Ammonia is present in the form of ammonium salts, and forms about 4% of the total urinary nitrogen. It may exceed this amount under certain conditions, for the most part pathologi cal. The ammonia is utilized by the body to neutralize acids which arise during the various metabolic processes.
Purim (uric acid, xanthin, hypoxanthin, etc.) are all members of a series which have as their common nucleus a body which E. Fischer called purin. The most important member of this series is uric acid. It forms about 2% of the total urinary nitrogen. Re cent work has shown that it has two quite definite sources of origin : from ingested food containing the precursors, and (2) from the tissue metabolism. The first is known as the exog enous source, and the second as the endogenous. This acid is chemically known as trioxy-purin, and may be regarded as the union of two urea molecules with a three-carbon chain fatty acid. All the uric acid formed in the body is not excreted as such, part being, as already mentioned, converted into urea. The amount which is converted into urea varies with the species of animal. The other chief purin bodies present in urine are xanthin and hypoxanthin, purins less oxidized than uric acid; the first is a dioxy-purin, and the second is a monoxypurin. The main source of total purin supply would seem to be muscle metabolism. The mother substances, from which all are derived in the body are the nucleo proteins. These complex bodies are apparently first broken down by enzyme action to aminopurins. These in their turn have their amino groups split off, and then, according to the degree of oxidation, the different purin bodies are formed.
Creatinine. The physiological significance of this substance is as yet unknown. The daily excretion varies little with the char acter of the diet, provided, of course, that the diet be creatinine free. It would seem to be derived from the creatine of muscle,
a substance which is very readily changed into creatinine outside the body. Whatever its source, after urea and ammonia it is one of the important nitrogenous substances excreted.
The sulphur excreted in the urine comes chiefly from the sul phur of the protein molecule. It is excreted in various forms: (I) as the ordinary preformed sulphates, i.e., sulphur in the form of sulphuric acid combined with the ordinary bases; (2) as ethereal sulphates, i.e., in combination with various aromatic substances like phenol, indol, etc.; (3) in the form of so-called neutral sul phur in such substances as cystin, which are intermediate products in the complete oxidation of sulphur.
Phosphorus appears linked to the alkalis and alkaline earths as phosphoric acid. A very small part of the phosphoric acid may be eliminated in organic combination such as the glycero-phosphates, etc.
Sodium (mostly as sodium chloride), potassium, calcium and magnesium are the common bases present in the urine.
The lungs are the important channel of excretion for the waste product of carbon metabolism, carbon dioxide or (see RES PIRATORY SYSTEM) ; and also a very important channel for the ex cretion of water. As regards the skin, the sweat carries off a large amount of the water. It has been estimated that about 9ooc.c. is excreted per diem under normal resting conditions, rising to 7,000c.c. and over when hard work is done. Sweat contains salts, chiefly sodium chloride, and organic waste products. Of the or ganic solids excreted from this source urea forms the most im portant under normal conditions. Under pathological conditions, especially when there is interference with free renal action, the amount of nitrogenous waste excreted may become important.