This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
74. Having traced the process of digestion, it would be natural to pursue the history of the new supplies of nourishment after their entrance into the economy from the alimentary tube. It will be found, however, to be more convenient, if, instead of adhering strictly to the course taken by these supplies, we first consider the blood, and afterwards the streams which fall into it, and its mode of elaboration.
On account of the extreme facility with which substances pass outwards and inwards between the minutest vessels and the tissues, and the impossibility of completely emptying the vascular system, even in the bodies of animals, it is exceedingly difficult to estimate the amount of the blood. In one observation, in which the blood was carefully washed from the bodies of two executed criminals, and the calculation based on the amount of solid matter obtained, the weight of blood was estimated as one-eighth of that of the body (Weber and Lehmann). According to other calculations founded on observations on animals, and made by mixing a portion of blood with a known amount of water, then washing out the vessels, and reducing the washings to the same tint as the standard solution, it was computed at about one-thirteenth of the weight of the body, or twelve pounds in a person eleven stones weight (Welcker).