The total amount of blood has been estimated to be from 1/13 to 1/14 of the body weight for an adult man, and somewhat less for a newborn child.

Much difficulty has been found in arriving at an accurate estimation of the amount of blood in the body. In the first place, all the blood cannot be made to flow out of the vessels of an animal when it is killed. Secondly, the quantity and quality of blood are constantly varying with the capacity of the blood vessels. Thirdly, when slowly withdrawn from the body during life it is rapidly replaced by more fluid passing into the blood vessels. This explains the enormous quantity of blood occasionally reported to be shed in cases of bleeding to death. In these cases, as quickly as the blood is lost, fluid is absorbed by the fine vessels to replace it, so that if the bleeding be gradual the standard quantity is still kept up in the vessels. Thus the very sudden loss of a comparatively small quantity of blood may cause death, whereas, if the bleeding go on sufficiently slowly and gradually, as much or even more in quantity than normally exists in the entire body may escape without fatal result. Of course much of this is fluid which has recently entered the vessels to replace the blood already lost.

Weber's Method

The percentage of solid matters in the blood is first carefully estimated. The absolute quantity of solids in all the blood drawn is then ascertained and added to the solids obtained by washing out the blood vessels. Here an error arises from the fact that, in washing out the blood vessels, much solid matter besides that belonging to the blood is taken from the tissues, and thus an excess is found.

Valentine's Method

A small measured quantity of blood is drawn from a vein and its percentage of solids accurately estimated; a known quantity of water is then injected into the vessels. When some time has been allowed for proper distribution of the water, a sample of the diluted blood is taken and its solids estimated. The difference in solid contents of the two samples shows the degree of dilution caused by a known quantity of water introduced into blood of ascertained strength, and thus the amount of the diluted fluid (the blood) may be calculated and added to the amount of the first sample to make the absolute quantity.

This method cannot give accurate results, because in the time necessary for the distribution and mixture of the water with the circulating blood much of the former is. excreted by the kidneys and skin, and the second sample of blood is more concentrated than should result from such dilution.

Welcker's Method depends upon the estimation of the coloring matter of the blood. He connected the carotid with a small piece, and allowed the animal to bleed into a bottle in which the blood could be defibrinated by shaking with pieces of glass. One cubic centimetre of this defibrinated blood was carefully measured off and saturated with carbon monoxide (CO), which gives a permanent and equally bright red color. It was diluted to 500 cc. with distilled water and kept as a standard color solution. The blood vessels of the animal were then washed out with.6 per cent, solution of sodium chloride until the solution flowing from the jugular vein was colorless. The tissues of the animal were chopped up, steeped in water and pressed. The washings of the vessels and the infusion from the tissues were added together and diluted until they had the same color intensity as a layer of the standard solution of the same thickness. Every 500 cc. of these diluted washings corresponds to 1 cc. of blood.

By this method the following estimates have been made of the relation of the blood to the body weight: -

Mouse,...................1/12 - 1/15

Guinea pig,.................1/17 - 2/22

Rabbit,...................1/12 - 1/22

Dog,....................1/11 - 1/15

Cat....................1/21

Bird..........................................................................................1/10 - 1/13

Frog.........................................................................................1/15 - 1/20

Only approximate estimates of the distribution of blood in the body during life can be made, since there can be no accurate method of investigation, and the amount varies considerably, according as the organ or part is in a state of rest or activity. It is supposed that a quarter of the entire amount is habitually flowing through each of the following regions: -

1. The heart, great vessels and lungs.

2. The skeletal muscles.

3. The liver.

4. Skin and other tissues.