In all animals, except those which form the lowest class (Protozoa), the distribution of the nutritious materials to the various parts of the body, as well as the collection of the effete matters prior to their expulsion, is carried on by the medium of a fluid which circulates through the different parts of the body. This fluid is the blood.

In vertebrate animals the blood passes through a closed system of elastic pipes and is kept in constant motion by the action of a muscular pump. It is first forced through strong, branching canals called arteries, whose walls gradually become thinner as the branches get smaller, and end in a network of delicate channels (capillaries), through which it slowly trickles into the wide, soft-walled veins by means of which it flows gently back again to the heart. In its course it receives the nutritive materials from the stomach and intestines after digestion, the specially elaborated substances from the liver, spleen and lymph glands, and the oxygen absorbed from the air in the lungs. In short, it contains and bears to their destination all the materials required for the chemical changes of the economy. While passing through the capillary networks of the various tissues, it takes up the waste materials resulting from the tissue changes and bears them to their proper point of exit from the body; at the same time the nutriment oozes through the delicate vessel walls and is diffused in the tissues.

General Characteristics Of The Blood

The blood of vertebrate animals is a bright scarlet color when exposed to the oxygen of the air, but when not in contact with oxygen it becomes a dark purplish red.

The blood is remarkably opaque, as may be seen by placing a thin layer on a piece of glass over the page of a book. This opacity depends on the fact that the blood, as will presently be seen, is not a red fluid, but owes its color to the presence of solid red particles or corpuscles which float in a clear, pale fluid. The blood has a peculiar smell (halitus) distinct in different animals and man, dependent on certain volatile fatty acids. Its specific gravity varies from 1045 to 1075,the average being 1055. The solid parts (corpuscles) are heavier (sp. gr. 1105) than the liquor sanguinis (1027).

When first shed the blood has a slippery feel, which it soon loses, becoming viscous as it passes into the first stage of coagulation.