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Urine Composition and Properties

For medical students2 min readUpdated 2026-10-10

Urine is a biological fluid through which the body excretes metabolic waste products. Its qualitative composition, physicochemical parameters, and total volume directly depend on the rate of metabolic waste accumulation in the blood, dietary habits, and the body's water balance.

Daily diuresisAverages 1.8 L, accounting for about 70% of daily water intake.

Obligate volume500 mL/day — the physiological minimum required to excrete all metabolic end-products.

Normal pH4.5 to 6.5. The acidic reaction is due to the presence of hydrogen ions.

PigmentationThe yellow color of the fluid is determined by pigments: urobilin and stercobilin.

Chemical Composition

Through urine, the body eliminates a variety of metabolic end-products. All dissolved substances are traditionally divided into organic and inorganic.

Organic components include several groups of compounds:

Nitrogenous products of protein catabolism: urea, uric acid, ammonia, purine bases, creatinine, and indican.
Products of intestinal protein putrefaction: indole, skatole, and phenol.
Other compounds: ketone bodies, lactic acid, and salts of oxalic acid.
Pigments: urobilin and stercobilin, which give urine its characteristic yellow color.

Inorganic components are predominantly salts. Primarily, these include sodium chloride (NaCl) and potassium chloride (KCl). In addition, urine contains sulfates and phosphates.

Physicochemical Properties

Under normal conditions, the reaction of urine is acidic due to the presence of hydrogen ions. The normal pH range is 4.5–6.5.

Qualitative composition and urine pH are highly labile and depend on diet and lifestyle:

Plant-based diets are rich in alkalis, so their predominance shifts urine pH toward alkalinity.
Meat-heavy diets promote acidity.
Intense physical exertion also increases acidity because skeletal muscles actively produce lactic and phosphoric acids, which are subsequently excreted by the kidneys.

The urine of a healthy individual is completely devoid of carbohydrates, proteins, and amino acids. Their appearance in urinalysis is considered pathological, though they may transiently appear during periods of severe physical or emotional stress.

Diuresis and Water Balance

Diuresis is the volume of urine excreted from the body over a specific period. To maintain normal diuresis, minimum water intake should be at least 800 mL per day.

The volume of excreted fluid is affected by dietary habits, fluid intake, environmental conditions, and the level of physical and emotional stress. For example, cold exposure increases diuresis, whereas fasting and sleep decrease it.

Normally, there is a distinct circadian rhythm: daytime diuresis exceeds nighttime diuresis by 3 to 4 times.

Terminology of Volume Disorders:

Polyuria — a pathological increase in urine output.
Oliguria — a significant decrease in daily urine volume.
Anuria — complete cessation of urine excretion.
Nycturia — a condition where nighttime diuresis exceeds daytime diuresis.

Based on the ratio of volume to osmotic pressure, three types of diuresis are distinguished: osmotic diuresis (large volume with a high concentration of osmotically active substances), water diuresis (large fluid volume with a low concentration of substances), and antidiuresis (small volume with a high concentration).

Mnemonic

To easily recall the pigments that color urine, use the association "US": Urobilin and Stercobilin.

Frequently asked questions

What is the normal relative density (specific gravity) of urine?

Normally, when assessing the concentrating ability of the kidneys, there is a pronounced diurnal variation in urine relative density (specific gravity). A normal state is considered when the maximum urine relative density exceeds 1.018. The volume of individual urine portions ranges from 50 to 250 mL.

In which nephron segments does hydrogen ion secretion occur to regulate urine pH?

Hydrogen ion secretion occurs in the proximal convoluted tubules, distal nephron segments, and collecting ducts. In these segments, the process has the following features:

Proximal convoluted tubule — secreted hydrogen ions combine with filtered sodium hydrogen phosphate, establishing the acidic reaction of final urine and preventing acidosis.
Distal convoluted tubule — hydrogen secretion proceeds alongside ammonia excretion.
Distal segments and collecting ducts — acidogenesis takes place, during which hydrogen ions are secreted in exchange for reabsorbed sodium.

What determines urine pH in a healthy person?

Urine pH directly depends on diet and physical activity. Meat consumption and intense muscle activity (due to the accumulation of lactic and phosphoric acids) acidify the environment, whereas a predominantly plant-based diet shifts the pH toward alkalinity.

Can proteins or glucose be normally detected in urine?

No, under normal conditions, proteins, amino acids, and carbohydrates are absent from urine. The only exception is their transient appearance during marked physical or psychoemotional stress.

What is the obligate urine volume and why is it important?

The obligate (minimum) volume is 500 mL per day. This is the critical minimum fluid volume required by the kidneys to dissolve and excrete all metabolic end-products generated by the body.

How does diuresis normally change over the course of a day?

In a healthy person, daytime diuresis exceeds nighttime diuresis by approximately 3 to 4 times. An increase in urine volume excreted at night is called nocturia (or nycturia) and indicates underlying abnormalities.

Go deeper

Mechanisms of osmotic and water diuresis formation
Physiological role of antidiuresis and its regulatory mechanisms
Pathophysiology of polyuria, oliguria, and anuria
Impact of emotional stress on transient proteinuria
Products of intestinal protein putrefaction: pathways of formation and excretion