The various waste products from this combustion of the digested nutrients and from the breaking up of the proteins within the animal body evidently must be disposed of in some manner. If not eliminated from the body, they would cause results of a most serious character, as, for instance, when an accumulation of urea in the blood produces ursemic poisoning. The blood, therefore, not only carries to the tissues the necessary nutrients and oxygen, but it has laid upon it the burden of taking into its currents the waste products of combustion and growth, and carrying them to the points where they are thrown off.

141. Urea

One of the branches of the arterial system of blood-vessels runs to the kidneys, and, by repeatedly rebranching, traverses all their substance. The main function of the kidneys is to secrete the urine, a liquid in which all the waste nitrogen from the digested protein finds its way out of the body in the form of urea and other nitrogen compounds. The blood that enters them carries with it the urea and uric acid which have resulted from a breaking down of protein, and, in a most wonderful manner, these compounds are filtered out so that they are not present in the outgoing blood.

142. Mineral Compounds

The mineral ingredients of the food in excess of storage in the body are excreted both through the kidneys and in the feces. Compounds of potassium and sodium appear almost wholly in the urine. Phosphorus, calcium, and magnesium are divided between the urine and feces, the two former being more largely excreted by way of the intestines, while practically all the iron goes out by way of the feces.

143. Carbon Dioxid

The carbon dioxid must in some way also be eliminated from the body. This is not accomplished to any extent until the venous blood containing it reaches the lungs, where it is exchanged for a new supply of oxygen and passes off in the expired air. In the case of man, the air " breathed out " is nearly a hundred times richer in carbonic acid than the air " breathed in."

144. Water

Water may be regarded from one point of view as a waste, for it is produced in the oxidation of the food, and this passes off from the lungs as vapor, through the skin as sensible or insensible perspiration, and in considerable quantities through the kidneys. Benedict and Carpenter have shown, on the basis of a large number of determinations, an average loss of 960 grams of water from the lungs and skin by a person at rest, during twenty-four hours; a little less than two-thirds of this loss coming from the surface of the body. This " insensible perspiration" is 60 per cent as much as the water excretion through the kidneys. A man at hard work may lose several times as much water from the lungs and skin as through the kidneys. To summarize, it may be said that the blood is constantly undergoing gain and loss. The gain comes from the food (including water and oxygen), and the loss consists of urea, carbonic acid, and water given off through various channels.