Urine: Composition, Constituents and Deposits

In this article we will discuss about:- 1. Composition of Urine 2. Constituents of Urine 3. Deposits.

Composition of Urine:

A. Volume:

1. A normal adult excretes daily from 1000 ml to 1800 ml of urine. The average is 1500 ml containing 60 gm. of solids.

2. The quantity depends on the water intake, external temperature, the diet and the in­dividual’s mental and physical condition.

3. A high protein diet increases excretion be­cause the urea formed as a result of catabolism of protein has a diuretic action.

4. The diuretic action of tea, coffee and co­coa is due to caffeine.

5. The decreased volume of urine in hot weather is due to an increased loss of wa­ter by perspiration.

6. Nervousness or excitement causes in­creased urinary volume.

7. Increased urine volumes are observed in diabetes insipidus, diabetes mellitus and certain types of kidney diseases and de­creased volumes are found in acute ne­phritis, fevers, diseases of the heart, diarrhea and vomiting.

B. Specific Gravity:

1. The specific gravity of urine in 24 hours lies between 1.003 and 1.030 and varies according to concentration of solutes in the urine. The figures in the second and third decimal places, multiplied by 2.66 (Long’s coefficient) give roughly the to­tal solids in the urine in gm./L. 50 gm. of solids are the average normal for the day.

2. The specific gravity of the urine varies with the food, water intake, and the activ­ity of the individual.

3. In chronic interstitial nephritis, the spe­cific gravity is lowered.

4. The specific gravity is increased in the ex­cretion of abnormal substances such as albumin or glucose (e.g., diabetes mellitus).

C. Colour:

1. Normal urine is pale yellow or amber. The colour is roughly proportional to the spe­cific gravity. Very dilute urine is colour­less.

2. Teroehrome, composed of a polypeptide and urobilin, is the chief pigment of urine. Traces of coproporphyrtn, urobilinogen and litroerytbrin are also found in urine.

3. Reddish urine is due to the ingestion of naturally coloured foods (e.g., beetroot, blackberries). In fever, the urine may be dark yellow or brownish because of con­centration. In liver disease, the urine may be green, brown, or deep yellow due to bile pigments. Blood or hemoglobin de­velops smoky to red colour. The urine is dark brown due to methemoglobin and homogentisic acid. Methylene blue gives the urine a green appearance.

4. The urine is transparent. A turbidity is de­veloped in alkaline urine by precipitation of calcium phosphate. Strongly acid urine is pink due to the precipitation of uric acid salts.

D. Odour:

1. Fresh urine is normally aromatic.

2. The odour is modified by the ingestion of certain foods or drugs. This is noticed af­ter eating asparagus; the odour is due to methyl mercaptan.

3. The ammoniacal smell of urine is due to the action of bacteria on urea.

4. In ketosis, the odour of excreted acetone is detected.

E. pH:

1. The mixed sample of normal urine in 24 hours has a pH 6.0. Individual samples vary from 4.6 to 8.0.

2. The urine is acid in high protein intake because excess phosphate and sulfate are formed in the catabolism of protein. Acid­ity is also increased in acidosis and in fe­ver.

3. The urine becomes alkaline on standing due to the conversion of urea to ammonia and loss of CO₂ to air. It may be alkaline in alkalosis such as after excessive vomit­ing and after meals due to H⁺ secretion in the stomach (the “alkaline tide”).

4. The acidity of urine is increased after strenuous muscular exercise (elimination of lactic acid), by ingestion of ammonium salts of strong acids. An alkaline urine may be produced by ingestion of sufficient NaHCO₃. Ammonium carbonate does not produce an alkaline urine because ammo­nia is rapidly converted into urea.

Constituents of Urine:

Normal Constituents of Urine:

A. Urea: Nitrogenous Constituents:

1. Urea is the main end product of catabo­lism of protein in mammals. Its excretion is directly proportional to the protein in­take. It consists of 80-90% of the total uri­nary nitrogen.

2. In fever, diabetes, or excess adrenocorti­cal activity, urea excretion is increased due to increased protein catabolism.

3. Decreased urea excretion is due to de­creased urea production in the last stages of fatal liver disease.

4. In acidosis, there is decreased urea excre­tion.

B. Ammonia:

1. Ammonia is formed by the kidney from glutamine or amino acids in acidosis.

2. There is a high ammonia output in the urine in uncontrolled diabetes mellitus in which renal function is unimpaired.

C. Creatinine and Creatine:

1. Creatine is excreted by children and preg­nant women and much smaller amounts in men. The excretion in men is 6% of the total excretion of creatinine.

2. Creatinine is formed from creatine. It is excreted in relatively constant amounts regardless of diet.

3. The creatinine coefficient is the ratio be­tween the amount of creatinine excreted in 24 hours and the body weight in kg. It is usually 20-26 mg/kg/day in normal men and 14-22 mg/kg/day in normal women.

4. Creatinine excretion is decreased in many pathological conditions.

5. Creatine excretion is also found in patho­logical states such as starvation, hyper­thyroidism, impaired carbohydrate me­tabolism and infections.

6. Creatine excretion is decreased in hy­pothyroidism.

D. Uric Acid:

1. It is the end product of the oxidation of purines in the body. It is not only formed from dietary nucleoprotein but also from the breakdown of cellular nucleoprotein in the body.

2. It is slightly soluble in water and precipi­tates readily from acid urine on standing.

3. Uric acid excretion is increased in leukemia, severe liver disease and vari­ous stages of gout.

4. The concentrated urine on cooling forms a brick-red deposit which is mainly acid urate.

5. Pure uric acid is colourless. Deposits of uric acid and urates are coloured by ab­sorbed urinary pigments, particularly the red uroervthrin.

6. The specificity of the analysis of uric acid is increased by treatment with uricase, the enzyme (from hog kidney) which converts uric acid to allantoin.

E. Amino Acids:

1. About 150-200 mg of amino acid nitro­gen is excreted in the urine of adults in 24 hours.

2. The infant at birth excretes about 3 mg amino acid nitrogen per pound of body weight, and up to the age of 6 months the value reaches to 1 mg/pound which is maintained throughout childhood. Prema­ture infants excrete 10 times amino acid nitrogen than that of full-term infant.

3. The low excretion of amino acid nitrogen is due to its high renal threshold value.

4. Increased amounts of amino acids are ex­creted in liver disease and in certain types of poisoning.

5. In cystinuria, 4 amino acids-arginine, cys­tine, lysine and ornithine are excreted in urine.

F. Allantoin:

1. It is the partial oxidative products of uric acid. Small quantities of the allantoin are excreted in human urine.

2. In other sub-primate mammals, allantoin, the principal end product of purine me­tabolism, is excreted.

G. Sulphates:

The urine sulphur is derived from sulphur contain­ing amino acids such as methionine and cystine and therefore, its output varies with protein intake.

The urine sulphur exists in 3 forms:

I. Inorganic (sulfate) sulfur:

1. This is the completely oxidized sulfur pre­cipitated from urine.

2. It is proportionate to the ingested protein with a ratio of 5 : 1 between urine nitro­gen and inorganic sulfate.

II. Ethereal sulfur (conjugated sulfates):

1. It is about 10% of the total excreted sulfur.

This includes the organic combination of sulfur excreted in the urine.

2. It consists of the sulphuric esters of cer­tain phenols.

3. It forms no precipitate on addition of acidi­fied BaCl₂. Some of the phenols are de­rived from putrefaction of protein in the large intestine.

4. Clinically, the ethereal sulfate is that of indoxyl indican which is formed from bac­terial decomposition of tryptophan in the large intestine.

5. Normally, 5-20 mg of indican are excreted and the amount increases in constipation. In cholera, typhus gangrene of lung, suffi­cient indican is excreted. Indoxyl liber­ated from indican is oxidized to indigo blue on exposure to air.

III. Neutral sulfur:

1. These are un-oxidized sulfur and contained in cystine, taurine, thiocyanate or sulfides.

2. They do not vary with the diet.

3. They are mainly the products of endog­enous metabolism.

Other organic compounds:

H. Chlorides:

These are excreted as NaCl and output varies with intake.

I. Phosphates:

1. The urine phosphates consist of sodium and potassium phosphates as well as cal­cium and magnesium phosphates.

2. The greater part of the excreted phos­phates is derived from ingested food which contains organic phosphates, e.g., nucleoprotein, phosphoprotein and phospholipids. Phosphates of food are not completely absorbed. Some phosphate is also derived from cellular breakdown.

3. Phosphate excretion is increased in cer­tain bone diseases such as osteomalacia, wasting diseases of the nervous system and in renal tubular rickets.

4. Marked increase of phosphate excretion is also observed in hyperparathyroidism and decrease in hypoparathyroidism and in infectious diseases.

J. Oxalates:

1. The amount of oxalate in the urine is low (20 mg/day) and found as calcium oxalate crystals in urinary deposits.

2. The excretion of oxalate is increased by ingestion of fruits and vegetables contain­ing high oxalates (spinach).

3. Large quantities of oxalate are excreted in urine in inherited metabolic diseases.

4. The oxalates present in urine are com­posed of partly unchanged ingested acid and partly oxidative products of other compounds.

K. Minerals:

1. The 4 cations of the extracellular fluid— sodium, potassium, calcium and magne­sium—are present in the urine.

2. Sodium content varies with intake. Urine potassium increases when the intake is increased or in excessive tissue catabolism. The excretion of potassium is affected by alkalosis. Sodium and potassium excre­tion are also controlled by the activity of the adrenal cortex.

3. Calcium and magnesium are not com­pletely absorbed and their presence in the urine is low. But their presence in the urine varies in certain pathological states, par­ticularly those involving bone metabolism.

L. Enzymes:

1. Traces of many enzymes are excreted in urine including pancreatic amylase, pep­sin, trypsin and lipase.

2. The pancreatic amylase excretion is in­creased in pancreatic disease.

M. Hormones and vitamins:

1. Certain hormones (sex hormones) and vi­tamins (e.g., B, and C) are found in urine.

2. The vitamin needs are assessed by study­ing the urinary output after test doses. The pregnancy test is also performed by the urinary sex hormones.

Abnormal Constituents of the Urine:

A. Proteins:

Proteinuria (albuminuria) is the presence of albu­min and globulin in the urine in abnormal concen­trations. The traces of protein (10-150 mg) present in normal urine cannot be detected by the ordinary simple tests. Pathologically, several proteins, such as serum albumin, serum globulin, hemoglobin, mucus, proteose, Bence-Jones proteins are found in urine.

1. Physiologic proteinuria:

In this condition, less than 0.5% protein is present in urine which occurs after severe exercise, after a high protein meal or as a result of some temporary impairment in renal circulation when a person stands erect. In 30-35% of pregnancy, there is proteinuria.

2. Pathologic proteinuria:

Proteinuria is marked in glomerulonephritis. In neph­rotic syndrome, a marked proteinuria oc­curs. The proteinuria increases with the increasing severity of the renal lesion. Pro­teinuria also results in poisoning of the renal tubules by heavy metals like mer­cury, arsenic or bismuth.

3. Hemoglobin is also present as a result of hematuria due to hemorrhage from the kidneys or urinary tract, clotting may oc­cur due to sufficient fibrinogen on pass­ing of much blood.

4. Mucus is the term for an unidentified pro­tein precipitated by acetic acid in the cold. It is mucin. The mucus is increased in in­fection of the bladder.

5. Proteose may be found which is of little clinical significance.

6. Bence-Jones proteins found in the urine are the peculiar proteins which are light chain fragments of globulins. Most com­monly they occur in multiple myeloma and rarely in leukemia. They are precipi­tated when the urine warmed to 50-60°C and re-dissolved almost completely at 100°C and precipitated again on cooling.

B. Glucose:

Normal individuals excrete not more than 16-300 mg of sugar per day which is diffi­cult to detect by simple test. It is said to be glycosuria when more than this quantity is found in urine. There are different causes of glycosuira.

Transient glycosuria is observed after emotional stress such as exciting atheletic contest. 15% of cases of glycosuria are not due to diabetes glycosuria suggesting dia­betes must be confirmed by blood glucose studies to eliminate the probability of re­nal glycosuria.

The presence of glucose must be tested by Benedict’s test. But in case of pregnant women and lactating mother, the Osazone test must be performed for urine glucose to eliminate the lactose present in urine.

C. Other sugars:

1. Fructosuria:

Fructosuria is due to the dis­turbance in fructose metabolism but no other carbohydrates.

2. Galactosuria and lactosuria:

These may occur occasionally in infants, pregnant women and lactating mother. Galactosuria may occur in inherited diseases due to the non-conversion of galactose to glucose.

3. Pentosuria:

This may occur transiently after intake of food containing large quan­tities of pentose’s, such as grapes, cherries and plums. It may take place in inherited diseases in which pentose’s are not metabo­lized. To detect these other sugars in urine it is wise to perform Osazone test.

D. Ketone bodies:

1. Only less than 1 mg of ketone bodies are excreted in urine normally in 24 hours.

2. Increased amount of ketone bodies are ex­creted in urine in starvation, diabetes mellitus, pregnancy, ether anesthesia, and some types of alkalosis.

3. Excess fat metabolism may induce a ketonuria in many animals.

4. Increased amount of ammonia is excreted in acidosis accompanying ketosis.

E. Bilirubin and Bile salts:

1. Bilirubin is found in the urine in cases of obstructive or hepatic jaundice.

2. Bilirubinuria is accompanied by the ex­cretion of bile salts.

3. Bile salts may be excreted in urine with­out bile pigment in certain stages in liver disease.

4. In excessive hemolysis, traces of bilirubin without bile salts are excreted in urine.

F. Blood:

1. In the lesion of the kidney or urinary tract blood is excreted in the urine in addition to its presence in nephritis.

2. Free hemoglobin is also found in urine after quick hemolysis, e.g., in black water fever (a complication of malaria) or after severe burns.

G. Urobilinogen:

1. In excessive hemolysis, e.g., hemolytic jaundice or pernicious anemia, part of the bile pigment formed by breakdown of hemoglobin is excreted in urine as urobilinogen.

2. Urobilin is formed from colourless uro­bilinogen when the urine is exposed to air. This gives the urine an orange colour.

3. In liver disease or temporarily in consti­pation, large amounts of urobilin are found in urine.

4. Urobilinogen excreted in urine normally are 1-3.5 mg/24 hrs.

H. Porphyrins:

1. Coprophyrins excreted in urine normally are 50-250 (µ gm./day.

2. Coproporphyria are excreted more in cer­tain liver diseases.

3. The increased amount of coproporphyrins in the urine is a characteristic of the urine of patients suffering from porphyria.

Urinary Deposits:

The commonest deposits are phosphates, oxalates and urates and are frequently seen in normal urine.

Phosphates:

1. They are usually found in alkaline urines. The commonest is ammonium magnesium phosphate which forms a characteristic crystal.

2. A less common form is calcium hydrogen phosphate which forms long prisms.

3. Amorphous calcium and magnesium phos­phates may be deposited from alkaline urines.

The deposition of phosphates is due to a change in pH after the urine has been passed.

Calcium Oxalate:

This is found in acid urine but may be found in alkaline urine. The crystals are of two types—octahedra, dumb-bells. Calcium oxalate is insoluble in acetic acid.

Urates:

1. They are usually found in acid urines.

2. Uric acid separates into different forms in­cluding prisms, barrels, hexagons and nee­dles which are always pigmented.

3. Urates are redissolved on warming the urine.

4. The cause of deposition of urates is the cooling of urine after it has been passed.

Feces:

Amount:

1. The quantity of feces varies from day to day and with the diet.

2. Vegetable food increases the bulk of feces but meat diet which is largely absorbed diminishes the bulk.

3. An adult taking a mixed diet passes from 60 to 250 gm of moist feces containing 25-45 gm of solids per day.

4. The young children release high bulk.

Composition:

1. Normal adult feces have a water content of 65-80%.

2. One-third of the dry matter of feces is rep­resented by bacteria. The rest are the remaining’s of intestinal secretions, sub­stances excreted by the large intestine (e.g., Ca, Fe) and small amounts of food residues. The food residues are cellulose, fruit skins and seeds.

3. The only substances in feces are fat, nitro­gen and mineral elements.

Dry feces con­tain the following:

Colour:

1. The normal colour of feces is brown which is due to stercobilin (urobilin).

2. Milk, cereals and meat, coffee, cocoa, black berries, etc. form darker stools.

3. Greenish colour due to excessive con­sumption of green vegetables.

4. The colour of the feces of newly-born in­fants is dark or blackish-green due to biliverdin and porphyrin.

5. The stools of young infants on a milk diet is yellow due to bilirubin because of non-development of intestinal bacterial flora.

6. The presence of blood in the stool gives the feces a red colour. Excessive hemoly­sis will give a dark-brown feces.

7. Iron or bismuth gives a black stool.

Odour:

1. The normal odour is due to indole and skatole.

2. Mercaptans and H₂S may produce odour.

3. A meat diet produces a more intense odour than a vegetable one and a milk diet least of all.

pH:

1. Feces are normally slightly alkaline, pH 7.0-7.5.

2. They may be slightly acid with a large proportion of carbohydrate or fat.

Fat:

1. The total fat of feces is divided into two:

(a) Split fat (fatty acids).

(b) Unsplit fat (neutral fats, phospholipids, sterols, pig­ments).

2. The amount of fat is independent of the diet. The split fat diminishes in amount on low-fat diets.

3. In diseases, fecal fat is increased when digestion or absorption of fat is im­paired.

Nitrogen:

1. Fecal nitrogen is very little affected by the amount of protein ingested if the pro­tein is well masticated and well assimi­lated.

2. An adult of a mixed diet usually excretes about lgm of fecal nitrogen per day.

3. In diseases, fecal nitrogen is greatly in­creased by failure of digestion or absorp­tion of protein.

Salts:

1. Moist feces contain about 2-3% of salts. Most abundant are calcium and phosphate.

2. Small amounts of magnesium, iron, so­dium, potassium, chloride and sulphate are also present in feces. The proportion of calcium is higher on a milk diet and that of magnesium on a meat diet.

Sweat:

1. Sweat is a more dilute fluid and is always hypotonic.

2. The pH is about 4.5. But if the skin is washed and dried, sweat secreted is slightly alkaline (pH 7.0-7.4).

3. It contains most of the diffusible constitu­ents of plasma. The most abundant con­stituent is NaCl.

4. The lactic acid content is more than nor­mally found in blood.