Blood is a fluid substance that circulates in the arteries
and the veins of the body. Blood is bright red when it has
been oxygenated in the lungs and passes into the arteries.
It becomes bluish red when it has given up its oxygen to
nourish the tissues of the body and is returning to the
lungs through the veins and the tiny vessels called capillaries.
In lungs, the blood gives up the carbon dioxide wastes it
has taken from the tissues and receives a new supply of
oxygen and then it begins a new cycle. This movement of
blood is brought about by the vital activities of the heart,
lungs, and blood vessels.
Composition of Blood
Blood is composed of a yellowish fluid which is called
plasma. The plasma is suspended the millions of cells that
constitute about 45 per cent by volume of whole blood. It
has a characteristic odour. In an average healthy adult,
the volume of blood is one-eleventh of the body weight,
or between 4.5 and 6 litres.
A great portion of the plasma is composed of water which
is a medium that facilitates the circulation of the many
indispensable factors of which blood is composed. A cubic
millimetre of human blood contains about 5 million red corpuscles
called erythrocytes and 5,000 to 10,000 white corpuscles
called leucocytes and also 200,000 to 300,000 platelets
called thrombocytes. The blood also carries many salts and
organic substances in the form of a solution.
The red corpuscles or red blood cells, are round discs
which are concave on two sides, and are approximately 7.5
thousandths of a millimetre in diameter. In humans, and
most other mammals, the mature red blood cell contains no
nucleus while in some vertebrates, it is oval and thus nucleated.
Haemoglobin a protein in the red blood cells, is the most
prevalent of the special blood pigments that transport oxygen
from the lungs to the body cells, where it picks up carbon
dioxide for transport back to the lungs to be expired.
The white blood cells are of two principal types: the granular,
which have multi lobed nuclei, and the non granular which
have rounded nuclei. The granular leucocytes include neutrophils,
which ingest and destroy bacteria , eosinophils are also
present which increase and become active in the presence
of certain infections and allergies; and basophils, which
secrete the anticoagulant heparin and the substance histamine
which further stimulates inflammation. The non granular
leucocytes are the lymphocytes and the less numerous monocytes
are both associated with the immune system. Lymphocytes
have an important role in producing antibodies and provide
cellular immunity. Monocytes ingest nonbacterial foreign
Blood platelets are small, round, non nucleated bodies
with a diameter about one-third that of red blood cells.
Thrombocytes adhere to the walls of blood vessels at the
site of an injury and so they plug the defect in the vascular
wall. While disintegrating , they release coagulating agents
that lead to the local formation of thromboplastin, which
helps to form a clot this is the first step in the healing
of an injury.
Plasma is a complex substance whose its principal component
is water. It also contains plasma proteins and inorganic
constituents such as sodium, potassium, calcium chloride,
carbonate, and Among the component plasma proteins are albumin,
the principal agent in maintaining the osmotic pressure
of the blood and therefore in controlling its tendency to
diffuse through the walls of blood vessels. A dozen or more
proteins, including fibrinogen and prothrombin, which participate
One of the most remarkable properties of blood is its ability
to clot, when it is withdrawn from the body. Inside the
body, a clot is formed in response to tissue injury, such
as a muscle tear. In the blood vessels, the blood remains
in a fluid condition even shortly after being withdrawn,
it becomes viscid and gelatinous and sets into a firm, jelly-like
mass. This mass then separates into two portions: a firm
red clot floating free in a transparent and a straw-coloured
fluid called serum.
A clot consists almost entirely of red corpuscles entangled
in a network of fine fibrils or threads, composed of a substance
called fibrin. This substance does not exist as such in
blood but is created by the action of thrombin which is
an enzyme that promotes the conversion of fibrinogen, one
of the plasma proteins, to fibrin in the clotting process.
Thrombin is not present in circulating blood; it is formed
from prothrombin, by a complex process involving blood platelets
and certain calcium salts. If any of these factors is deficient,
clot formation is defective. The addition of sodium citrate
removes calcium ions from the blood and thus prevents a
clot from forming. Lack of vitamin K makes impossible the
maintenance of the proper amount of prothrombin in the blood.
Certain diseases may lower the concentration of the various
clotting proteins or of the platelets of the blood.
In the case that you have a doubt
on a something listed here or want to know something that
has not been mentioned please use our Reference