The composition of urine varies greatly from person to person. It can also vary in the same person depending on their diet or the time of day.
Urine is about |95\%| water, but also contains various solutes such as urea (a nitrogenous waste product), minerals (e.g., potassium and sodium) and other substances (vitamins and urobilin, or urochrome).
Urobilin, also called urochrome, is the pigment that gives urine its yellow colour. It is the result of the breakdown of hemoglobin in the liver.
In the case of certain diseases or infections, urine may also contain substances that are not normally there. For example, the presence of glucose (sugar) in the urine, called glycosuria, can be a sign of diabetes. This is why urine analysis, also called urinalysis, is often part of a medical examination, as it can detect certain conditions.
Urine usually has a faint odour when it leaves the body. However, if the urine is left to stand, bacteria converts the nitrogenous waste into ammonia, and the smell becomes stronger. Certain foods, such as asparagus, as well as drugs and toxins can alter the smell, as most of the toxins are excreted through the kidneys.
The pH of urine is usually around 6, or between 4.5 and 8 depending on metabolism and diet. A high-protein diet produces acidic urine, while a vegetarian diet produces more alkaline urine.
The amount and composition of urine varies according to several factors: the amount of fluids ingested, diet, sweating, stress and the use of certain substances, such as medication. Several drugs promote water retention, while others, called diuretics, increase the excretion of water through the kidneys and, therefore, the production of urine.
The concepts covered in this section are more advanced than those covered in secondary school. This is enrichment information for students who want to find out more.
The animal kingdom has 3 types of nitrogenous waste, which is a by-product of protein and amino acid metabolism. As they are highly toxic, eliminating them is often a challenge for living organisms. When these proteins are converted into lipids or carbohydrates, or broken down to create energy (catabolism), nitrogenous waste is produced in the form of ammonia.
Ammonia
In aquatic and marine animals, nitrogen is excreted directly as ammonia |(\text{NH}_3).| This ammonia is usually excreted as ammonium ions |({\text{NH}_4}^{+}),| directly into the water through the gills of fish and through the skin of soft-bodied invertebrates.
Mammals excrete urea, which is the main component of urine. Urea is composed of about |5\%| ammonia, which gives it its characteristic strong odour.
Uric acid
Birds, insects, snails and some reptiles use uric acid |(\text{C}_5\text{H}_4\text{N}_4\text{O}_3)| to excrete their nitrogenous waste products.
For reference, uric acid is the whitish substance found in bird droppings, while the brown substance is the feces. The advantage of this substance is that it is thousands of times less soluble in water than ammonia, which allows the body to reabsorb all the water from the urine before excreting a precipitate. It is excreted into the cloaca and expelled from the body along with the feces. Ideally, terrestrial animals want to avoid water loss. Uric acid has an important benefit for oviparous animals, since rigid egg shells do not allow this waste to be eliminated. Precipitating uric acid, present in smaller quantities, prevents the embryo from being poisoned.
Urea
Mammals, most adult amphibians, some types of fish and tortoises prioritize urea |(\text{CON}_2\text{H}_4)| to eliminate their nitrogenous waste.
Removing this waste in the form of pure ammonia would be virtually impossible since it is highly toxic. Ammonia is about |100\ 000| times more toxic than urea. Consequently, the amount of water required to dilute it would be far too great for a terrestrial organism. Since urea is less toxic, the concentration of nitrogenous waste in the urine can be higher and the water loss much lower.
Urea is produced in the liver from ammonia and carbon dioxide. The bloodstream transports ammonia to the liver and urea to the kidneys.
Humans can excrete a small amount of their urea through sweat. A more significant amount of urea is excreted immediately to the bladder and from there to the ureter, urethra and outside the body, while some will be retained in the kidneys. This retention is necessary for osmoregulatory purposes, to maintain a specific osmotic gradient in order to reabsorb water. For some marine animals, such as sharks, urea can be retained in the blood, to maintain osmotic equilibrium with the ocean water.