If the albuminoid food element has entered the blood, whether or not it has been built up into a constituent part of the structure of the body, its waste products, which contain nitrogen, are in the main expelled through the kidneys, so that the latter become the principal channels for the expulsion of all nitrogen-containing waste.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
While much of the waste material containing nitrogen leaves the body by the bowels, this is virtually only such of the albuminoid food as has failed to be fully digested and absorbed; this has never formed a true constituent part of the body itself or of the blood, but is so much waste food, like that which has come to the table and again carried away unused.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
All living organisms contain albumin; this is probably due to the fact that albuminoid matter is particularly adapted for the formation of osmotic membranes.
"The Mechanism of Life"
Stéphane Leduc