Urine is an aqueous solution of electrolytes and metabolites excreted by mammals, birds, reptiles, fish and amphibians. Although urine is excreted as a paste (uric acid) by most birds, it is commonly excreted as a fluid varying in color from clear, when dilute, to a dark amber, when concentrated. Urine is produced by the kidneys, and plays a vital role in maintaining homeostasis by removing excess water, electrolytes such as sodium, chloride, potassium, and calcium ions, urea and other metabolites from the blood. Urine excreted by healthy kidneys is sterile. The production of urine is called diuresis.

Composition

Water

The main constituent of urine is water. All vertebrates must carefully maintain the volume of fluid in their extracellular space in order to prevent fluid overload or dehydration. Some water is inevitably lost during solute excretion, and represents an unavoidable fluid loss. However, the majority of water excreted in the urine is lost to prevent fluid overload. Different animals have different renal physiologies depending on their need to retain water. Freshwater fish, for example, produce very large amounts of very dilute urine, whereas desert-dwelling animals such as the meerkat have evolved very effective renal systems, allowing them to conserve water by producing small amounts of extremely concentrated urine. By comparison, even the most concentrated human urine is relatively dilute. The excretion of water is called aquaresis.

Electrolytes

Along with volume regulation, urine regulates the osmolarity of an animal's internal space. The concentration of ions in the blood and extra-cellular fluid must stay within a fairly tight range to maintain health and avoid dehydration. Although some ions are lost to normal bodily function (such as sweating), most animals have a large surplus of ions in their diets and must excrete them. Most humans, for example, ingest many more sodium and chloride ions than they need in the form of [salt] - it is secretion of these surplus ions which makes urine taste salty.

Nitrogen

Urine production and excretion is a vertebrate's primary method for removal of nitrogen. This is a waste product, produced in the form of ammonia by the liver. Excess nitrogen is found in the diet, and released into the blood during the deamination of amino acids in protein metabolism. In fish, where water conservation is not an issue, ammonia is excreted in dilute urine. However, at higher concentrations it is toxic, and so in mammals' urine, this mainly is in the form of urea, produced from ammonia in the liver. Birds generally excrete uric acid as a paste, to further conserve water.

Acid

The kidneys play a vital role in regulating body pH, preventing acidosis or alkalosis by excreting excess hydrogen ions or bicarbonate ions as required. When it leaves the body, urine is usually around pH 6, though it may be as low as 4.5 or as high as 8.2. As urea—the compound which accounts for 75–90% of the nitrogen in urine—begins to decay, hydroxide ions form, raising the pH as high as 9–9.3.

The decay of urea into carbon dioxide is catalyzed by urease:

(NH2)2CO + H2O → CO2 + 2NH3

Metabolites

Animals ingest a wide variety of compounds daily. Not least are humans, who consume an incredible array of natural and artificial chemicals in the form of food, drink and pharmaceutical products. With the exception of vitamins, minerals and other micro-nutrients such as essential fatty acids, none of these are needed or desirable within the body. All are either metabolised by the liver, excreted in bile or filtered from the blood by the kidneys, and excreted in urine.

Dissolved heavy metals

Studies of urine in organic cattle farms in Sweden in 1999 and 2002 yielded the following concentrations of heavy metals (all in μg/kg wet weight):

Copper: 67
Zinc: 30
Chromium: 5
Lead: 1
Cadmium: 0

Glucose

Glucose is constantly lost from the blood into the filtrate at the kidneys - however, active reuptake in the proximal tubule usually prevents any being excreted. This is desirable, as glucose is a valuable source of energy, not a waste product. However, in hyperglycaemia - most commonly arising from diabetes mellitus in humans - the tubular limit on glucose reabsorbtion may be breached, in which case some glucose will be lost in the urine.

Bacteria

Urine excreted by healthy kidneys is sterile. When it leaves the body, however, the urine can pick up bacteria from the surrounding skin, which would contaminate it.

Characteristics

Color

Urine is normally yellow because it contains urobilin. It can also be a clear color if an excessive amount of water is consumed.

Uses

Diagnostic tests

Testing urine for its constituents is a cost-effective and non-invasive means of assessing the internal situation of an animal. It is commonly used to test for pregnancy, by measuring levels of hormones excreted. Urinalysis can also be used to test for the metabolites of illegal drugs, if substance abuse is suspected. It is also an invaluable first-line investigation in clinical medicine, where pH, glucose, protein, white blood cell, bacteria and blood content can all be tested to aid in making a diagnosis.

Fertilizer

Urine has been and is used extensively as fertilizer. Its high nitrogen content allows increased amino acid synthesis by plants. During WWII, Japanese farmers used urine collected in so-called "honeypots" as cheap fertilizer for their crops.

Animal repellent

Taking advantage of the scents of male animals' urine, some companies sell animal urine, usually coyote or fox, to cities and other organizations to repel those animals by essentially "marking their territory". The scents of carnivore urine (bobcat, mountain lion, and wolf, in addition to coyote and fox) are also sold to the public in pelletized form to repel garden browsing by herbivores such as squirrels and rabbits, as well as deterring domestic or feral cats from marking territory, or catching birds, in gardens. When the pellets are sprinkled on a target area, the intruding animal will instinctively recognize the territorial urinary scent of its predators and avoid the area.

Munitions

In historical times, urine was collected and used in the manufacture of gunpowder. Stale urine was filtered through a barrel full of straw and allowed to continue to sour for a year or more. After this period of time, water was used to wash the resulting chemical salts from the straw. This slurry was filtered through wood ashes and allowed to dry in the sun. Saltpeter crystals were then collected and added to sulfur and charcoal to create black powder.http://www.foxfire.org/foxfire5.aspx http://en.wikipedia.org/wiki/Potassium_nitrate

Textiles

Urine has often been used as a mordant to help prepare textiles, especially wool, for dyeing. Urine was used for dyes such as indigo where the urea in the urine reacted with the insoluble dye to form a soluble solution.

Hormone replacement therapy

Steroid hormones extracted from the urine of pregnant mares are used in a drug sold under the trade name Premarin (a neologism derived from 'pregnant mare urine'). The drug, manufactured and sold by Wyeth, is an estrogen replacement therapy used in the treatment of menopause symptoms.

References

Synonyms, Antonyms and Related Words

chyle, colostrum, discharge, gleet, humor, ichor, lachryma, lactation, leukorrhea, lymph, matter, micturition, milk, mucor, mucus, peccant humor, pee, pee-pee, phlegm, piddle, pish, piss, purulence, pus, rheum, saliva, sanies, serous fluid, serum, snot, stale, suppuration, sweat, tear, teardrop, the whites, urea, urination, water, wee-wee

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