The kidneys are paired organs, which are responsible for the constancy of the internal environment in the organism and elimination of the metabolism end-products.The kidneys regulate water-electrolyte balance, acid-base balance, excretion of metabolic wastes, osmotic pressure. Besides, they take part in the regulation of blood pressure and R.B.C production.


1.   Structure and function of the kidneys


Each kidney is composed of 2 layers: the cortex or outer layer is brownish- red and the medulla or inner layer is lighter in colour. The nephron is the functioning unit of the kidney. Each kidney contains more than a million of nephrons.

It consists of renal corpuscle, which contains glomerulus surrounded by hollow capsule (Bowman’s capsule). Besides each nephron contains: proximal convoluted tubules, a descending limb of the loop of Henle, collecting tubules and distal convoluted tubules.

The 2 principal types of nephron are classified according to their position in the kidneys:

1.    Cortical nephrons (85%), which are situated in the cortex.

2.    Yuxtamedullary nephrons (15%)


The kidneys are the most important organs of excretion. A human dies when the kidneys are not functioning for 4-6 days.

A. Renal hormones

In addition to their involvement in excretion and metabolism, the kidneys also have endocrine functions. They produce the hormones erythropoietin and calcitriol and play a decisive part in producing the hormone angiotensin II by releasing the enzyme renin. Renal prostaglandins have a local effect on Na+ resorption.

Calcitriol (vitamin D hormone, 1α,25-dihydroxycholecalciferol) is a hormone closely related to the steroids that is involved in Ca2+

homeostasis. In the kidney, it is formed fromcalcidiol by hydroxylation at C-1.

The activity of calcidiol-1-monooxygenase [1] is enhanced by the hormone parathyrin  (PTH).

Erythropoietin is a peptide hormone that is formed predominantly by the kidneys, but also by the liver. Together with other factors known as “colony-stimulating factors” (CSF), it regulates the differentiation of stemcells in the bone marrow.

Erythropoietin release is stimulated by hypoxia (low pO2). Within hours, the hormone ensures that erythrocyte precursor cells in the bone marrow are converted to erythrocytes, so that their numbers in the blood increase.

Renal damage leads to reduced erythropoietin release, which in turn results in anemia. Forms of anemia with renal causes can now be successfully treated using erythropoietin produced by genetic engineering techniques.

The hormone is also administered to dialysis patients. Among athletes and sports professionals, there have been repeated cases of erythropoietin being misused for doping purposes.

Renin–angiotensin system

The peptide hormone angiotensin II is not synthesized in a hormonal gland, but in the blood. The kidneys take part in this process by releasing the enzyme renin.

Renin [2] is an aspartate proteinase. It is formed by the kidneys as a precursor

(prorenin), which is proteolytically activated into renin and released into the

blood. In the blood plasma, renin acts on angiotensinogen, a plasma glycoprotein in the α2-globulin group, which like almost all plasma proteins is synthesized in the liver. The decapeptide cleaved off by renin is called angiotensin I. Further cleavage by peptidyl dipeptidase A (angiotensin-converting enzyme, ACE), a membrane enzyme located on the vascular endothelium in the lungs and other tissues, gives rise to the octapeptide angiotensin II [3], which acts as

a hormone and neurotransmitter. The lifespan of angiotensin II in the plasma is only a few minutes, as it is rapidly broken down by other peptidases (angiotensinases [4]), which occur in many different tissues.

The plasma level of angiotensin II is mainly determined by the rate at which renin is released by the kidneys. Renin is synthesized by juxtaglomerular cells, which release it when sodium levels decline or there is a fall in blood pressure.

Effects of angiotensin II.

Angiotensin II has neffects on the kidneys, brain stem, pituitary gland, adrenal cortex, blood vessel walls, and heart via membrane-located receptors. It increases

blood pressure by triggering vasoconstriction (narrowing of the blood vessels). In

the kidneys, it promotes the retention of Na+ and water and reduces potassium secretion. In the brain stem and at nerve endings in the sympathetic nervous system, the effects of angiotensin II lead to increased tonicity (neurotransmitter

effect). In addition, it triggers the sensation of thirst. In the pituitary gland,

angiotensin II stimulates vasopressin release (antidiuretic hormone) and corticotropin (ACTH) release. In the adrenal cortex, it increases the biosynthesis and release of aldosterone, which promotes sodium and water

retention in the kidneys. All of the effects of angiotensin II lead directly or indirectly to increased blood pressure, as well as increased sodium and water retention. This important hormonal system for blood pressure regulation

can be pharmacologically influenced by inhibitors at various points:

• Using angiotensinogen analogs that inhibit renin.

• Using angiotensin I analogs that competitively inhibit the enzyme ACE [3].

• Using hormone antagonists that block the binding of angiotensin II to its receptors.


Mechanism of the urine formation

How urine is formed?

There are 3 basic renal processes: filtration, reabsorption and secretion. Glomerular filtration is caused by difference between glomerular pressure (70 mm Hg), colloid oncotic pressure (30 mm Hg) and capsular pressure (20 mm Hg).

The effective filtration pressure is approximately 70mmHg -(30mmHg + 20mmHg) =20mmHg . Oncotic + capsular pressure must be lower than glomerular pressure.  As a result of the filtration primary urine is formed. Assuming that the kidneys are healthy and filter approximately 20% of the plasma they receive each minute, they will produce 180 to 200 l of filtrate per day. This fluid is essentially protein-free and contains mostly crystalloids in the same concentrations as in the plasma. Approximately 99% of the filtrate must be returned to the vascular system, while 1% is excreted in the urine. The return flow of filtered molecules from the tubules to the blood is called reabsorption. Some substances are not being reabsorbed; such as: urea, uric acid, creatinine etc. Tubules reabsorb 179 l  of water, 1kg of  NaCl, 500g of NaHCO3, 250g of  glucose, 100g of amino acids per day. In addition, some substances are secreted by tubular cells (bases, acids, drugs, etc.). As a result of the reabsorption, secondary urine is formed (1-2 l per day).

Some of the blood that passes through the kidneys, in the other words, is “cleared” from waste products.

If a substance is neither reabsorbed nor secreted by the tubules, the amount of excreted per minute in the urine will be equal to the amount that is filtered out of the glomeruli.

The renal plasma clearence is the volume of plasma from which a substance is completely removed in 1 minute by excretion in the urine.

Renal plasma clearence is calculated using formula:

C = V*U


Where C-clearence

V-urine formation volume per minute (ml per min)

U-concentration of substance in urine (mg%)

P-concentration of substance in plasma (mg%)


For example: clearence of inulin, creatinine is equal to 125 ml per min (because they are not being reabsorbed);  These substances are used for the determination of renal plasma clearence in medicine.

If clearence >125, it means that substance is intensively secreted in tubules.

If clearence <125, it means that there is some inflammatory process of the kidneys (nephritis), which caused azotemia.

Tests of glomerular function

The GFR depends on the net pressure across the glomerular membrane, the physical nature of the membrane and its surface area, which in turn reflects the number of functioning glomeruli. All three factors may be modified by disease, but in the absence of large changes in filtration pressure or in the structure of the glomerular membrane, the GFR provides a useful index of the numbers of functioning glomeruli. It gives an estimate of the degree of renal impairment by disease.

Accurate measurement of the GFR by clearance tests requires determination of the concentra­tions, in plasma and urine, of a substance that is filtered at the glomerulus, but which is neither reabsorbed nor secreted by the tubules; its concen­tration in plasma needs to remain constant throughout the period of urine collection. It is convenient if the substance is present endoge-nously, and important for it to be readily mea­sured. Its clearance is given by

Clearance = U- V/P

where U is the concentration in urine, V is the volume of urine produced per minute and P is the concentration in plasma. When performing this calculation manually, care should be taken to ensure consistency of units, especially for the plasma and urine concentrations.

Inulin (a complex plant carbohydrate) meets these criteria, apart from the fact that it is not an endogenous compound, but needs to be adminis­tered by IV infusion. This makes it completely impractical for routine clinical use, but it remains the original standard against which other measures of GFR are assessed.

Measurement of creatinine clearance

Creatine is synthesised in the liver, kidneys and pancreas, and is transported to its sites of usage, principally muscle and brain. About 1-2% of the total muscle creatine pool is converted daily to creatinine through the spontaneous, non-enzymatic loss of water. Creatinine is an end prod­uct of nitrogen metabolism, and as such undergoes no further metabolism, but is excreted in the urine. Creatinine production reflects the body's total muscle mass.

Creatinine meets some of the criteria mentioned above. Creatinine in the plasma is filtered freely at the glomerulus, but its concentration may not remain constant over the period of urine collec­tion. A small amount of this filtered creatinine undergoes tubular reabsorption. A larger amount, up to 10% of urinary creatinine, is actively secreted into the urine by the tubules. Its measurement in plasma is subject to analytical overestimation. In practice, the effects of tubular secretion and ana­lytical overestimation tend to cancel each other out at normal levels of GFR, and the creatinine clearance is a fair approximation to the GFR. As the GFR falls, however, creatinine clearance progres­sively overestimates the true GFR.

Creatinine clearance is usually about 110 mL/min in the 20-40-year-old age group. Thereafter, it falls slowly but progressively to about 70 mL/min in people over 80. In children, the GFR should be related to surface area; when this is done, results are similar to those found in young adults.

Creatinine clearance or plasma [creatinine]?

Measurement of plasma [creatinine] is more precise than creatinine clearance, as there are two extra sources of imprecision in clearance measurements, that is, timed measurement of urine volume and urine [creatinine]. Accuracy of urine collections is very dependent on patients' cooperation and the care with which the procedure has been explained or supervised. The combination of these errors causes an imprecision (1 SD) in the creatinine clear­ance of about 10% under ideal conditions with 'good' collectors; this increases to 20-30% under less ideal conditions. This means that large changes in creatinine clearance may not reflect any real change in renal function.

It will be apparent that creatinine clearance mea­surements are potentially unreliable. Although cre­atinine clearance measurements are commonly made, accurate measurement of GFR is not often required. Indications for its measurement include determining the dose of a number of potentially toxic drugs that are cleared from the body by renal excretion, investigation of patients with minor abnormalities of renal function and assessment of possible kidney donors.

In most circumstances, however, assessment of glomerular function can be made and changes in GFR over time can be monitored, biochemically, by measurement of plasma [creatininej rather than by measurement of creatinine clearance, because

1 plasma   [creatinine]   normally  remains   fairly constant throughout adult life, whereas creatinine clearance declines with advancing age;

2 plasma [creatinine] correlates as well with GFR as does creatinine clearance in patients with renal disease;

3 measurements  of  plasma   [creatinine]   are  as effective in detecting early renal disease as creati­ nine clearance, despite the form of the relation­ ship described above, because of the imprecision in measuring creatinine clearance;

4 sequential plasma  [creatinine]  measurements enable the progress of renal disease to be followed with better precision than creatinine clearance.



Low plasma [creatinine]

A low [creatinine] is found in subjects with a small total muscle mass (Table 4.1). A low plasma [creati­nine] may therefore be found in children, and val­ues are, on average, normally lower in women than in men. Abnormally low values may be found in wasting diseases and starvation, and in patients treated with corticosteroids, due to their protein catabolic effect. Creatinine synthesis is increased in pregnancy, but this is more than offset by the ncombined effects of the retention of fluid and the physiological rise in GFR that occur in pregnancy, so plasma [creatinine] is usually low.

High plasma [creatinine]

Plasma [creatinine] tends to be higher in subjects with a large muscle mass (Table 4.1). Other non-renal causes of increased plasma [creatinine] include the following:

1 A high meat intake can cause a temporary increase.

2 Transient, small increases may occur after vigor­ ous exercise.

3 Some analytical methods are not specific for creatinine. For example, plasma [creatinine] will be overestimated by some methods in the pres­ence of high concentrations of acetoacetate or cephalosporin antibiotics.

4 Some drugs (e.g. salicylates, cimetidine) com­pete  with  creatinine  for  its  tubular  transport mechanism, thereby reducing tubular secretion of
creatinine and elevating plasma [creatinine].

If non-renal causes can be excluded, an increased plasma [creatinine] indicates a fall in GFR. The renal causes of this include:

1 any disease in which there is impaired renal perfusion (e.g. reduced blood pressure, fluid depletion, renal artery stenosis);

2 most diseases in which there is loss of func­tioning    nephrons    (e.g.    acute    and    chronic glomerulonephritis);

3 diseases where pressure is increased on the tubular side of the nephron (e.g. urinary tract obstruction due to prostatic enlargement).

Other tests of glomerular function

Isotope tests

A number of isotopic markers (e.g. 51Cr-EDTA, 99Tc-DTPA) are almost entirely cleared from the circulation by glomerular filtration. They are injected or infused, and the measurement of their disappearance from the circulation or appearance in urine can be used to calculate the GFR. These tests have largely superceded the use of inulin clearance, but are not widely used in routine clinical practice.


ß2-microglobulin is a small (11.8 kDa) protein found on the cell surface of all nucleated cells, as part of the class 1 major histocompatibility com­plex. It is shed into the blood, where it is normally present in low concentrations. Its small size allows it to pass freely through the glomerular membrane, following which it is reabsorbed and catabolised in the proximal tubules. As glomerular filtration falls, the concentration of ß2-microglobulin rises, making it a good indicator of GFR in normal peo­ple, since it is not affected by muscle mass or diet. However, its concentration also increases in a number of malignancies and inflammatory condi­tions. The prognosis in multiple myeloma is adversely influenced by increasing tumour mass and by declining renal function, both of which cause ß2-microglobulin to rise, making it a helpful prognostic indicator in this condition.

Cystatin C

Cystatin C is a cysteine protease inhibitor produced by all nucleated cells. It is a small (13 kDa) basic protein which is freely filtered by the glomerulus and almost completely reabsorbed and catabolised by the proximal tubules. Serum levels of cystatin C are independent of weight, height, muscle mass, age (over 1 year) or sex, and it has a stable produc­tion rate. Serum levels correlate well with GFR, per­forming at least as well as creatinine, and being less subject to confounding influences. However, at present its measurement is much more expensive and not as rapid as the measurement of creatinine, so despite promise as a measurement, it is unlikely to be widely adopted in the near future.


2.   Mechanisms of reabsorption in kidneys’ tubules.


The biggest part of primary urine during its transference through kidney tubules (the length of all kidney tubules is more than 100km) return many components into blood. Approximately all important for organism substances are reabsorbed. The mechanisms involved in this process may be divided into 2 categories : simple diffusion and active transport.

The main portion of substances is reabsorbed by active transport which requires the use  of metabolic energy. That’s why system of active transport is very developed in kidneys tubules. High activity of Na+/K+ ATPase creates Na+/K+ gradient for secondary active transport of different substances. All the substances are divided into 3 groups due to their extent of reabsorption in proximal tubules :

1.Substances which are actively reabsorbed

2.Substances which are reabsorbed not enough

3.Substances which are not reabsorbed   

Ions of sodium, chloride, magnesium, calcium, water, glucose and other monosaccharides, amino acids, phosphates, hydrocarbonates, proteins, etc are actively reabsorbed.

Glucose and proteins are reabsorbed approximately all, amino acids -  up to 93%, water – up to 96%, NaCl- up to 70%, the other substances- more than 50%. Reabsorption of Na ions by the tubular epithelial cells is generally regarded as an active transport. Firstly Na ions pass from the kidney tubules into the epithelial cells and from there- into extracellular space.

Tubular reabsorption of Cl and HCO3- occurs passively in association with reabsorption of Na+. Water is absorbed isoosmotically with Na and also by flowing along the osmotic gradient due to increase of osmotic pressure in extracellular space. From there substances pass into capillaries.

Glucose and amino acids are transported by the special mediators in association with Na. They use energy of Na+ - gradient on membrane Ca and Mg are reabsorbed by the help of special ATPase. Protein is reabsorbed by endocytosis.

Urea and uric acid belong to substances which are being reabsorbed not enough. They are transported by simple diffusion into extracellular space, and from there-in loop of Henle.

Creatinine, mannitol, inulin - are substances which are not being reabsorbed.

Functional significance of different parts of kidney tubules in the urine formation is heterogeneous. Descending and ascending limbs of the loop of Henle form the countercurrent system which takes part in concentration and dilutation of the urine due to the normal range for the specific gravity of urine which is from 1.002 to 1.030.

Liquid, which is transferred from the proximal tubule to descending limb of the loop of Henle, passes in kidney zone where concentration of osmoactive substances is higher, than in cortex. The walls of the ascending limb of the loop of Henle are not permeable to water. Salt (NaCl) is extruted into the surrounding tissue fluid. The descending limb does not actively transport salt. It is however, permeable to water.

Since the surrounding interstitial fluid is hypertonic to the filtrate in the descending   limb, water is drawn out of the descending limb by osmosis and enters blood capillaries. This system results in a gradually increasing concentration of renal tissue fluid from the cortex to the inner medulla; the osmolality of tissue fluid increases from 300mOsm/l to 1450mOsm/l.

Tests of tubular function

Specific disorders affecting the renal tubules may affect the ability to concentrate urine or to excrete an appropriately acidic urine, or may cause impaired reabsorption of amino acids, or glucose, or phosphate, etc. In some conditions, these defects occur singly; in others, multiple defects are present. Renal tubular disorders may be congeni­tal or acquired, the congenital disorders all being very rare. Chemical investigations are needed for specific identification of these abnormalities and may include amino acid chromatography, or investigation of calcium and phosphate metabo­lism, or an oral glucose tolerance test (OGTT). The functions tested most often are renal concentrating power and the ability to produce an acid urine.

The healthy kidney has a considerable reserve capacity for reabsorbing water, and for excreting H+ and other ions, only exceeded under excep­tional physiological loads. Moderate impairment of renal function may reduce this reserve, and this is revealed when loading tests are used to stress the kidney. Tubular function tests are only used when there is reason to suspect that a specific abnormality is present.

Fluid deprivation test

This test is effectively a bioassay of vasopressin, which is itself difficult to measure. The test can be hazardous in a patient excreting large volumes of dilute urine, and requires close supervision. There are a number of ways of performing a fluid deprivation test, differing in detail but all involv­ing fluid deprivation over several hours, ensuring that the patient under observation takes no fluid, and that excessive fluid losses do not occur. Local directions for test performance should be followed. For instance, beginning at 10 pm, the patient is told not to drink overnight, and urine specimens are collected while the patient continues not to drink between 8 am and 3 pm the next day. During the test, the patient should be weighed every 2 h, and the test should be stopped if weight loss of 3-5% of total body weight occurs. Blood and urine speci­mens are collected for measurement of osmolality. Normally, there is no increase in plasma osmo­lality (reference range 285-295 mmol/kg) over the period of water deprivation, whereas urine osmo­lality rises to 800 mmol/kg or more. A rising plasma osmolality and a failure to concentrate urine are consistent with either a failure to secrete vasopressin or a failure to respond to vasopressin at the level of the distal nephron. When this pat­tern of results is obtained, it is usual to proceed immediately to perform the DDAVP test.

DDAVP test

The patient is allowed to drink a moderate amount of water at the end of the fluid depriva­tion test, to alleviate thirst. An intramuscular injection of DDAVP is then given, and urine spec­imens are collected at hourly intervals for a fur­ther 3 h and their osmolality measured.

Interpretation of tests of renal concentrating ability

These tests are of most value in distinguishing among hypothalamic-pituitary, psychogenic and renal causes of polyuria (Table 4.3).


Patients with diabetes insipidus of hypothala­mic-pituitary origin produce insufficient vaso­pressin; they should therefore not respond to fluid deprivation, but should respond to the DDAVP. As a rule, these patients show an increase in plasma osmolality during the fluid deprivation test, to more than 300 mmol/kg, and a low urine osmo­lality (200-400 mmol'Kg). There is a marked increase in urine osmolality, to 600 mmol/kg or more, in the DDAVP test.

Patients with psychogenic diabetes insipidus should respond to both fluid deprivation and DDAVP. In practice, however, renal medullary hypo-osmolality often prevents the urine osmolal­ity from reaching 800 mmol/kg after fluid depriva­tion or DDAVP injection in these tests, as is normally performed. Also, the chronic suppression of the physiological mechanism that controls vaso­pressin release may impair the normal hypothala-mic response to dehydration. These patients have a plasma osmolality that is initially low, but which rises during the tests. However, fluid deprivation may have to be continued for more than 24 h in these patients before medullary hyperosmolality is restored; only then do they show normal responses to fluid deprivation or to DDAVP injection.

Polyuria of renal origin may be due to inability of the renal tubule to respond to vasopressin, as in nephrogenic diabetes insipidus. In this condition, there is failure to produce a concentrated urine in response either to fluid deprivation or to DDAVP injection, the urinary osmolality usually remain­ing below 400 mmol/kg; in these patients, plasma osmolality increases as a result of fluid deprivation.

Fanconi's syndrome

Fanconi's syndrome may be inherited (e.g. in cystinosis) or secondary to a number of other dis­orders (e.g. heavy metal poisoning, multiple myeloma). The syndrome comprises multiple defects of proximal tubular function. There are excessive urinary losses of amino acids (gener­alised amino aciduria), phosphate, glucose and sometimes HCO3, which gives rise to a proximal renal tubular acidosis. Distal tubular functions may also be affected. Sometimes globulins of low molecular mass may be detectable in urine, in addition to the amino aciduria




The kidneys help to regulate the blood pH, together with respiratory system and the blood buffer systems. Blood buffer systems very quickly react to violation of pH (in 0.5-1 min); lungs influence on hydrogen ions concentration in 1-3 min ; and kidney is the latest regulator of pH (in 10-20 hours). There are 2 main mechanisms which are responsible for the kidneys regulation of blood pH: reabsorption of sodium and secretion of hydrogen ions.

1) Reabsorption of sodium ions during transformation the alkaline phosphate Na2HPO4 of the blood to the acidic phosphate (NaHPO4) which is eliminated in the urine.

2) When the urine is acidic, HCO3- combines with H+ to form carbonic acid. Carbonic acid in the filterate is then converted to CO2 and H2O by the action of carbonic anhydrase. Carbonic acid dissociates to HCO3- and H+. Then H+ (acid) excreted in the urine and HCO3-(base) passes in to the blood as NaHCO3 and decreases the acidity.

3) Ammonia (NH3) is a base that is formed from the amino acid glutamine within the tubular cells. It crosses into tubular lumen to combine with H+ to form ammonium (NH4). This effectively prevents accumulation of H+ ions in the fluid, and therefore permits continued exchange of H+ for Na+ ions. The amount of Na+ ions abbsorbed in the distal tubule is consequently reflected in the amount of both H+ and NH4+ ions in the urine.

8.    Properties and urine’s composition

The amount of urine (diuresis) excreated by a healthy man is 1000-2000 ml per 24 hours. Daily amount of urine, which is lower than 500 ml and higher than 2000 ml, of adults is considered to be pathological. Men's diuresis is a little bit higher than women's one, and it is 1500-2000 ml, and women's diuresis is 1000-1600 ml. Twenty four hour's diuresis can change depending on the kind of a diet, conditions of work, the temperature of the environment and ets.

Drinking a lot of water causes the increase of diuresis to 2000-3000 ml, and decrease of water drinking causes the decrease of diuresis to 700 ml and even less. Consuming of fruits, berries and vegetables, rich in water also increase diuresis, but dry products, especially salted, lower it. The volume of urine is also lowered during a work in hot shops when a man loses water mostly through  sweating.

Diuresis's increase (poliuria) is observed with many diseases and while using  different diuretics. A lot of urine is excreted by the patient who are ill with diabetes mellitus  and  diabetes insipidus.

Twenty four hour's decrease of urine excretion (oliguria) is observed while having fever, diarhea, nausea, acute nephritis, heart deficieny and in some other cases.

When a man is lead or arsenic poisoned, is upset, has nephritis, the full stop of urine excretion (anuria) is observed. Prolonged anuria causes uremia. According to standard, urine is discharged 3-4 times more by day – light time than at night. But in some pathological conditions (the beginning of heart decompensation, diabetes, nephritis) become apparent by predominance of night discharge compare to day time. Such condition is called nicturia.

Urine’s colour. Usually urine is straw -yellow. It's main pigment is urochrome which is formed from urobilin or urobilinogen during their interaction with some peptides. Some other pigments influence on the urine's colour, that's uroerytryn which is obviously derivate of melanin, uroporphyrins, riboflavin and others. During the conservation obviously as a result of urobilinogen oxidation, urine darkens. Such urine is observed during bilirubin's excretion when a man is ill with obstructive or hepatic jaundice.

Concentrated urine, which is excreted in large quantities and has high specific gravity, is of bright-yellow color.

Pale urine has low specific gravity and is excreted in large quantities.

Urine can become of different colour shades when a patient has pathological changes. Urine is red or pink-red when a patient is ill with hematuria, hemoglobinuria, when he takes amidopirin, santonin and other medicines. High concentration of  urobilin and bilirubin can cause dark-red colour of urine.Green or blue colour of urine is observed while albumin is rotting in the bowels and as a result, indoxylsulphuric acids are produced. The last ones while decomposing produce indigo.

Transparency. Fresh urine is transparent. Not fresh urine opacificates because of mucins and the epithelium of the mucosal membrane of  urethras. Urine's opacification is caused also by the crystals of oxalic acid (oxalates) and uric acid (urates). During durable urine standing mostly urates are in fall-our, which, adsorpting pigments, cause its opacification. Calcium and magnesium phosphates  are in fall-out in urine with alkaline reaction. Alkaline character of urine which is falling out  is caused by the decomposition of urea under the influence of urine's microflora to ammonia. Ammonia makes urine alkaline that causes the fall-out of mentioned solts and urine's darkening.

Urine also becomes turbid when a patient is ill with inflamatory process of urethra ducts while pus, proteins, blood cells falling into urine.

For the diagnostics of some diseases urine is acidified and warmed up. If after this process cloudiness disappears it means that it is caused by calcium or magnesium phosphates or urates. If cloudiness  doesn't  disappear it means that it is caused by pus, epitheliym cells and by other admictures.

Urine's specific gravity  depends on the concentration of  dissolving substances. During twenty four hours urine's density changes from 1.002 to 1.035 g /sm3 which is connected with periodical food and water intake and water output . 60-65 grams of hard substances are discharged with urine per day, specifically about 20 grams of  mineral left-overs. Under usually conditions an awerage level of  urine's density of  helthy person is 1.012-1.020.

 Increase of the density during a normal diuresis or poliuria is observed with that patients who discharge a great amount of organic and nonorganic substanses. Urine of the person with diabetes mellitus contains   sugar, ketone bodies and other  substances, which cause not only poliuria, but a high density ( to 1.035). Daily diureses with low specific density of urine is observed among the patients with diabetes insipidus. Urine with low density which is similar to primary urine (1.010) is constantly discharged  when a person has a complicated form of renal failure. Such  condition is called  sthenuria, and it speaks about the disturbance of the concentrational functions of kidneys.

Low density of urine which have patients with diabetes incipidus (1.001-1.004) is the result of the disturbance of  reverseble reabsorption of water in kidney's canaliculi becouse of lack of  antidiuretic hormone.

Oliguria which accompanies acute nephritis is characterized with high urine's density

Urine's reaction.   Normally, having  mixed food urine is acidic or light acidic (pH=5.3-6.8). Urine with pH=6 is usually taken as the norm. Eating mostly meat food and proteins gives urine acidic reaction, while eating vegetables it become alkaline. Acidic reaction of urine is mainly caused by onesubsubstituted phosphaties, mostly NaH2PO4 and KH2PO4.  Twosubstituted phosphaties or biocarbonate potassium or sodium predominate in alkaline urine. Considerable emimence of alkaline substances in blood is accompanied with biocarbonates excretion with urine that raises pH from 6.0 to 7,5-7.7.

Alkaline reaction of urine is observed in patients who are ill with the cystitis (inflammation of urinary bladder) which is connected with urea decomposition and ammonia formation.

The same reaction is observed after vomiting, drinking of alkaline mineral waters and so on.

Clearly acidic reaction is notable for patients who are ill with diabetes mellitus, during fever and starvation.

Urine's smell. Fresh urine has a specific smell mainly caused by volatile acids which are available in it. Urine which is preserved, is influenced by microorganisms, specifically by the decomposition of urea with ammonia forming. The last one causes acute ammonia smell. Healthy people's urine can have different smell, depending on kind of meals. Having some garlic, horseradish, onion gives urine specific smell. Taking medicines and also some diseases can give urine specific smell to.


8.1 Chemical composition of the urine


There are a lot of different organic and non- organic substances in the urine (about 200 ).

They are metabolism end- products  in the kidneys and other organs and tissues of the organism.


8.2 Organic components of the urine


            Proteins. Healthy man excretes about 30 mg of proteins with urine per day (0,033 g/l). This quantity of the protein is not determined by ordinary lab. methods. As a rule low molecular proteins are eliminated , such as enzymes( pepsin, trypsin , amilase, ets.) , albumins. The increasing of protein level in urine is called proteinuria. There are 2 kinds of proteinuria: renal (real) and extrarenal (unreal or false).

Renal proteinuria is caused by organic demage of nephrons , due to blood proteins  (albumins and globulins) occur in urine. For example inflammation of glomeruluses (glomerulonephritis )or nephrosis (violations of proteins reabsorption in tubules).

Extrarenal proteinuria- availability of proteins in urine due to diseases of urinary tract. (inflammation of urinary bladder, urethritis). Patients with such diseases may loose 20 -40 g of protein with urine per day .

     Urea is the main  end- product of the catabolism  of amino acids and is the substance in which is incorporated , for purposes of excretion, the bulk of the nitrogen provided to the organism in excess of its needs.  Nitrogen of urea is equal to 80 -90 %of total nitrogen in urine.

  An adult eliminates 20-35 g of urea with urine per day.

The increasing of urea concentration:

 1. Excess of proteins in the diet

 2. Diabetes mellitus

 3. Cancer

 4. Fever


The decreasing   of urea concentration:

1. Lack of proteins in the diet

2. Liver diseases

3. Acidosis

4. Intensive growth of the organism


Pre-renal uraemia may develop whenever there is impaired renal perfusion, and is essentially the result of a physiological response to hypovolaemia or a drop in blood pressure. This causes renal vaso-constriction and a redistribution of blood such that there is a decrease in GFR, but preservation of tubular function. Stimulation of vasopressin secre­tion and of the renin-angiotensin-aldosterone system causes the excretion of small volumes of concentrated urine with a low Na content. This reduced urine flow in turn causes increased pas­sive tubular reabsorption of urea. Thus shock, due to burns, haemorrhage or loss of water and elec­trolytes (e.g. severe diarrhoea) may lead to increased plasma [urea]. Renal blood flow also falls in congestive cardiac failure, and may be further reduced if such patients are treated with potent diuretics. If pre-renal uraemia is not treated ade­quately and promptly by restoring renal perfu­sion, it can progress to intrinsic renal failure.

Increased production of urea in the liver occurs on high protein diets, or as a result of increased protein catabolism (e.g. due to trauma, major surgery, extreme starvation). It may also occur after haemorrhage into the upper GI tract, which gives rise to a 'protein meal' of blood.

Plasma [urea] increases relatively more than plasma [creatinine] in pre-renal uraemia. This is because tubular reabsorption of urea is increased significantly in these patients, whereas relatively little reabsorption of creatinine occurs.

Renal uraemia may be due to acute or chronic renal failure, with reduction in glomerular filtra­tion. Plasma [urea] increases until a new steady state is reached at which urea production equals the amount excreted in the urine, or continues to rise in the face of near-total renal failure. Although frequently measured as a test of renal function, it is always important to remember that plasma [urea] may be increased for reasons other than intrinsic renal disease (pre-renal and post-renal uraemia).

Post-renal uraemia occurs due to outflow obstruc­tion, which may occur at different levels (i.e. in the ureter, bladder or urethra), due to various causes (e.g. renal stones, prostatism, genitourinary can­cer). Back-pressure on the renal tubules enhances

back-diffusion of urea, so that plasma [urea] rises disproportionately more than plasma [creatinine]. Impaired renal perfusion and urinary tract obstruction, each in itself possible causes of uraemia, may in turn cause damage to the kidney and thus renal uraemia.


Uric acid:     

An adult eliminates 0.6  - 1g  of uric acid with urine per day.

 The increasing of uric acid concentration:

1. Feeding products, which contain many nucleoproteins(meat , fish eggs etc)

2. Leucosis, burns

3. Some Drugs (Aspirin)

4. Violations of proteins metabolism (gout)


The decreasing of uric acid concentration:

1.    Diet poor in proteins and rich in carbohydrates

Intermediate products of purine metabolism are also excreated with urine (xanthine, hypoxanthine – 20-50 mg per day).


Creatinine and Creatine:

An adult excretes of 1-2 g creatinine with urine per day. The excretion of the creatinine is constant from day to day and being determined chiefly by the muscle mass. The term “creatinine coefficient” is applied to the number of mg of creatinine nitrogen excreted daily per kilogram of body weight. Normal values are 18-32 for men and 10-25 for women.

Synthesis of creatine, from which creatinine is formed, is in the kidneys and liver. The decreasing concentration:

1. The kidneys and the liver lesion

2. Violations of protein metabolism

3. Atrophy of muscle


Creatinine is neither reabsorbed nor secreted by the tubules, the amount excreted per minute in the urine will be equal to the amount that is filtered out of the glomeruli.

Creatinine is used for the determination of renal plasma clearance.

The increasing of the creatinine concentration:

-Some infections



The decreasing of creatinine concentration:

- Violations of filteration in kidneys. Children excrete more creatine than adults, females- more, than males.

Cretinuria takes place in old people due to muscle atrophia .



Amino  Acids:

An adult excretes about 2-3 g amino acids with urine per day. Twenty different amino acids and their metabolites are determined in the urine. The increasing of amino acids concentration:

1. Splitting of the tissue proteins

2. Violations of liver functions


There are some genetic defects in the metabolism of seperate amino acid. For example:

1) Phenylketonuria: Which is caused by enzyme phenylalanine-4-monoxygenase absence. In this case a pathway of phenylalanine breakdown and tyrosine is not formed. To determine phenylketonuria is used FeCl3 ( fresh urine +2-3 drops of FeCl3 solution and in 2-3 min observe appearance of dark-green colour).


2) Alkaptonuria: The urine of people genetically defective in homogentisic acid 1,2-dioxygenase contains homogentisic acid, which when made alkaline and exposed to oxygen, turns dark because it is oxidised and polymerized. to a black melanine pigment.


Paired compounds:

Hippuric acid (benzoglycine) is formed by the conjugation in peptide linkage of benzoic acid and glycine. In a man this occours largely in the liver and also in the kidneys. Benzoglycine is excreted in the urine in amounts ranging from 0.6 to 1.5 g daily, depending largely on the dietary intake . It is present in many vegetables and fruits.


Indican  : It is excreted in the urine in amounts ranging from 10-25 mg daily.

       The increasing of indican concentration :

       1. Intestinal obstruction

       2 Generalized peritonitis.

       3. Decomposition of tissue protein, for example, tuberculosis


Organic compounds:

In the urine of healthy man some organic acids are usually observed (for example, acetoacetate ). Some lipids (cholesterol) are present in urine in small amounts.



Allmost all vitamins are excreted with urine. Most of all water-soluble vitamins such as thiamine: 0.1-0.3mg, riboflavin: 0.5-0.8mg ,ascorbic acid: 20-30mg . In medicine wide-spread is a method of determining quantity mg. of vitamin C,which is excreted in urine per hour.In a person 1mg of vitamin C is excreted per hour.



 Some hormones are present in urine. Androgenic compounds of 17- ketosteroids structure are found in the urine of normal person in amounts ranging from 15-25 mg. The increasing of this quantity may be caused by adrenocortical tumours.


   Urobilin (stercobilin): always is present in small amounts in urine. It's concentration increases when liver looses property to decompose urobilinogen from intestine(haemolytic jaundice and hepatic jaundice).



   Urine of healthy individual contains a small amount of bilirubin, which is not determined by ordinary lab. methods.

 Causes of bilirubinuria:

1. Obstruction of bile canaliculi and bile duct.

2. Damage of liver cells.

  Urine will have special colour like dark beer, then it becomes yellow-green, due to oxidation of bilirubin into biliverdin.



 Urine of healthy person contains small amounts of glucose, which is not determine by urinary lab methods.

 Glucose is normally completely reabsorbed in the proximal tubule. But in the patient with diabetes mellitus , content of glucose in urine may be 5-10%.



Galactose is metabolised mainly by liver. Alimentary galactosuria, which is related to the ingestion of milk and milk products.

                  In new borns galactosuria very often combines with  lactosuria.

Galactose tolerance:

  After ingestion of 40 mg. of galactose , quantity of galactose is detected in urine per every hour.

  In normal conditions galactose is excreted in urine in first 2 hours.



  Fructose may appear in the urine under the following circumstances;

1. Alimentary fructosuria(fruits, berries, honey).

2. Unsatisfactory hepatic function.

3. Diabetes mellitus.



Pentose may appear in the urine under the following circumstances;

1. Alimentary pentosuria, occuring in normal individuals after the ingestion of large quantities of fruts which have high quantity of pentose content (cherries, grapes, plums).

2.Essential Pentosuria.

      It is genetically determined, has a familial incidence.


 Ketone bodies:

 In normal conditions daily urine content is 20-50mg of ketone bodies. It is not determined by ordinary lab. methods.

  Ketonuria may occur in a variety of clinical conditions;

1. Diabetes mellitus (20-50mg per day).

2. Carbohydrate starvation.

3. Thyrotoxicosis and fever.



  When red blood cells appear in urine ( hematuria), it means that there are some damage of  the kidney or urinary tract. Hemoglobinuria -- presence of free hemoglobin in urine , is a result of hemolysis (renal infarction, poisons).



 These are red pigments with a pyrolle structure , which are important components of hemoglobin ,, myoglobin, cytochrome and catalysts.In normal conditions daily urine contain very small amount of poryphyrin type I (300 mkg).

 There are 3 isomeric etioporyphyrins , designated typeI, II, III.

Porphyrinuria may occur in a variety of clinical conditions :

- some liver diseases

- intoxication

- intestine bleeding

- pernicious anemia



8.3 Mineral components of the urine


  In normal conditions daily urine contains 15 to 25 g of mineral components.

Sodium chloride is the most wide-spread non-organic substance in urine. It is excreted in amounts ranging from 8 to 16 g per day by the kidneys.

About 1 kg of sodium chloride passes through the glomeruluses every day and only 1 % of this quantity is eliminated from the organism.

In normal conditions daily urine contains 2 to 5 g of potassium. Potassium and sodium are excreted paired with an anion (for example Cl-).

Calcium and magnesium:                                                                                                                                                                                    In normal conditions daily urine contains 0,1-0,3 g of calcium. Excreation calcium in urine depends from its blood concentration.  When blood concentration of Ca is less than 8 mg % calcium is not excreated in urine (for example hypoparathyroidism, pregnancy).

In normal conditions daily urine contains 0,03-0,18 g of magnesium.

Such low Ca and Mg concentration is because their salts are poorly soluble in water.



In normal conditions daily urine contains about 1 mg of iron.

Excessive breakdown of erythrocytes in hemolytic types of anemia causes the increasing iron concentration.



  Phosphorus is excreted in urine as KH2PO4 or NaH2PO4. 

  Quantity of excreted phosphate depends on blood pH:

1. Acidosis: Alkaline phophate (NaH2PO4) react with acids and are transformed into acid phosphates(NaHPO4) which are eliminated in the urine.  

2. Alkalosis: Acidic phosphates (NaHPO4) react with bases and are transformed into alkaline phosphates (Na2HPO4 ) which are eliminated in the urine.



 Sulphur is excreted in the urine as sulphates and paired compounds. In normal conditions daily urine contains 2-3 g of sulphur.



The ammonia which is present in the urine is formed in the kidneys from amino acids , such as glutamine and asparagine, for purpose of neutralization of excreted acid.

 Quantity of ammonia salts is equal to 3-6% of total urinary nitrogen. Urinary ammonia is increased in many conditions associated with acidosis( diabetes mellitus, starvation , dehydration, etc.)

Microscopic Examination of Urine


The third part of routine urinalysis is the microscopic examination of the urinary sediment. Its purpose is to detect and to identify insoluble materials present in the urine.

Red Blood Cells

In the urine, RBCs appear as smooth, non-nucleated, biconcave disks measuring approximately 7 mm in diameter.  In concentrated (hypersthenuric) urine, the cells shrink due to loss of water and may appear crenated or irregularly shaped. In dilute (hyposthenuria) urine, the cells absorb water, swell, and lyse rapidly, releasing their hemoglobin and leaving only the cell membrane. These large empty cells are called ghost cells and can be easily missed if specimens are not examined under reduced light. The presence of RBCs in the urine is associated with damage to the glomerular membrane or vascular injury within the genitourinary tract.

Normal RBCs

Microcytic and crenated RBCs

White Blood Cells

WBCs are larger than RBCs, measuring an average of about 12 mm in diameter.

The predominant WBC found in the urine sediment is the neutrophil. Neutrophils are much easier to identify than RBCs because they contain granules and multilobed nuclei.

WBC clump

Epithelial Cells

It is not unusual to find epithelial cells in the urine, because they are derived from the linings of the genitourinary system. Unless they are present in large numbers or in abnormal forms, they represent normal sloughing of old cells. Three types of epithelial cells are seen in urine: squamous, transitional (urothelial), and renal tubular.

Sediment-containing squamous, caudate transitional, and RTE cells

Squamous epithelial cells

Syncytia of transitional epithelial cells


Bacteria are not normally present in urine. However, unless specimens are collected under sterile conditions (catheterization), a few bacteria are usually present as a result of vaginal, urethral, external genitalia, or collection-container

contamination. These contaminant bacteria multiply rapidly in specimens that remain at room temperature for extended periods, but are of no clinical significance. They may produce a positive nitrite test result and also frequently result in a pH above 8, indicating an unacceptable specimen. Bacteria may be present in the form of cocci (spherical) or bacilli (rods).


The most frequent parasite encountered in the urine is Trichomonas vaginalis. The Trichomonas trophozoite is a pearshaped flagellate with an undulating membrane. It is easily identified in wet preparations of the urine sediment by its rapid darting movement in the microscopic field. Trichomonas is usually reported as rare, few, moderate, or many per hpf. When not moving, Trichomonas is more difficult to identify and may resemble a WBC, transitional, or RTE cell. Use of phase microscopy may enhance visualization of the flagella or undulating membrane.

T. vaginalis is a sexually transmitted pathogen associated primarily with vaginal inflammation. Infection of the male urethra and prostate is asymptomatic. The ova of the bladder parasite Schistosoma haematobium will appear in the urine. However, this parasite is seldom seen in the United States. Fecal contamination of a urine specimen can also result in the presence of ova from intestinal parasites in the urine sediment. The most common contaminant is ova from the pinworm Enterobius vermicularis.


Mucus is a protein material produced by the glands and epithelial cells of the lower genitourinary tract and the RTE cells. Immunologic analysis has shown that Tamm-Horsfall protein is a major constituent of mucus. Mucus appears microscopically as thread-like structures with a low refractive index. Subdued light is required when using bright-field microscopy. Care must be taken not to confuse clumps of mucus with hyaline casts. The differentiation can usually be made by observing the irregular appearance of the mucous threads. Mucous threads are reported as rare, few, moderate, or many per lpf. Mucus is more frequently present in female urine specimens. It has no clinical significance when present in either

female or male urine.

Mucus threads


Casts are the only elements found in the urinary sediment that are unique to the kidney. They are formed within the lumens of the distal convoluted tubules and collecting ducts, providing a microscopic view of conditions within the nephron. Their shape is representative of the tubular lumen, with parallel sides and somewhat rounded ends, and they may contain additional elements present in the filtrate. The most frequently seen cast is the hyaline type, which consists almost entirely of Tamm-Horsfall protein. The presence of zero to two hyaline casts per lpf is considered normal, as is the finding of increased numbers following strenuous exercise, dehydration, heat exposure, and emotional stress. Pathologically, hyaline casts are increased in acute glomerulonephritis, pyelonephritis, chronic renal disease, and congestive heart failure.

Hyaline casts appear colorless in unstained sediments and have a refractive index similar to that of urine; thus, they can easily be overlooked if specimens are not examined under subdued light. The morphology of hyaline casts is varied, consisting of normal parallel sides and rounded ends, cylindroid forms, and wrinkled or convoluted shapes that indicate aging of the cast matrix.

Convoluted hyaline cas

Hyaline cast containing occasional granules


RBC Casts

Whereas the finding of RBCs in the urine indicates bleeding from an area within the genitourinary tract, the presence of RBC casts is much more specific, showing bleeding within the nephron. RBC casts are primarily associated with damage to

the glomerulus (glomerulonephritis) that allows passage of the cells through the glomerular membrane; however, any damage to the nephron capillary structure can cause their formation.

RBC casts associated with glomerular damage are usually associated with proteinuria and dysmorphic erythrocytes. RBC casts have also been observed in healthy individuals following participation in strenuous contact sports.

RBC cast. Notice the presence of hypochromic and dysmorphic free RBCs


WBC Casts

The appearance of WBC casts in the urine signifies infection or inflammation within the nephron. They are most frequently associated with pyelonephritis and are a primary marker for distinguishing pyelonephritis (upper UTI) from lower UTIs. However, they are also present in nonbacterial inflammations such as acute interstitial nephritis and may accompany RBC casts in glomerulonephritis.

Waxy Casts

Waxy casts are representative of extreme urine stasis, indicating chronic renal failure. They are usually seen in conjunction with other types of casts associated with the condition that has caused the renal failure. The brittle, highly refractive cast matrix from which these casts derive their name is believed to be caused by

degeneration of the hyaline cast matrix and any cellular elements or granules contained in the matrix.

Granular cast degenerating into waxy cast


Urinary Crystals

Crystals frequently found in the urine are rarely of clinical significance. They may appear as true geometrically formed structures or as amorphous material. The primary reason for the identification of urinary crystals is to detect the presence of the relatively few abnormal types that may represent such disorders as liver disease, inborn errors of metabolism, or renal damage caused by crystallization of iatrogenic compounds within the tubules. Crystals are usually reported as rare, few, moderate, or many per hpf. Abnormal crystals may be averaged and reported per lpf. Crystals are formed by the precipitation of urine solutes, including inorganic salts, organic compounds, and medications (iatrogenic compounds). Precipitation is subject to changes in temperature, solute concentration, and pH, which affect solubility. Solutes precipitate more readily at low temperatures. Therefore, the majority of crystal formation takes place in specimens that have remained at room temperature or been refrigerated prior to testing. Crystals are extremely abundant in refrigerated specimens and often present problems because they obscure clinically significant sediment constituents. As the concentration of urinary solutes increases, their ability to remain in solution decreases, resulting in crystal formation. The presence of crystals in freshly voided urine is most frequently associated with concentrated (high specific gravity) specimens.

A valuable aid in the identification of crystals is the pH of the specimen because this determines the type of chemicals precipitated. In general, organic and iatrogenic compounds crystallize more easily in an acidic pH, whereas inorganic salts are less soluble in neutral and alkaline solutions. An exception is calcium oxalate, which precipitates in both acidic and neutral urine.


Uric acid crystals




Classic dihydrate calcium oxalate crystals.




Amorphous urates



Amorphous phosphates




“Coffin lid” and other forms of triple phosphate crystals


Cholesterol crystals. Notice the notched corners





Renal Disease



Primary Urinalysis Results





Microscopic hematuria

Mild proteinuria

Increased pH

Acute pyelonephritis




WBC casts

Bacterial casts

Microscopic hematuria


Chronic pyelonephritis




WBC casts

Bacterial casts

Granular, waxy, broad casts



Acute interstitial nephritis




WBC casts

Acute glomerulonephritis

Macroscopic hematuria


RBC casts

Granular casts

Chronic glomerulonephritis




Cellular and granular casts

Waxy and broad casts

Nephrotic syndrome

Heavy proteinuria

Microscopic hematuria

Renal tubular cells

Oval fat bodies

Fat droplets

Fatty and waxy casts




Renal failure

 Acute renal failure

By definition, there is renal disease of acute onset, evere enough to cause failure of renal homeostasis. Often oliguric, diuretic and recovery phases can be recognised, although a few patients maintain a normal urine volume throughout the course of the illness. Chemical investigations help to determine the severity of the disease and to follow its course, but do not help much in determining the cause. Proteinuria is present, and haem pig­ments from the blood may make the urine dark.

Oliguric phase

In this phase, less than 400 mL urine is produced each day; if the renal failure is due to outflow restruction, there may be anuria. The oliguria is mainly due to a fall in GFR. The urine that is formed  usually  has   an  osmolality  similar  to

plasma and a relatively high [Na+], since the composition of the small amount of glomerular filtrate produced is little altered by the damaged tubules.

Plasma [Na+] is usually low due to a combination of factors, including intake of water in excess of the amount able to be excreted, increase in metabolic water from increased tissue catabolism, and possibly a shift of Na+ from ECF to ICE. Plasma [K+], on the other hand, is usually increased due to the impaired renal output and increased tissue catabolism, which aggravated by the shift of K+ out of cells that acompanies the metabolic acidosis that develops due to failure to excrete H+ and also due to the increased formation of H+ from tissue catabolism.

Retention of urea, creatinine, phosphate, sulphate and other waste products occurs. The rate at which plasma [urea] rises is affected by the rate of tissue catabolism; this, in turn, depends on the cause of the acute renal failure. In renal failure due to trauma (including renal failure developing after surgical operations), plasma [urea] tends to rise more rapidly than in patients with renal failure due to medical causes such as acute glomerulonephritis.

To differentiate the low urinary output of sus­pected acute renal failure from that due to severe circulatory impairment with reduced blood volume, the tests summarised in Table 4.4 may be helpful. However, none of these tests can be com­pletely relied upon to make the important and urgent distinction between renal failure and hypo­volaemia. Careful assessment of the patient's fluid status, possibly including measurement of the central venous pressure, is also required.

For monitoring patients in the oliguric phase of acute renal failure, plasma [creatinine] or [urea] and plasma [K+] are particularly important, and need to be determined at least once daily. Decisions to use haemodialysis are reached at least partly on the basis of the results of these tests. The volume of urine and its electrolyte composition (and the volume and composition of any other measurable sources of fluid loss) should also be assessed in order to determine fluid and elec­trolyte replacement requirements.


Diuretic phase

With the onset of this phase, urine volume increases, but the clearance of urea, creatinine and other waste products may not improve to the same extent. Plasma [urea] and [creatinine] may therefore continue to rise, at least at the start of the diuretic phase. Large losses of electrolytes may occur in the urine and require to be replaced orally or parenter-ally. Measurement of these losses is needed so that correct replacement therapy can be given; this requires urine collections, for urine [Na+] and [K+] measurement, and calculation of daily outputs. Plasma [K+] tends to fall as the diuretic phase continues, due to the shift of K+ back into the cells and to marked losses in urine resulting from impaired conservation of K+ by the still-damaged tubules. Usually, Na+ deficiency occurs also, due to failure of renal conservation. Throughout the diuretic phase, therefore, it is important to mea­sure plasma [creatinine] or [urea] and both plasma [Na+] and [K+] at least once daily, and to monitor the output of Na+ and K+ in the urine.

Chronic renal failure

Most of the functional changes seen in chronic renal failure can be explained in terms of a full solute load falling on a reduced number of normal nephrons. The GFR is invariably reduced, associ­ated with retention of urea, creatinine, urate, vari­ous phenolic and indolic acids, and other organic substances. The progress and severity of the dis­ease are usually monitored by measuring plasma [creatinine] or [urea], or both.

Sodium, potassium and water

The renal handling of Na+, K+ and water by nor­mal kidneys and in chronic renal failure has already been considered above.

Acid-base disturbances

The total excretion of H+ is impaired, mainly due to a fall in the renal capacity to form NH4+. Metabolic acidosis is present in most patients, but its severity remains fairly stable in spite of the reduced urinary H+ excretion. There may be an extrarenal mechanism for H+ elimination, possibly involving buffering of H+ by calcium salts in bone; this would contribute to the demineralisation of bone that often occurs in chronic renal failure.

Calcium and phosphate

Plasma [calcium] tends to be low, often due, at least partly, to reduced plasma [albumin]. Plasma [phos­phate] is high, mainly due to the reduction of GFR. Virtually all patients with chronic renal failure have secondary or, much less often, tertiary hyper-parathyroidism,  and they may develop osteitis fibrosa. Plasma [calcium], which is decreased or close to the lower reference value in patients with secondary hyperparathyroidism, increases later if tertiary hyperparathyroidism develops. Many patients with a low plasma [calcium] have reduced activity of renal cholecalciferol la-hydroxylase, the enzyme responsible for the synthesis of the most active form of vitamin D. They can poten­tially develop osteomalacia or rickets, but this would be uncommon in adequately treated patients. A few patients show a third type of bone abnormality: increased bone density (osteosclero-sis). It is not clear why any particular one of these various types of renal osteodystrophy should develop in an individual patient.

Renal stones

Physicochemical principles govern the formation of renal stones, and are relevant to the choice of treatment aimed at preventing progression or recurrence. Stones may cause renal damage, often progressive.

The solubility of a salt depends on the product of the activities of its constituent ions. Frequently, the solubility product in urine is exceeded without the formation of a stone, provided there is no 'seeding' by particles present in urine, such as debris or bacteria, which promote crystal forma­tion. Formation of stones can also be prevented by inhibitory substances that are normally present in the urine, such as citrate, which can chelate cal­cium, keeping it in solution.

People living or working in hot conditions are liable to become dehydrated, and show a greater tendency to form renal stones, as the urine becomes more concentrated. There are also several metabolic factors.that can cause stones to form in the renal tract. However, in many patients, no cause can be found to explain why stones have formed.




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