|Composition of blood|
|Physiology of blood|
Blood is composed of two principal components :
In the living body :
The blood circulates as a fluid. During certain illnesses, one notices :
In the dead body :
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.