What is urine? Physiology, Composition etc

What is urine?

Urine is a liquid waste product of the body secreted by the kidneys by a process of filtration from blood called urination and excreted through the urethra. Cellular metabolism generates numerous waste compounds, many rich in nitrogen, that require elimination from the bloodstream. This waste is eventually expelled from the body in a process known as micturition, the primary method for excreting water-soluble chemicals from the body. These chemicals can be detected and analyzed by urinalysis. In pregnant women, amniotic fluid is closely related to urine, and can be analyzed by amniocentesis.


Main article: Renal physiology

To eliminate soluble wastes, which are toxic, most animals have excretory systems. In humans soluble wastes are excreted via the urinary system, which consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys extract the soluble wastes from the bloodstream, as well as excess water, sugars, and a variety of other compounds. The composition of urine is adjusted in the process of re-absorption whereby certain solutes, such as glucose, are reabsorbed back into the blood stream via carrier molecules.[citation needed] The remaining fluid contains high concentrations of urea and other substances, including toxins. Urine flows through these structures: the kidney, ureter, bladder, and finally the urethra. Urine is produced by a process of filtration, re-absorption, and tubular secretion.


Urine is a transparent solution that can range from colorless to amber but is usually a pale yellow. Urine is an aqueous solution of metabolic wastes such as urea, dissolved salts, and organic compounds. Fluid and materials being filtered by the kidneys, destined to become urine, come from the blood or interstitial fluid.

Except in cases of kidney or urinary tract infection (UTI), urine is virtually sterile and nearly odorless. Subsequent to elimination from the body, urine can acquire strong odors due to bacterial action. Most noticeably, the asphyxiating ammonia is produced by breakdown of urea. Some diseases alter the quantity and consistency of the urine, such as sugar as a consequence of diabetes.


Urea is toxic and can be irritating to skin and eyes. High concentrations in the blood can cause damage to organs of the body.


The typical bright yellow color of urine is caused by the pigment urochrome as well as the degradation products of bilirubin and urobilin.[citation needed] It can range from clear to a dark amber, depending mostly upon the level of hydration of the body, among other factors.

Chemical analysis

Main article: Urinalysis

Urea structure

Urine contains a range of substances that vary with what is introduced into the body. Aside from water, urine contains an assortment of inorganic salts and organic compounds, including proteins, hormones, and a wide range of metabolites.

Unusual coloration

Clear- indicates over-hydration, which is usually considered much healthier than dehydration. In the context of a drug test, it could indicate a potential attempt to avoid detection of illicit drugs in the bloodstream through over-hydration[1].

Yellowing/light Orange may be caused by removal of excess B vitamins from the bloodstream.

Certain medications such as rifampin and pyridium can cause orange urine.

Bloody urine is termed hematuria, potentially a sign of a bladder infection.

Consumption of beets can cause urine to have a pinkish tint; the condition is harmless and temporary.

Dark orange to brown urine can be a symptom of jaundice, rhabdomyolysis, or Gilbert’s syndrome.

Black or dark-colored urine is referred to as melanuria and may be caused by a melanoma.

Reddish or brown urine may be caused by porphyria. Again, the consumption of beets can cause the urine to have a harmless, temporary pink or reddish tint.

Fluorescent Yellow / Greenish urine may be caused by dietary supplemental vitamins, especially the B vitamins.

Dark yellow urine is usually indicative of dehydration.


Usually odorless, urine can be pungent after the consumption of certain foods. Eating asparagus is known to produce a strong odour in human urine. This is due to the body’s break down of asparagusic acid. Although odorous urine is a universal consequence of eating asparagus, the odor is not universally detectable.[2] Other foods that contribute to odor include Sugar Puffs, curry, alcohol, artichokes, and turkey.[3][4]


Turbid urine may be a symptom of a bacterial infection, but can also be due to crystallization of salts such as calcium phosphate.


The pH of urine is close to neutral (7) but can normally vary between 4.5 and 8. Strongly acidic or alkaline urine may be symptomatic of disease,[5] and may also contribute to disease. In persons with hyperuricosuria, acidic urine can contribute to the formation of stones of uric acid in the kidneys, ureters, or bladder.[6] Urine pH can be monitored by the physician[7] or at home.


The amount of urination produced depends on numerous factors including state of hydration, activities, environmental factors, size, and health. In adult humans the average production is about 1 – 2 L per day. Producing too much or too little urine needs medical attention: Polyuria is a condition of excessive production of urine (> 2.5 L/day), in contrast to oliguria where < 400 mL are produced per day, or anuria with a production of < 100 mL per day.