# The New Student's Reference Work/Blood

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​Blood, the circulating fluid of animals. It is a nutrient fluid of varied composition. In the course of its circulation it is continually giving nourishment to the tissues ​and carrying away from them worn-out material. Therefore, it carries on a sort of exchange with the tissues, in which there are certain sources of loss and certain sources of gain to the blood. For example, it loses to the tissues soluble food material and oxygen; in the lungs it loses carbon dioxid (CO₂); through the skin it loses water and certain salts in solution; and through the kidneys it loses the broken-down protoplasm that contains nitrogen and water. But the gain keeps pace with the loss; in the lungs it receives oxygen; from the digestive system it receives soluble food material and water; from the tissues it receives the broken-down material in the form of carbon dioxid, water and nitrogenous waste. From these statements it will be seen that the blood is the agent of exchange between the tissues and the outside world.
There is a similar circulating fluid in insects, mollusks, worms and other simple animals as well as in the vertebrated animals. Contrary to the usual statement, this fluid in the simpler animals, also, contains solid particles or corpuscles, though they correspond with the white blood corpuscles rather than with the red ones. In the fluid part of the blood of the crayfish, also, for example, there is a substance which acts as an oxygen carrier.
As everyone knows, the blood undergoes changes from arterial, bright red color to venous of dark blue tint, and back again from venous to arterial; but we should understand clearly where these changes take place. It is not in the ordinary blood vessels which have thick walls, but in the network of capillaries—those small tubes with very thin walls that connect the arteries and veins. For example, the blood is rendered arterial in the capillaries of the lungs and venous in the capillaries of the tissues; also, all the exchanges spoken of above take place in the capillaries.
Blood is made up of a fluid plasma, in which float minute corpuscles. In higher animals there are two kinds, red and white. The red ones are much more numerous than the white (in the human body about 355 red corpuscles for one white). They are oxygen carriers and their color is due to the presence of the substance (hæmoglobin) which holds the oxygen. They have a regular life-history; they live a few weeks and break down, disappearing principally in the liver and the spleen. It follows, since they disappear, that they must be renewed, and new ones are being continually formed in the red marrow of bones. The white blood corpuscles are larger in size; they possess the power of changing form and creeping. They gather particles of foreign substance, and creep with them out of the blood vessels, and even to the surface of the body. They also feed upon bacteria, and help to rid the body of harmful kinds. The white blood corpuscles are, in a sense, eating cells. They are renewed from the spleen and the lymphatic glands. Physiologists now recognize several kinds of white blood corpuscles.
The clotting of blood prevents profuse bleeding. After a cut the blood tends to form a stringy fibrin in which the corpuscles get entangled and this forms a sort of plug to the smaller vessels and prevents further bleeding. The cut ends of larger blood vessels cannot be stopped in this way.