It is by the formation of a clot that bleeding from a vessel that has been opened is stopped naturally. The clot closes the opening, and gradually fibrous tissue becomes formed at that part of the vessel, which thus becomes per manently closed. When a wounded artery is tied the same thing happens. A clot forms at the tied part, and finally fibrous tissue takes its place and the vessel is completely sealed up.
The Chemical Constitution of the Blood.
—Human blood has an average specific gravity of 1055; it has an alkaline reaction, which in shed blood quickly diminishes up to the moment of clotting. We have seen that circulating blood consists of corpuscles (red and white) floating in plasma. Of the total weight of blood more than one-third, and less than one half, is made up of the corpuscles, and the rest of the weight is made up of plasma. The plasma contains fibrin and serum, the fibrin forming about '2 per cent, a remarkably small quantity considering the part it plays in coagulation.
The red corpuscles consist of 57 parts of water and 43 parts of solid matters in 100 parts of the wet corpuscles. The solid matter is mainly a substance called form ing 90 per cent of the solids of the corpuscles, the remainder being proteid substances, traces of other organic substances (cholesterin and lecithin), and inorganic salts—the salts of po tassium and phosphates. Haemoglobin consists of an albuminous body, globulin, and a colour ing matter, haematin, and it is capable of crys tallizing in various shapes. It is remarkable for containing 4 parts of iron in 1000. Haemo globin forms a combination with oxygen gas called oxy-haemoglobin, which is of a bright scarlet colons. But the oxygen may be removed from the haemoglobin, which then becomes of a darker and more purple hue; or, if carbonic acid gas be substituted for the oxygen, a dark colour is produced. This is the essence of the explanation offered for the difference between the scarlet blood found flowing in arteries, and the more purplish blood of the veins. This is explained at greater length in Section XVI. If haemoglobin, dark in hue because it has been deprived of oxygen, be exposed to an atmos phere containing that gas, it seizes upon it greedily, and speedily becomes of a bright hue.
This is the reason why blood, which may have been dark when shed, becomes of a bright scarlet colour, at least on the surface, if exposed to the air. It seizes on the oxygen of the air. Thus the red blood corpuscles consist mainly of a substance greedy for oxygen, and thus these bodies become the oxygen carriers of the body, seizing upon the oxygen which they get in the air in the lungs, and hurrying with it to the remotest parts of the body. (See Section XVI.) Haemoglobin, it may be added, is also called hmmatoglobin or hmmatocrystallin.
The serum of blood, that is, the plasma de prived of its fibrin, consists of water 90 per cent, albuminous or proteid substances 8 to 9 per cent, and 1 to 2 per cent of fats and saline matters, and substances capable of being dis solved out by water, alcohol, or ether, and hence called extractives, such as urea, kreatiu, sugar, lactic acid. The saline matters are chiefly salts of sodium. Thus the serum differs from the corpuscles in which the potash salts are found. • The blood also contains gases, which may be separated from it by allowing the blood to flow into a vacuum at a temperature higher than body heat Fahr.). At the ordinary pres sure of the atmosphere, 30 inches of mercury,and a temperature of 32° Fahr., the quantity of gas separated from 100 volumes of blood is about 60 volumes. It consists of oxygen, carbonic acid, and nitrogen, in different proportions ac cording to whether the blood was arterial or venous. Thus the 60 volumes are distributed as follows:— Of Oxygen Of Carbonic Acid Of Nitrogen In arterial blood,... )20 volumes 39 volumes 1 to 2 voluines.
In venous to 12 „ 46 „ 1 to 2 „ blood,...
The significance of these figures will be com mented on at greater length in the section in which breathing and its purposes are discussed (Section XVI.); but it is well to note now that arterial blood contains more oxygen and less carbonic acid gas than venous blood. With that fact we at once associate the bright scarlet hue of arterial blood, and the purplish colour of venous blood, remembering the love of the hEemoglobin of the red blood corpuscles for oxy gen, and the bright colour resulting from its satisfaction, and the dark colour resulting from deprivation.