Chemical composition
The body rids itself of numerous final metabolic products via urine. All substances dissolved in it are traditionally divided into organic and inorganic.
Organic components include several groups of compounds:
- Nitrogen-containing protein oxidation products: urea, uric acid, ammonia, purine bases, creatinine, and indican.
- Products of intestinal protein putrefaction: indole, skatole, phenol.
- Other compounds: ketone bodies, lactic acid, and oxalic acid salts.
- Pigments: urobilin and stercobilin, which give urine its characteristic yellow color.
Inorganic components consist predominantly of salts. First and foremost, these are sodium chloride (NaCl) and potassium chloride (KCl). In addition, urine contains sulfates and phosphates.
Physicochemical properties
Normally, the urine reaction is acidic due to the presence of hydrogen ions. The normal pH range is 4.5–6.5.
The qualitative composition and reaction of urine are highly labile and depend on diet and lifestyle:
- Plant-based foods are rich in alkalis, so their predominance shifts the urine reaction toward the alkaline side.
- Meat-based foods contribute to increased acidity.
- Intensive muscular work also increases acidity due to the active formation of lactic and phosphoric acids in skeletal muscles, which are subsequently excreted by the kidneys.
Carbohydrates, proteins, and amino acids are completely absent in the urine of a healthy person. Their appearance in lab tests is considered pathological, though they may be detected transiently during severe physical or emotional stress.
Diuresis and water balance
Diuresis is the amount of urine excreted from the body over a given period of time. To maintain normal diuresis, minimum water intake must be at least 800 mL per day.
The volume of excreted fluid is affected by dietary habits, water intake, environmental conditions, and the level of physical and emotional stress. For example, body cooling increases diuresis, while fasting and sleep decrease it.
Normally, there is a clear circadian rhythm: daytime diuresis should exceed nighttime diuresis by 3–4 times.
Terminology of volume disorders:
- Polyuria — a pathological increase in the volume of excreted urine.
- Oliguria — a significant decrease in the daily urine volume.
- Anuria — complete cessation of urine excretion.
- Nocturia — a condition in which nighttime diuresis exceeds daytime diuresis.
Based on the ratio of volume to osmotic pressure, three types of diuresis are distinguished: osmotic (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).
Briefly: answers to topical questions
What is the normal relative density (specific gravity) of urine?
Normally, when assessing the concentration function of the kidneys, there is a marked variation in the relative density (specific gravity) of urine throughout the day. It is considered normal when the maximum relative density of urine 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 tubules — hydrogen secretion occurs concurrently with ammonia excretion.
- Distal segments and collecting ducts — acidogenesis takes place, where hydrogen ions are secreted in exchange for reabsorbed sodium.
To quickly recall the pigments that color urine, use the mnemonic "US": Urobilin and Stercobilin.
Class and exam questions
What determines the urine pH in a healthy person?
Urine reaction directly depends on diet and physical activity levels. Meat-based foods and intensive muscular work (due to the accumulation of lactic and phosphoric acids) render the environment acidic, while a predominance of plant-based foods shifts the pH toward the alkaline side.
Can proteins or glucose be normally detected in urine?
No, proteins, amino acids, and carbohydrates are normally absent in urine. The only exception is their transient appearance during marked physical or psychoemotional stress.
What is the obligatory urine volume and why is it important?
The obligate (minimum) volume is 500 mL per day. This is the critical fluid minimum required by the kidneys to dissolve and excrete all final metabolic products generated in the body.
How does diuresis normally change throughout the day?
In a healthy person, daytime diuresis exceeds nighttime diuresis by approximately 3–4 times. An increase in the volume of urine excreted at night is called nocturia and indicates abnormalities.
What else to study on this topic
- Mechanisms of osmotic and water diuresis formation
- Physiological role of antidiuresis and its regulatory mechanisms
- Pathophysiology of polyuria, oliguria, and anuria
- Effect of emotional stress on transient proteinuria
- Products of intestinal protein putrefaction: pathways of formation and excretion
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