Blood is the fluid which circulates through the
heart, arteries, and veins. It is mixed with lymph, its corpuscles
being suspended in a fluid called blood-plasm (or plasma). The arterial
blood is of a lighter red than the venous, which is more purple blood.
Blood shows the following composition:-
1. Red blood corpuscles or cells,
oval, flat disks, with rounded-off margins and a central nucleus which
forms a slight swelling: they contain a substance known as haemoglobin,
which, combining with the oxygen of the blood, causes the latter's red
colour. These red corpuscles are present even in a small drop of blood
in innumerable numbers. In birds, red corpuscles are largest in the Cassowary, smallest in
Humming-bird, their smallest axis measuring about 1/110
or 1/175 mm, their largest axis from 1/59
to 1/105 mm.
2. White blood or lymph corpuscles;
by far less numerous, colourless and of very variable size (from 1/500
mm to 1/100 mm) showing lively amoeboid motions.
These aid in the body's defence against infection and diseases.
3. The blood plasm or plasma, consisting
of fibrin and serum. The blood plasma is the liquid transportation for
the corpuscles or cells. Serum is a fluid, freequently yellowish, and is
composed of water, albumen, and various salts.
4. Platelets, which are fragments
of blood cells and essential for repairing wounds by the process of clotting.
Function of Blood
The function of the blood is this: The arterial blood
in the capillaries of the body gives off its oxygen to the tissues of the
body; the lymph, charged with the nutritive elements derived through the
process of digestion, bathes the same tissues, by leaving the capillaries,
and is collected again into lymphatic vessels, being ultimately emptied
into the big veins of the body, to be mixed again with the deoxydized blood
returning likewise through the veins from the capillaries of the whole
body. All this exhausted blood is, together with the lymph, received into
the right auricle of the heart, thence pumped through the right ventricle
and the pulmonary arteries into the capillaries of the lungs, there to
give up its carbonic acid, and to be charged again with oxygen. Returning
through the pulmonary veins into the left auricle, and thence into the
left ventricle, it is forced by the contraction of the latter into the
arteries of the body to commence its circulation anew.
The lymph is a fluid like the blood-plasm, slightly
yellowish or colourless and containing only white, but no red, blood-corpuscles.