorption into the blood stream--indeed in the case of living
organisms such an absorption would in many instances rapidly prove
fatal, and special provision is made to prevent such an accident. These,
therefore, are made to travel along the lymphatic channels, and so,
before gaining access to the blood stream and thus to the body
generally, have to run the gauntlet of the protective mechanism provided
by the lymphatic glands, where in the major number of cases they are
readily destroyed.
Hence we see that first and foremost we have to regard the blood as a
food-carrier to all the cells of the body; in the second place as the
vehicle carrying away most if not all the waste products; in a third
direction, it is acting as a means for transmitting chemical substances
manufactured in one tissue to distant cells of the body for whose
nutrition or excitation they may be essential; and in addition to these
important functions there is yet another whose value it is almost
impossible to overestimate, for it plays the essential role in rendering
the animal immune to the attacks of invading organisms. The question of
immunity is discussed elsewhere, and it is sufficient merely to indicate
the chief means by which the blood subserves this essential protective
mechanism. Should living organisms find their way into the surface cells
or within the tissue spaces, the body fights them in a number of ways,
(1) It may produce one or more chemical substances capable of
neutralizing the toxic material produced by the organism. (2) It may
produce chemical substances which act as poisons to the micro-organism,
either paralysing it or actually killing it. Or (3) the organism may be
attacked and taken up into the body of wandering cells, _e.g_. certain
of the leucocytes, and then digested by them. Such cells are therefore
called phagocytes ([Greek: Phagein], to eat). Thus, by its power of
reacting in these ways the body has become capable of withstanding the
attacks of many different varieties of micro-organisms, of both animal
and vegetable origin.
_General Properties._--Blood is an opaque, viscid liquid of bright red
colour possessing a distinct and characteristic odour, especially when
warm. Its opacity is due to the presence of a very large number of solid
particles, the blood corpuscles, having a higher refractive index than
that of the liquid in which they float. The specific gravity in man
averages about 1.055. The specific gravity of the liquid portion, the
plasma (Gr. [Greek: plasma], something formed or moulded, [Greek:
plassein], to mould), is about 1.027, whilst that of the corpuscles
amounts to 1.088. To litmus it reacts as a weak alkali.
_Blood Plasma._--The plasma is a solution in water of a varied number of
substances, and as a solvent it confers on the blood its power of acting
as a carrier of food stuffs and waste products. One important food
substance, oxygen, is, however, only partly carried in solution, being
mainly combined with haemoglobin in the red corpuscles. The food stuffs
carried by the plasma are proteins, carbohydrates, salts and water. The
main waste products dissolved in it are ammonium carbonate, urea,
urates, xanthin bases, creatin and small amounts of other nitrogenous
bodies, carbonic acid as carbonates, other carbon compounds such as
cholesterin, lecithin and a number of other substances. Thus, if we take
mammalian blood as a type, the plasma would have the following
approximate composition:--
In 1000 grms. plasma--
Water 901.51
Substances not vaporizing at 120 deg. C.--
Fibrin 8.06
Other proteins and organic substances 81.92
Inorganic substances--
Chlorine 3.536
Sulphuric acid 0.129
Phosphoric acid 0.145
Potassium 0.314
Sodium 3.410
Calcium 0.298
Magnesium 0.218
Oxygen 0.455
----- 8.505
----- 98.49
-------
1000.00
_Proteins._--The proteins of the blood plasma belong to the two classes
of the albumins and the globulins. The globulins present are named
fibrinogen and serum-globulin; as its name implies, the chief
physiological property of fibrinogen is that it can give rise to fibrin,
the solid substance formed when blood clots. It possesses the typical
properties of a globulin, i.e. it coagulates on heating (in this
instance at a temperature of 56 deg.C.), and is precipitated by half
saturating its solution with ammonium sulphate. It differs from other
globulins in that it is less soluble. It is only present in very small
quantities, 0.4%. The other globulin, serum-globulin, is not coagulated
until 75 deg.C. is reached, and we now know that it is in reality a
mixture of several proteins, but so far these have not been completely
separated from one another and obtained in a pure form. On dialysing a
solution of serum-globulin a part is precipitated, and this portion has
been termed the eu-globulin fraction, the remainder being known, in
contradistinction, as the pseudo-globulin. Again, on diluting a solution
and adding a small amount of acetic acid a precipitate is formed which
in some respects differs from the remainder of the globulin present.
Whether in these two instances we are dealing with approximately pure
substances is extremely doubtful. A further important point in connexion
with the chemistry of the globulins is that dextrose may be found among
their decomposition products, i.e. that a part of it, or possibly the
whole, possesses a glucoside character.
Serum-albumin gives all the typical colour and precipitation reactions
of the albumins. If plasma be weakly acidified with sulphuric acid, then
treated with crystals of ammonium sulphate until a slight precipitate
forms, filtered and the filtrate allowed to evaporate very slowly,
typical crystals of serum-albumin may form. According to many it is a
uniform and specific substance, but others hold the view that it
consists of at least three distinct substances, as shown by the fact
that if a solution be gradually heated coagulation will occur at three
different temperatures, viz. at 73 deg., 77 deg. and 84 deg. C. On the
other hand the close agreement between different analyses of even the
amorphous preparations points to there being but one serum-albumin.
When blood clots two new proteins make their appearance in the fluid
part of the blood, or serum, as it is now called. The first of these is
fibrin ferment (for its origin see section on _Clotting_ below). The
other, fibrinoglobulin, possesses all the typical characteristics of the
globulins and coagulates at 64 deg. C.
_Carbohydrates._--Three several carbohydrates are described as occurring
in plasma, viz. glycogen, animal gum and dextrose. If glycogen is
present in solution in the plasma it is there in very small quantities
only, and has probably arisen from the destruction of the white blood
corpuscles, since some leucocytes undoubtedly contain glycogen. A small
amount of carbohydrate having the formula for starch and yielding a
reducing sugar on hydrolysis with acid has also been described. The
constant carbohydrate constituent of plasma, however, is dextrose. This
is present to the approximate amount of 0.15% in arterial blood. The
amount may be much greater in the blood of the portal vein during
carbohydrate absorption, and according to some observers there is less
in venous than in arterial blood, but the difference is small and falls
within the error of observation. The statement that when no absorption
is taking place the blood of the hepatic vein is richer in dextrose than
that of the portal vein (Bernard) is denied by Pavy.
_Fats._--Plasma or serum is as a rule quite clear, but after a meal rich
in fats it may become quite milky owing to the presence of neutral fats
in a very fine state of subdivision. This suspended fat rapidly
disappears from the blood after fat absorption has ceased. To some
extent it varies in composition with that of the fat absorbed, but
usually consists of the glycerides of the common fatty acids--palmitic,
stearic and oleic. In addition, there is a small amount of fatty acid in
solution in the plasma. As to the form in which this occurs there is
some uncertainty. It is possibly present as a soap or even as a neutral
fat, since a little can be dissolved in plasma, the solvent substance
being probably protein or cholesterin. Fatty acids also appear to be
present to some extent combined with cholesterin forming cholesterin
esters (about 0.06%).
_Other Organic Compounds._--In addition to the substances above
described, belonging to the three main classes of food stuffs, there are
still other organic bodies present in plasma in small amounts, which for
convenience we may classify as non-nitrogenous and nitrogenous. Among
the former may be mentioned lactic acid, glycerin, a lipochrome, and
probably many other substances of a similar type whose separation has
not yet been effected.
The non-protein nitrogenous constituents consist of the following:
ammonia as carbonate or carbamate (0.2 to 0.6%), urea (0.02 to 0.05%),
creatine, creatinine, uric acid, xanthine, hypoxanthine and occasionally
hippuric acid. Three ferments are also described as being present: (1) a
glycolytic ferment exerting an action upon dextrose; (2) a lipase or
fat-splitting ferment; and (3) a diastase capable of converting starch
into sugar.
_Salts._--The saline constituents of plasma comprise chlorides,
phosphates, carbonates and possibly sulphates, of sodium, potassium,
calcium and magnesium. The most abundant metal is sodium and the most
abundant acid is hydrochloric. These two are present in sufficient
amount to form about 0.65% of sodium chloride. The phosphate is present
to about 0.02%. Sulphuric acid is always present if the blood has been
calcined for the purposes of the analysis, and may then be present to
about 0.013%. This is, however, probably produced during the destruction
of the protein, since it has been shown that no sulphate can be removed
from normal plasma by dialysis. The amount of potassium present (0.03%)
is less than one-tenth of that of the sodium, and the quantities of
calcium and magnesium are even less.
_Formed Elements._--When viewed under the microscope the main number of
these are seen to be small yellow bodies of very uniform size, size and
shape varying, however, in different animals. When observed in bulk they
have a red colour, their presence in fact giving the typical colour to
blood. These are the _red blood corpuscles_ or _erythrocytes_ (Gr.
[Greek: erythros], red). Mingled with them in the blood are a smaller
number of corpuscles which possess no colour and have therefore been
called _white blood corpuscles_ or _leucocytes_ (Gr. [Greek: leukos],
white). Lastly, there are present a large number of small lens-shaped
structures, less in number than the red corpuscles, and much more
difficult to distinguish. These are known as _blood platelets_.
_Red Corpuscles._--These are present in very large numbers and, under
normal conditions, all possess exactly the same appearance. With rare
exceptions their shape is that of a biconcave disk with bevelled edges,
the size varying somewhat in different animals, as is seen in the
following table which gives their diameters:--
Man 0.0075 mm.
Dog 0.0073 mm.
Rabbit 0.0069 mm.
Cat 0.0065 mm.
Goat 0.0041 mm.
The coloured corpuscles of amphibia as well as of nearly all vertebrates
below mammals are biconvex and elliptical. The following are the
dimensions of some of the more common:--
Pigeon 0.0147 mm. long by 0.0065 mm. wide.
Frog 0.0223 " " 0.0157 " "
Newt 0.0293 " " 0.0195 " "
Proteus 0.0580 " " 0.0350 " "
Amphiuma 0.0770 " " 0.0460 " "