Composition of
blood 
Blood is composed of two principal components :
- The cells (the 3 main types of which are red cells or erythrocytes, white
cells or leucocytes, and platelets).
- The plasma, itself composed of :
An ionic solution : Water (90%) containing ions of sodium,
potassium, calcium, magnesium, chlorine, etc. )
Proteins, etc : albumin, globulin, coagulants such as fibrinogen,
prothrombin...)
Blood plasma which has lost its fibrinogen is called blood plasma.

Physiology
of the blood 
In the living body :
The blood circulates as a fluid. During certain illnesses, one notices :
- either a tendency to increased coagulaton : (thrombosis)
- which can be arterial, causing strokes and heart attacks,
or;
- which can occur in the veins, giving rise to thrombophlebitis,
- or a tendency to reduced coagulation which can result in haemorrhage.
In the dead body :
- If there is a wound in the wall of the blood vessel the blood will
coagulate, (just as in a living person), in the minutes immediately after death.
- If there is no wound in the walls of the blood vessel, the blood will
remain liquid for several days, after which it starts to decompose. The blood in the
arteries tends to drain into the capillaries and veins. (This phenomenon probably gave
rise to the ancient belief that the arteries carried air and played a part in respiration,
and it is interesting to note that modern thinking on the circulation and physiology of
the blood started only around the beginning of the XVIIth century, about 300 years after
the Shroud was first known to have been put on display). If a big vein is cut after death,
e.g. during an autopsy, the blood runs ; and if this is done very shortly after death this
blood will coagulate. In effect, the blood cells continue to live for several hours after
the person has died. In the past, in Russia, successful blood transfusions have been made
using blood taken from people who had just died.
Coagulation

In the case of a cut or
rupture of the blood vessel wall, the escaping blood comes into contact with collagen
(contained in the tissues which surround the blood vessel). This contact triggers off the
coagulation process. Initially this is a series of complex chemical reactions, lasting
about three minutes, and involving trace protein and tissue factors. Then the soluable
Fibrinogen gets converted into the insoluble protein Fibrin., and in 10 to 12 minutes,
this Fibrin has formed an initial barrier over the wound, initially a soft clot containing
serum and blood cells. Under the action of coagulation factor XII, this clot shrinks,
expelling the liquid serum that it contains. In 1 to 3 heures, the quantity of serum
ejected corresponds to about half the volume of the initial clot. If the coagulation takes
place on the exterior surface of the body, the clot dries to form a scab (we have all seen
this on our own skin after a scratch or cut). If the coagulation takes place inside the
body, the clot stays humid, and does not dry to a scab.
Most of us will have noticed, on the dressings and
bandages that have protected wounds, that there is often a sort of clear halo around the
central point ; this halo is formed by exuded serum, containing mineral salts, proteins
and water.
The blood cells in the clot contain most of the elements
that were present in the cells in the blood, notably haemoglobin, the main constituant of
red cells (Note that haemoglobin is rich in iron), and some antigens, such as those
responsible for the different blood groups A, B, AB, rhesus, that we normally find in the
red blood cells. (Note that Group O signifies only the absence of the antigens A or B),...
Footnote 1 -- When the old-time local butchers made black pudding, they needed
blood that did not coagulate, so they whisked vigorously the blood that ran from a stuck
pig (a pig with its throat cut). By doing so they broke up the fibrin as it formed, and
apparently prevented the blood from clotting. In reality it still coagulated, in the
bio-chemical meaning of the word, but the clots were mini-clots and not visible.
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