Metaphor: One part doesn't make a whole. Flour isn't a cookie.
According to the European Food Information Council, "The salivary glands in our mouth produce about 1-2 litres of saliva daily. Blood plasma is used as the basis, from which the salivary glands extract some substances and add various others." What are some of those ingredients (according to the Journal of Contemporary Dental Practice):
Salivary fluid is an exocrine secretion consisting of approximately:
- 99% water
- A variety of electrolytes (sodium, potassium, calcium, chloride,
magnesium, bicarbonate, phosphate) and proteins, represented by
enzymes, immunoglobulins and other antimicrobial factors
- Mucosal glycoproteins
- Traces of albumin and some polypeptides and oligopeptides of
importance to oral health
- Glucose and nitrogenous products, such as urea and ammonia
Since blood plasma is the catalyst for saliva creation, our other cells do not have much use for nitrate - which is why dietary nitrate floats unused in our blood until we excrete it via urine.
So yes, our saliva contains a component of urine - but one component of urine doesn't make it urine.
Urea vs. Urine
Urea is a nitrogenous waste - but it isn't urine... yet. Urea's molecular formula is CH4N2O. It doesn't have the sulfates and phosphates that urine does.
Urine is nitrogenous waste - but it has much other stuff. Human urine consists primarily of water, with organic solutes including1:
- uric acid
- trace amounts of enzymes
- fatty acids
- inorganic ions such as sodium (Na+), potassium
(K+), chloride (Cl-), magnesium
(Mg2+), calcium (Ca2+), ammonium
(NH4+), sulfates (SO42-),
and phosphates (e.g., PO43-)
Just because we urinate phosphates, nitrates, and sulfates, doesn't mean we are "peeing liquefied mustard gas."
1Composition and Concentrative Properties of Human Urine