Urine - Characteristics & Chemical Constiuents

   General Characteristics of Urine

The general characteristics of Urine are as follows:

1. Volume: The urinary volume is not constant and varies according to many factors even in health. In the normal adult, 600 to 2500 ml of urine formed daily. The factors which modify urinary volume are water intake, exercise, climate, diet and physical state. Urine volume is less in summer or in warm climates. Coffee, tea and alcohol have a diuretic effect. Eating high protein diet increases urinary volume because of excessive urea formation which produces osmotic dieresis. Normally about half as much urine is formed during sleep as during activity. In renal disease, relatively more urine may be produced during night; this is termed nocturia.

An abnormally increased urinary volume is called polyuria; oliguria means the passage of lesser amount of urine. Anuria means the complete cessation of urine formation.

2. Specific gravity: This also varies greatly even normally and ranges from 1.003 to 1.030 or higher according to concentration of solutes in the urine. The last two digits of the specific gravity at 25^oC multiplied by 2.66 (Long’s coefficient) give roughly the total solids in the urine in grams per liter. This principle can be used to find out the amount of solids in the urine. 60 grams of solids in the urine are an average normal value per day.
   
   
3. Reaction: The normal urine is usually acidic with a pH of about 6.0 (range is from 4.7 to 7.5). A high protein diet results in an acidic urine because excess phosphate and sulfate are produced in the catabolism of protein. The urinary acidity, it also increased in acidosis and in fevers. The urine may become alkaline in alkalosis, e.g. after intake of bicarbonates, in hyperpna etc. The pH of the urine should be taken immediately the urine is passed because the urine becomes alkaline after some time due to conversion of it urea to NH₃.
   
   
4. Color: Normal urine is pale yellow or amber yellow. The color varies with the quantity and concentration of urine voided. The principal pigment is urochrome which is a compound of a peptide with urobilin and unibilinogen. Small amounts of urobilinogen, coproporphyrin, uroporphyrine and uroerythrin are also present. The urine may be colored green, brown or deep yellow if bile pigments are present in the urine as occurs in post-hepatic jaundice. Presence of blood or hemoglobin makes the urine smoky or red colored. A dark brown urine may indicate methemoglobinuria or alkaptonuria. Excessive excretion of porphyrins in urine gives rise to pink or brown urine. The highly concentrated urine produced in fever may look dark yellow or brownish. Methylene blue intake results in a green colored urine.
   
   
5. Odor: Fresh normal urine is said to have an aromatic odor. In ketosis, the fruity odor of excreted acetone may be present. A strongly ammoniacal odor means infection in the urinary tact.

   Normal Constituents of Urine

Normal constituents of Urine are as follows:

1. Urea: This is the chief end-product of protein catabolism in man. Its excretion is directly proportional to the protein intake. Normally about 30 grams of urea are excreted in the urine per day and it comprises 85% to 92% of the total urinary N but on a low protein diet, this value is decreased to about 60%.

Urea excretion is increased in conditions associated with an increased protein catabolism, e.g. fever, diabetes mellitus or excessive adrenocortical activity. A decreased urea production leading to a decreased urea excretion is found in severe liver disease prior to death.

2. Ammonia: Normally very little ammonia is excreted in urine which amounts to about 0.7 grams per day representing 2.5 to 4.5% of the total urinary N. The ammonia formed in the kidneys is utilized to buffer H+ ions and its formation is increased in conditions associated with acidosis, e.g. diabetic ketosis.
   
   
3. Creatinine and creatine.
   
   
4. Uric Acid: It is a tribasic acid in its lactim form but it acts as a weak dibase acid because only two of the three hydrogens are dissociated. It is only slightly soluble in water and still less soluble in acidic solutions. However, its alkali salts with sodium, potassium and ammonium are more soluble. Being insoluble in acid solutions, uric acid precipitates in acidified urine or even in the normal slightly acidic urine. Uric acid is the most important end-product of the catabolism of purines in the body. The urinary excretion of uric acid is increased in gout, leukemias, polycythemia and in starvation. Uricosurtic drugs also increase its excretion in the urine.
   
   
5. Amino acids: Small amounts of all the amino acids occurring in the body are excreted in urine through in different amounts. The infant, specially prematurely born one, excretes relatively larger amounts of amino acids in the urine but the amount excreted falls with increasing age. Women excrete relative more amino acids (2.3 mg/Kg body weight) than men (1.4 mg/Kg body weight). Glycine and histidine are the main amino acids occurring in urine. Pathologically, extensive liver disease results in a greater excretion of amino acids in the urine.
   
   
6. Allantoin: Only small traces of allantoin are excreted in human urine. It is produced by the partial oxidation of uric acid.
   
   
7. Chloride: The amount of chloride excreted in urine is proportional to its dietary intake which as NaCL is 10 to 15 grams per day. NaCl excretion in urine is increased in adrenocortical deficiency, i.e. in Addison’s disease and is decreased in salt deprivation as in heat exhaustion.
   
   
8. Sulfates
   
   
9. Phosphates: The kidneys excrete 1.1 grams of P most of which occurs as inorganic phosphates (H₂PO₄^- and HPO₄^-) which are in combination with Na⁺ and K⁺ (the alkaline phosphates) as well as with Ca⁺⁺ and Mg⁺⁺ (earthy phosphates). The earthy phosphates are precipitated in an alkaline urine. A crystalline precipitate of magnesium ammonium phosphate (Mg.NH₄.PO₄) also called triple phosphate, is found in conditions in which abnormally high amounts of phosphates are found in urine.

The phosphate excretion is increased in hyperparathyroidism, osteomalacia and renal tubular rickets. A decreased P in the urine occurs in hypoparathyroidism. The phosphate excretion is also decreased in renal insufficiency which leads to its retention in the body.

10. Oxalates: Normally the oxalate of the urine is only 10 to 30 mg per day. Most of the oxalates are derived from fruits and vegetables especially grapes, apples asparagus, spinach, cabbage, tomatoes, lettuce and rhubarb.
    
    
11. Minerals: Na⁺, K⁺, Ca⁺⁺ and Mg++ are the cations of the plasma and extracellular fluid which are excreted in urine in appreciable amounts.

The Na and K contents vary considerably according to their amounts taken in diet and physiological requirements. Urine K is increased in conditions in which there takes place an excessive tissue breakdown and in alkalosis; in the latter condition, the increased excretion of K is due to a deficiency of H+ ions in the tabular cells. The hormones of the adrenal cortex also are important in the regulation of Na and K excretion in urine.

The amount of Ca and Mg excreted in urine is less than Na and K. Large amounts of Ca are lost in urine in hyperparathyroidism.

12. Vitamins, hormones and enzymes: These are excreted in urine in small amounts only.