Blood is composed of two parts, the cellular elements, and the fluid, plasma or liquor sanguinis. Although its reaction to litmus is alkaline, the use of more delicate reagents would indicate that the blood may be considered as approximately a neutral fluid. Its specific gravity varies between 1055 and 1065, the average being 1060. The odor is characteristic in different animals. The color is due to the presence in the erythrocytes of an iron-containing albuminous substance, hemoglobin, which has a marked affinity for oxygen and other gases. Arterial blood is freshly aerated and is bright red in color, venous blood is bluish. The variations in color depend upon the relative proportions of oxygen and carbon dioxid in the two varieties of blood.

The chief gaseous constituents and their proportions are as follows:

Coagulation

On standing, except under certain conditions, the blood will tend to separate into its two portions, the cellular elements and the plasma. In a short time, the period varying according to the composition of the blood, the phenomenon of coagulation will occur. In this process the plasma changes into serum and the clot, this latter being composed of minute threads of fibrin which hold the corpuscles in a network. In normal blood a substance called fibrinogen is present in the plasma. Under the action of the ferment 20 305 thrombase, which is present as prothrombase in the leukocytes, the fibrinogen is converted into fibrin. The prothrombase is changed into thrombase by the action of calcium compounds, which appear to be the active agents in causing coagulation.

Normally coagulation should take place in from three to eight minutes, but it may vary considerably under pathologic conditions.

Hemoglobin

The color of the blood is due to the combination of oxygen and hemoglobin, the latter owing its ability to take up oxygen to the presence of iron in the molecule. The amount of the coloring matter may be much diminished either by a decrease in the actual number of red cells, or by each cell containing less than its normal amount.

Under certain conditions the hemoglobin may be dissolved out of the cells and escape into the plasma, hemoglobinemia. The cells that have lost the coloring matter are sometimes referred to as shadow corpuscles.

A diminution in the amount of hemoglobin without a decrease in erythrocytes is known as oligochromemia, the individual cells containing less than normal. In some diseases there is an absolute reduction of hemoglobin, but the individual red cells may contain an increased amount, the diminution being due to an actual decrease in the number of erythrocytes.

Methemoglobin is closely related to oxyhemoglobin but differs in that the oxygen in the former is in a more stable combination and cannot be made use of by the tissues. It gives a chocolate color to the blood and occurs in various forms of poisoning as from the chlorates, nitrites, acetanilid, antipyrin, and some others.

In poisoning by carbon monoxid the blood is a bright cherry red in color. In carbon dioxid poisoning the blood is dark in color.

Color-Index

This term is used to indicate the amount of hemoglobin contained in each red cell, as compared with the amount present in a normal erythrocyte. It is obtained by dividing the hemoglobin percentage by the percentage of red cells. This latter is procured by dividing the number of red cells found in the blood by 5,000,000, the number accepted as normal. This percentage can be gotten readily by multiplying the number of hundred thousands of red cells by two. The color-index is normally one; a hemoglobin value of 100 per cent, is associated with a blood count of 5,000,000 red cells.

In some conditions the amount of hemoglobin is greatly reduced without a corresponding diminution in the amount of reds. In such the index is less than one. This occurs particularly in chloroses and splenic anemia. At other times the diminution of the reds is much greater proportionately than that of the hemoglobin, the color-index will then be high. This is the condition found in the pernicious types of anemia.