Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Kidneys and Urine
General Properties and Constituents of Urine
General Properties of Urine

The normal volume of urine excreted per 24 hours (diuresis) in adults ranges from 1,000 to 2,000 mL, averaging about 50–80% of the fluid intake. A daily urine output below 500 mL or above 2,000 mL is considered pathological in adults. An increased urine volume (polyuria) is observed after consuming large amounts of fluids or foods that promote diuresis (such as watermelon or pumpkin). Pathologically, polyuria is associated with Kidney disorders (such as chronic nephritis and Pyelonephritis), Diabetes Mellitus, and other pathological conditions. A massive volume of urine is excreted in diabetes insipidus—reaching 15 liters or more per day.

A decrease in daily urine output (oliguria) occurs due to insufficient fluid intake, febrile states (when significant amounts of Water are lost through the Skin), vomiting, diarrhea, toxicosis, acute nephritis, etc. In cases of severe renal parenchymal damage (such as acute diffuse nephritis), urolithiasis (ureteral obstruction), lead, mercury, or arsenic poisoning, or severe psychological trauma, a near-complete cessation of urine production (anuria) may occur. Prolonged anuria leads to uremia.

Normally, more urine is excreted during the day than at night, with a daytime-to-nighttime diuresis ratio ranging from 4:1 to 3:1. Under certain pathological conditions (such as Cytology/cytology/16.html">Early stages of cardiac decompensation or cystopyelitis), a larger volume of urine is passed at night than during the day. This condition is known as nocturia.

Normal urine color ranges from straw-yellow to deep yellow. The coloration depends on the presence of specific pigments: urochrome*, urobilin, uroerythrin, urosein, and others.

* It is believed that normal urine color is 95% attributable to the presence of urochrome. The Chemical Structure of urochrome has not yet been fully elucidated. Apparently, this pigment is formed in the body during The breakdown of Tryptophan.

Deep yellow urine is typically concentrated, has a high specific gravity, and is excreted in relatively small amounts. Pale (straw-colored) urine generally has a low relative density and is excreted in large volumes.

Under pathological conditions, urine color may turn red, green, brown, etc., depending on the presence of abnormal coloring agents. For instance, a red or pinkish-red color is observed in Hematuria and hemoglobinuria, as well as after the administration of antipyrine, amidopyrine, santonin, and other medications. A brown or red-brown hue occurs when high concentrations of urobilin and bilirubin are present in the urine.

Small amounts of stercobilinogen enter the urine of a healthy person via the hemorrhoidal Venous system. Upon exposure to light and air, colorless stercobilinogen is oxidized into the colored pigment stercobilin (see Chapter 16). As noted, in clinical practice urinary stercobilin is frequently referred to as urobilin. In Liver diseases, when the liver loses its ability to degrade mesobilinogen (urobilinogen) absorbed from the Small Intestine into di- and tripyrroles, large amounts of urobilinogen appear in the urine (transforming into urobilin upon exposure to light and air). In such cases, the urine darkens significantly.

A green or blue urine color is observed following the administration of methylene blue or when protein putrefaction processes are intensified in the intestine. In the latter case, elevated levels of indoxyl sulfate appear in the urine, which can break down to form indigo.

Normal urine is transparent. Turbidity can be caused by salts, cellular elements, Bacteria, mucus, or fat (lipuria). The cause of urine cloudiness can be determined either microscopically (by examining the urinary sediment) or through chemical analysis.

The relative density (specific gravity) of adult urine fluctuates within a fairly wide range over the course of a day (from 1.002 to 1.035), owing to the periodic intake of food and water, as well as fluid loss by the body (via sweating, etc.). Most commonly, it ranges between 1.012 and 1.020. Urine density provides a general indication of the concentration of dissolved solutes. The daily urinary excretion of solid solutes ranges from 50 to 75 g. An approximate calculation of the solid residue content in urine (in grams per liter) can be obtained by multiplying the last two digits of the relative density by a factor of 2.6.

In severe renal failure, urine is continuously excreted with a constant relative density equal to that of primary urine or ultrafiltrate (~ 1.010). This condition is termed isosthenuria.

Consistently low urine specific gravity indicates impaired renal concentrating function* associated with chronic nephritis or primary/secondary granular contracted kidney. Diabetes insipidus also presents with low-density urine (1.001–1.004) due to impaired water reabsorption in the renal tubules. In oliguria (reduced daily urine volume), such as in acute nephritis, urine exhibits a high density. High specific gravity is also characteristic of diabetes mellitus complicated by polyuria, in which case it is caused by high glucose content in the urine.

* The ability of the Kidneys to concentrate and dilute primary urine is of paramount importance for maintaining the constancy of the Osmotic Pressure of the Blood.

The normal pH of urine on a mixed diet is acidic or mildly acidic (pH 5.3–6.5)*. Typically, 40 to 75 mEq of acids are excreted in the urine per 24 hours. Urine pH is influenced by dietary habits. A predominantly meat-based diet results in a more acidic urine reaction, whereas a plant-based (vegetarian) diet yields an alkaline reaction.

The acidic reaction of human urine is primarily due to the presence of primary phosphates (e.g., KH₂PO₄ or NaH₂PO₄). In alkaline urine, secondary phosphates or potassium/sodium bicarbonates predominate.

A sharply acidic urine reaction is observed in febrile states, diabetes mellitus (especially when Ketone Bodies are present in the urine), starvation, etc. An alkaline urine reaction occurs in cystitis and pyelitis (microorganisms can break down urea to form ammonia directly within the Urinary Bladder), following severe vomiting, the administration of certain medications (such as sodium bicarbonate), or the consumption of alkaline mineral waters, etc.