MLS 313 Renal Function PDF

**MLS 313 RENAL FUNCTION** **Brief description of the Kidney** The kidneys are bilateral organs that are placed retroperitoneally (meaning that they sit behind a lining in the abdominal cavity) in the upper left and right abdominal quadrants. The kidneys are bean-shaped and consist of the cortex a...

**MLS 313 RENAL FUNCTION** **Brief description of the Kidney** The kidneys are bilateral organs that are placed retroperitoneally (meaning that they sit behind a lining in the abdominal cavity) in the upper left and right abdominal quadrants. The kidneys are bean-shaped and consist of the cortex and medulla. They are part of the urinary system, and they empty urine into the ureter which carries urine to the urinary bladder. The functional unit of the kidney called the **nephron** consist of the glomerulus and the tubule. There are about 1.2 million of these structures in each kidney. It is important to note that all of these structures are not working at any one time, but their presence gives the kidney considerable reserve capacity **in the event of stress, disease or injury**. Renal function is also called kidney function. It is a term used to describe how well the kidneys work. The kidney has multiple functions, but the principal role of the kidney in the body metabolism is the formation of urine. *Formation of urine entails the following:* - excretion of **waste products** from the blood, - provision for the preservation of **essential solutes**, and - regulation of **hydration and electrolytes**. The kidney is a very complex organ with rich blood supply. While the **Vasa recta capillaries** (which specifically arise from the efferent arteriole of juxtamedullary nephrons and wind around the loops of Henle) feed medulla of the kidney with oxygen and nutrients, the **peritubular capillaries** provide nutrients and oxygen to the renal cortex. **Brief history** **Claude Bernard,** a nineteenth century French Scientist recognized that it is of vital importance that the body's internal environment (homeostasis) be maintained. He was among the first to point out that the integrity of the body was dependent upon **selective excretion of metabolites** which could not be allowed to accumulate within the body else they would cause harm to the individual. This excretion of metabolic waste products must be sufficiently selective so that substances that are utilized or required by the body are not lost. Another Scientist, **Homer Smith** put it this way, ***"the composition of the blood is determined not by what the mouth ingests but by what the kidney keeps."*** **How the kidneys' function.** The production of urine involves highly complex steps of **filtration, reabsorption, secretion and excretion**. The processes are necessary in maintaining a stable balance of the body chemicals. The renal artery enters the kidney, while the renal vein and ureter leave the kidney. A nephron consists of a filtering unit (of tiny blood vessels) called a glomerulus which is attached to a tubule. Each nephron contains a **renal corpuscle**, which is the initial component that filters the blood, and a **renal tubule** that processes and carries the filtered fluid to the system of calyces (chambers of the kidney through which urine passes). The renal corpuscle has two components: the glomerulus and the glomerular (Bowman's) capsule in which sits the glomerulus. **The glomerulus is actually a web of arterioles and capillaries,** with the **glomerular membrane which is a special filter** which filters the blood that runs through the capillaries. The vessel which brings blood into the glomerulus is the **afferent arteriole**, whereas the vessel that carries the rest of the blood that has not been filtered out of the glomerulus is called the **efferent arteriole**. Each nephron is supplied by a small blood vessel called the **afferent arteriole** which carries blood from a branch of the renal artery into the nephron at the rate of 1,200 ml/min which is the total renal blood flow (**RBF**). Of this renal blood flow (RBF), only the plasma (which contains nutrients, water, ions, gases, wastes) can cross the structures comprising the glomerulus. The **glomerular membrane** is designed in a way in which it is not permeable for big and important molecules in blood, such as plasma proteins, but it is permeable to the smaller substances such as sodium, potassium, amino acids and many others. It is also permeable for the products of the metabolism such are creatinine and drug metabolites. The afferent arteriole enters into the Bowman's capsule, and within the capsule the small blood vessel breaks up into a **plexus of capillaries** which ultimately recombines to form an **efferent arteriole**. This efferent arteriole then joins with other efferent arterioles to carry blood from the nephron to the renal tubular area. The capillary plexus and its afferent and efferent arterioles are often referred to as the **glomerular tuft**. The Bowman's capsule envelopes the glomerular tuft and is connected with **the tubule where *concentration and modification of the filtrate occurs.*** Schematic diagram illustrating the anatomical relationship \...  Filtration Process (System) When blood enters the glomerulus, it is filtered. Filtration is accomplished through the thin walls of the fenestrated capillaries that make up the plexus. **There is a difference in size of the two vessels. The blood flows into the plexus from the relatively large afferent vessel and leaves the plexus through the efferent vessel with smaller lumen.** The difference in size of the vessels is critical because it produces an increase in hydrostatic pressure (about 75 mmHg) within the capillaries. The relatively high hydrostatic pressure **forces the filtrate through the thin capillary epithelium** and is caught in the Bowman's capsule. The forces that govern filtration in the glomerular capillaries are the same as any capillary bed. **The glomerular filtration rate (GFR) represents the flow of plasma from the glomerulus into Bowman's space over a specified period, and is the chief measure of kidney function.** GFR is the rate in milliliters per minute (ml/min) at which substances in plasma are filtered through the glomerulus; in other words, the clearance of a substance from the blood. The normal GFR for an adult male is 90 to 120 mL per minute. It estimates the volume of blood/plasma that passes through the glomeruli per minute. **Evaluating the GFR is a test used to check how well the kidneys work.** The normal GFR is approximately 120 mL per min (180 L per day). The average urine output, on the other hand, averages only about 1.5 L daily. The reabsorption of 178.5 L requires a sophisticated tubular network. Tubular Processes (System) **The concentration and modification of the filtrate is an active process of selective secretion and reabsorption by the tubular epithelium of the kidney.** Solutes and metabolites that are required by the body are conserved, while molecules that must be eliminated e.g., creatinine, urea and metabolites not needed by the body are disposed. **The filtrate** passes into the tubular system, and must not contain large protein like albumin or red blood cells. Presence of albumin or red blood cells suggest a disease process often within the glomerulus. The tubular system is considered to be divided into **four major sections** namely, the proximal convoluted tubule (PCT), the loop of Henle, the distal convoluted tubule (DCT) and the collecting tubules. Each section of the nephron contains **different types of cells.** The filtrate contains water, electrolytes, glucose, amino acids, creatinine and urea. The PCT is concerned with the reabsorption of large volume of the glomerular filtrate back into the blood. About 65% of water, sodium chloride, potassium, 90% of bicarbonate, and approximately 100% of glucose and amino acids are reabsorbed at this section of the tubular system. **Also, uric acid and organic acids such as antibiotics are secreted at the PCT into the tubules.** The modified filtrate passes into the loop of Henle, where about 25% of water and sodium chloride are further reabsorbed. Urine concentration takes place in this section of the tubular system. At the DCT, more water (5%) is reabsorbed. It is also at this point in the tubular system that **potassium and hydrogen are secreted into the tubule**, exchange of Na^+^, K^+^, and H^+^ occurs, and **the kidney forms ammonia from amides.** The urine has now assumed its final composition. It passes via the collecting tubule (where some urea, sodium chloride (5%) and water are reabsorbed), to the pelvis of the kidney, to the ureter, and finally into the bladder. Note that some substances present in the glomerular filtrate are known as ***threshold substances***. Such substances appear in the urine only after they have reached certain minimum concentrations in the blood e.g., glucose. Glucose does not appear in urine until the plasma glucose levels reach about 180mg/100ml. Creatinine, a waste product of the breakdown of creatine phosphate from the muscle, may be excreted in urine without appreciable reabsorption. The polysaccharide, inulin, a foreign substance behaves similarly. Therefore, both creatinine and inulin are useful in measuring GFR.) **Properties of the glomerular filtrate** include: Specific gravity = 1.010; pH = 7.4; and it is isosmotic with plasma. This remarkable process of filtration, reabsorption, secretion and elimination by the kidneys, is subject to many types of disturbances. **Therefore, the knowledge of renal physiology, and the results of some kidney function tests, would help in the diagnosis of the type of lesion.** **It is important to note that the characteristics of an ideal marker of GFR are as follows:** 1\. It should appear endogenously in the plasma at a constant rate; 2\. It should be freely filtered at the glomerulus; 3\. It can be neither reabsorbed nor secreted by the renal tubule; 4\. It should not undergo extra renal elimination. **KIDNEY (RENAL) FUNCTIONS** The kidneys are powerful chemical factories that perform the following functions: - **A-Acid Base Balance** The **kidneys** play an important role in maintaining **acid-base homeostasis** by regulating the pH of the blood plasma. The kidneys have **two** very important roles in maintaining the **acid-base balance**: - to reabsorb **bicarbonate (base)** from urine, and - to excrete **hydrogen ions** into urine. Accumulation of certain substances in the blood could cause it to become **acid or alkaline.** Wastes from protein metabolism such as urea, uric acid and creatinine are **substances of acid reaction**, while salts of sodium, potassium, calcium, magnesium and phosphorus are **substances of alkaline reaction**. The kidneys are responsible for removing them if the concentration is too high in the blood. In doing so, the normal pH of the blood 7.35 -- 7.45 which is slightly alkaline is maintained. The kidneys are **slower to compensate than the lungs**, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. **The major homeostatic control point** for maintaining a stable balance **is renal excretion**. - **W-Water Balance** By removing just, the right amount of **excess fluid (water)**, healthy **kidneys** maintain what is called the **body's fluid** balance. The **kidneys** maintain proportions by balancing the amount of fluid (water) that leaves the body against the amount entering the body. To understand this concept further, let's look at the osmotic pressure of plasma. Normally, the osmolarity of the plasma varies only slightly despite wide variations in fluid and electrolytes intake of the body. In the case of **excess water intake**, it tends to dilute the plasma and reduce its osmotic pressure. The kidneys' response would lead to an excretion of increased volume of urine with an osmolarity less than that of plasma -- water is excreted in excess of solutes. Likewise, in **fluid restriction** situation, the plasma osmotic pressure increases leading to an excretion of 'concentrated' urine with an osmolarity higher than that of the plasma -- more solute is excreted in excess of water. **By excreting water in excess of solutes or more solute in excess of water as the case may be,** the kidneys maintain the water balance of the body. - **E-Erythropoeisis** Healthy kidneys produce a **hormone** known as **erythropoeitin (EPO)**, which is carried in the blood to the **bone marrow** where it stimulates the production of red blood cells. These cells carry oxygen throughout the body. Without enough healthy red blood cells, you develop anemia, a condition which makes you feel weak, cold, tired, and short of breath. Other hormones are also produced, e.g., the juxtaglomerular cells in the afferent arteriole release renin. The renin-angiotensin-aldosterone system (RAAS) acts to preserve glomerular filtration rate (GFR). - **T-Toxin Removal** The kidneys **remove wastes and excess water (fluid)** collected by, and carried in, the blood as it flows through the body. About **190 liters (335 pints)** of blood enter the kidneys every day via the renal arteries. **Millions of tiny filters**, called **glomeruli**, inside the kidneys' separate wastes and water from the blood. The kidneys automatically remove the right amount of salt and other minerals from the blood to leave just the quantities the body needs. After the metabolism of drugs and detoxification of toxins, the waste products are excreted by the kidneys either unchanged or as metabolites. The cleansed blood returns to the heart and recirculates through the body. Excess wastes and fluid leave the kidneys in the form of urine. - **B-Blood Pressure Control** Healthy **kidneys** make hormones such as renin and angiotensin. These hormones regulate how much **sodium (salt)** and fluid the body keeps, and how well the blood vessels can expand and contract. This, in turn, helps control blood pressure. They do this by regulating the amount of water in the body and the width of the arteries. - **E-Electrolytes Balance** The kidneys help maintain **electrolyte concentrations** by filtering electrolytes from blood, returning some electrolytes to the blood, and **excreting any excess into the urine.** Thus, the kidneys help maintain a balance between daily consumption and excretion. In the event that any fluctuation in the amounts of water and/or electrolytes occur in the body, the kidneys respond adequately with the help of antidiuretic hormone (ADH - produced by the posterior pituitary) and aldosterone (produced by the adrenal gland), thereby restoring the body fluids to the normal composition and volume. - **D-Vitamin D Activation** Healthy kidneys **keep bones** strong by producing the hormone calcitriol. **Calcitriol** maintains the right levels of calcium **and phosphate** in the blood and bones. Calcium and phosphate balance are important to keep bones healthy. **EVALUATION OF KIDNEY FUNCTION** The diagnosis of the functional renal disease is to a great extent made in the clinical laboratory. It is indeed fortunate that the laboratory has a battery of kidney function tests available, which when properly applied can give information on the individual's kidney function **status and the location** of the defect. It is important to remember that the kidney has a considerable functional reserve and kidney function tests may be normal even in the presence of a relatively severe renal lesion. Clinical signs and symptoms may be minimal or absent entirely and, even when present, will not always reflect the severity of the disease or the prognosis for the patient. **Kidney function tests can be affected by prerenal, renal or postrenal phenomena.** A considerable rise in the blood non-protein nitrogen (NPN) level e.g., urea and creatinine in a kidney insufficiency is referred to as **azotemia. Azotemia** can be caused by: - dehydration, - medication side effects or - a very high protein diet, which are not as a result of underlying disease. **Prerenal** causes include dehydration which may be found in pyloric and intestinal obstruction and in prolonged diarrhea, conditions of shock and excessive blood loss such as seen in severe intestinal bleeding or cardiac failure. Decreased kidney function may occur in these conditions either due to decreased plasma volume or decreased blood flow. **Increased protein catabolism** as seen in severe burns, stress and crush injury also contributes to prerenal phenomena. **Intra-Renal** causes for decreased kidney function include diseases affecting the glomerular filtration rate, the tubular function, or alterations in the vascular system of the kidney that decrease the blood flow. **Acute state** includes glomerulonephritis, nephrotoxic drugs, or renal cortical necrosis. **Chronic state** includes glomerulonephritis, pyelonephritis, diabetes mellitus, renal tubular diseases. **Post renal** causes for decreased kidney function include conditions that lead to obstruction of the urine flow either due to enlargement of the prostate, stones in the urinary tract (calculi), or tumors of the bladder. The post renal causes achieve the decrease in renal function **by reducing the effective filtration pressure of the glomeruli.** +-----------------------+-----------------------+-----------------------+ | Pre-renal | Intra-renal | Post-renal | +=======================+=======================+=======================+ | Dehydration () | Glomerulonephritis | Obstructions by | | | | kidney stones | | Shock () | Nephrotoxins | | | | | Prostatic enlargement | | Hypovolemia () | Aminoglycosides | | | | | | | Increased protein | Drugs | | | catabolism | | | | | | | | Hypotension | | | | | | | | Low cardiac output | | | | | | | | Burns | | | | | | | | Septicemia | | | +-----------------------+-----------------------+-----------------------+ **LABORATORY TESTS FOR KIDNEY FUNCTION** There are several clinical laboratory tests that are useful in investigating and evaluating kidney function. Clinically, the most practical tests to assess renal function is to get an estimate of the glomerular filtration rate (GFR) and to check for proteinuria (albuminuria). **Urine Albumin and Protein** Urine albumin or protein may be increased in the presence of conditions not related to renal disease, for example, posture, fever, and exercise. Furthermore, in the presence of a urinary tract infection, urine protein levels may be raised without any intrinsic renal pathology present. Albuminuria refers to the abnormal presence of albumin in the urine. **Albuminuria is used as a marker for the detection of incipient nephropathy in diabetics.** Kidney function tests are therefore separated into **three major groups**: 1\) those measuring glomerular filtration, 2\) those measuring renal blood flow, and 3\) those measuring tubular function. 1. **Tests Measuring Glomerular Filtration Rate (GFR)** **Clearance Tests** **Renal clearance tests** are very useful in measuring the **actual capacity of the kidney** to eliminate waste products, and certain substances present in the plasma must be determined. Granted that there are group of tests involved, **the selection of the particular test** to be conducted is based on the consideration of **the aspect of the kidney physiology** that is being evaluated. For instance, a substance may be excreted by the glomerular filtration alone, or it may be filtered by the glomerulus and also excreted by the tubules (depending on the substance, tubular excretion may be more or less dominant), or it may be filtered by the glomerulus but subsequently reabsorbed by the tubules either in whole or in part. **If the facet of renal physiology to be studied is the glomerular filtration rate**, then a substance that is excreted (filtered) completely or predominantly by the glomeruli without being either excreted or reabsorbed by the tubules should be selected. **Inulin** is such a substance but in practice it is rarely used. **Creatinine** is a substance that behaves similar to inulin. The procedure is readily available and it has become one of the most popular tests for glomerular filtration rate. **Urea clearance test has considerable historical significance since it was the first of the clearance tests to be used widely,** but it is partially reabsorbed by the tubules after being filtered by the glomeruli. Again, the rate of reabsorption of urea is a process of passive diffusion and varies with the amount of water reabsorbed. **However, urea clearance is an index of the overall renal function.** The **screening tests for measuring renal function** involve the determination of blood (plasma/serum) constituents such as non-protein nitrogen (NPN) which includes urea, urea nitrogen, and creatinine. **The sensitivity of these screening tests is limited while the clearance tests provide a much more sensitive measure of renal function.** If the appropriate clearance test is used, the result obtained is **a measure of the functional capacity of a specific part of the nephron.** **Usefulness of Clearance Test** The usefulness of the clearance tests is due to the fact that **we can relate the quantity** of certain substances excreted in urine to the quantity of the same substance in plasma. **The amount of substance cleared by the kidney is expressed as** that volume of plasma which contain the quantity of the substance excreted in urine in a period of one minute: **clearance (ml/min) = U/P×V.** Where: U = the concentration of the substance in urine; P = the concentration of the substance in plasma; and V = the volume of urine per min expressed in ml. U and P must be expressed in the same unit. **All other factors being equal, the clearance rate is roughly proportional to the size of the kidney, and the body surface area (BSA) of the individual.** Therefore, to correct for deviations from the average adult body surface area, multiply the clearance by the factor 1.73/A, where 1.73 is the generally accepted average body surface in square meter, and A is the body surface of the patient under investigation. The formula for calculating renal clearance is expanded thus, Clearance (ml/min/std. surface area) = U/P×V×1.73/A a. **Inulin Clearance Test** Inulin is a polysaccharide obtained from dahlias and artichokes. It has a molecular weight of about 5100 Daltons. This substance is filtered freely through the glomeruli and it is neither secreted nor reabsorbed by the tubules. It is therefore a substance of choice for precise investigative work, however, because of the discomfort of the procedure to the patients the use of inulin for measuring GFR **cannot be regarded as a routine laboratory test.** For the Procedure: An adequate fluid intake of 1000 ml of water is maintained during the hour before the test. No fasting is required. Blood specimens are taken at beginning and at the end of the urine collection period. **Clinical Significance** Inulin is cleared by glomerular filtration at an average rate of 125 ml/min. The amount of inulin filtered is not reabsorbed by the tubules and thus, the amount excreted is quantifiable. **Although it is the most accurate measure of GFR, the procedure is time consuming, expensive, cumbersome, and uncomfortable to the patient (invasive).** Therefore, the clinical application is limited. It is best suited for research institutions and kidney disease study centers. b. **Creatinine Clearance Test** Creatinine is a waste product from the metabolism of creatine and creatine phosphate, and it is excreted by the kidneys. Creatinine is removed from plasma by glomerular filtration and excreted in urine without being appreciably reabsorbed by the tubules. As a result, Creatinine has a relatively high clearance rate of 125 ml/min compared to urea with 70 ml/min. **When plasma levels increase above normal, the kidneys can also excrete creatinine through the tubules.** For this reason, the exogenous clearance test in which creatinine is administered either orally or intravenously is rarely employed. Creatinine clearance test is one of the most sensitive tests for measuring GFR. It is relatively accurate and useful, and has replaced the less accurate urea clearance test in measuring the GFR. However, urea clearance test had considerable historical significance since it was the first of the clearance tests to be widely in use. For the Procedure: the patient should be well hydrated to ensure a urine output of at least 2 ml/min. Patients should be adequately prepared before test is carried out. Administer a minimum of 600 ml of water. Withhold tea, coffee and drugs on the day of the test. Before timed urine collection begins, patient should void and discard urine. Collect a 4-, 12-, or 24-hour urine specimen. Record the time of starting and completing the collection (a precisely timed urine specimen is required). Keep patient well hydrated to ensure urine output of at least 2 ml/min. Collect blood specimen in plain container (clotted specimen i.e., allow blood specimen to clot before separation), at any time during the urine collection. Store specimens in the refrigerator and analyze within 24 hours. This would slow equilibrium reaction between creatinine and creatine (which is accelerated by hydrogen and hydroxyl ions), as well as retard bacterial decomposition. **Clinical Significance** In renal disease, serum or blood creatinine levels do not increase until renal function is substantially impaired. **The advantage of creatinine determination over urea determination lies in the fact that creatinine levels are not affected by a high protein diet as in the case of urea levels.** Determination of urine creatinine levels is not helpful in evaluating renal function unless it is done as part of creatinine clearance test. 2. **Renal Plasma Flow Function** a. **Renal Plasma Flow Test** Para-amino hippuric acid (PAH) measures the renal plasma flow, which is a measure of renal function. PAH is filtered by the glomerulus and secreted by the proximal tubule of the nephron such that, in one pass, it is entirely cleared by the kidney. The rate at which the kidneys clear PAH from the blood reflects the total RPF. Therefore, its clearance approximates RPF. The para-aminohippurate (PAH) test is one of the tests that is conducted to measure the excretory ability of the tubules. The substance ρ-aminohippurate is foreign to the human body. It is injected into the blood, and in addition to being filtered through the glomerulus, they are also predominantly removed by the kidney via the tubules. About 90% of the PAH within the limits of the plasma concentration, are removed from the plasma through the kidney. **Since renal function is dependent on the renal blood flow, a clearance value of PAH will provide a measure of the effective *renal plasma flow* in the absence of tubular functional impairment and vice versa.** The clearance value of PAH is 600-700 ml/min/m^2^ of body surface area. Note that the **actual renal blood flow is about 1200 ml/min and plasma flow is about 750 ml/min.** For the Procedure: The procedure for performing PAH clearance test is essentially similar to that of Inulin clearance test. Because of the technical difficulties of performing test, this substance is not widely used. A priming dose is given to the subject by intravenous injection, and this is followed by a slow continuous administration of solution of low concentration of PAH in order to maintain a constant level. The ***principle*** of the assay is a coupling of PAH with diazotized N-(1-naphthyl) ethylenediamine dihydrochloride. 3. **Renal Tubular Function** **Test Measuring Renal Tubular Function** Since the renal tubules are engaged in a wide variety of activities, therefore several groups of tubular function tests are conducted in the clinical laboratory. **The first group of tests** measure the excretory ability of the tubules. In this group of tests, substances are injected into the blood **which are cleared either exclusively or predominantly** by the tubules. **The second group of tubular function tests** is concerned with the concentrating ability of the tubules. **In tubular damage** the concentrating ability is the first function to be decreased. a. **Dye Excretion Test** **Phenolsulfonphthalein (PSP)** Test PSP determines how well the kidneys are working by measuring how well the kidneys excrete the dye. **This test is mainly a measure of the secretory capacity of the tubules.** PSP is a dye that is removed from the plasma through the kidney. **The renal clearance is approximately 400 ml/min.** About 20% of the dye is removed by the liver. Of the 60 - 70% that is removed by the kidney, 6% is excreted by glomerular filtration and about 94% by tubular excretion. For the Procedure: The test is performed as a 2-hour test. The patient is hydrated by giving about 600 ml of water prior to administering PSP. Measure the quantity of PSP excreted spectrophotometrically/colorimetrically after ***alkalizing*** the specimen to convert the dye to the colored form. b. **Tubular Function - Concentration and Dilution Tests** Principle In the ***concentration tests***, the ability of the kidneys to concentrate urine is tested by measuring specific gravity or osmolality of urine voided at intervals in the morning, following an overnight period of fluid restriction. Clinical Significance The production of a dilute urine or the formation of a concentrated urine is a tubular function. The concentration tests are used for the assessment of the renal tubular function. They have been gradually replaced by tests measuring plasma constituents of a normal patient. Of all the procedures available for concentration tests, the Fishberg test procedure had the widest application. The tubular system can concentrate urine of normal patient to a specific gravity of 1.025, and often gets to 1.032, and the respective values for the osmotic concentration are \> 900 mOsm/kg water or 855 mOsm/L plasma. However, the ability to concentrate urine is lost to some extent with age, so that the elderly usually show values in the lower portion of the normal even when the kidney function is apparently normal. **In tubular damage** the concentrating ability is **the first function to be decreased.** Other conditions that may cause impairment of the concentrating ability of the tubular cells include: - Tubular epithelial damage as may be seen after the intake of nephrotoxic drugs - Severe alkalosis - Shock syndrome - Impairment of tubular blood supply In severe tubular function impairment as in the case of acute nephritis, the specific gravity values below 1.020 are observed, and when healing occurs, the concentrating power of the kidneys is the **last function to return** probably due to functional inadequacy of the newly regenerated tubular epithelial cells. **Dilution test** In the ***dilution test***, the ability of the kidneys to excrete dilute urine is tested by administering a massive fluid load and testing the specific gravity and osmolality of the urine specimens for a period of 3 to 4 hours. Clinical Significance In the case of dilution tests as an index of tubular function, the usefulness is limited by several factors: - Other physiological processes involved in the regulation of salt retention limit its usefulness. - Such non-tubular phenomena involved are adrenal insufficiency, hepatic disease, and cardiac failure. - The emotional state of the patient may affect the result. - Clinical contraindications arising from administering large amount of fluid in the case of patients with decreased ability to excrete fluids must be considered. It is therefore, seldom used clinically. Normally the specific gravity of at least one of the urine specimens collected should be less than 1.003 (approx. 50 mOsm/kg H~2~O) and at least half of the water ingested should have been excreted in 3 to 4 hours period. **Specific Gravity (Sp. Gr.)** Specific gravity is the ratio of the weight of a substance to the weight of equal volume of water (at a specific temperature) that is (wt. of a substance / wt. of equal vol. of water). It measures the concentration of chemical particles in urine. It is one measure of the function of the kidney. It is a direct function of the number of particles in the urine, but because each substance contributes differently to the sp. gr., this function is not strictly a function of the number of particles as osmolality is. However, it is performed more frequently than the osmolality because of its simplicity and speed. Clinical Significance The determination of sp. gr., has clinical value **as a screening test.** It measures the concentrating ability of the renal tubules. In the case of renal tubular damage, this function is generally the first to be lost. **Elevation in urine sp. gr., in the absence of dehydration** is commonly observed in patients with uncontrolled diabetes with glycosuria. **Extremely high values (\> 1.050)** are seen in patients who have had urinary tract diagnostic study (that used iodine-containing x-ray contrast media). There is considerable fluctuation of urine sp. gr., from day to day, and also during the course of a day. It is still an important part of the routine urinalysis. Random as well as 24-hour urine specimen can be used in carrying out this test. With timed specimen after water restriction, more exact information is derived. **Osmolality** Measuring the **osmotic concentration of body fluids** such as serum and urine gives information about the physiological processes involved in the transport of solutes and solvent across membranes which separate fluid compartments. The main solute components contributing to serum osmotic concentration are **sodium, chloride, bicarbonate ions,** and to a **lesser degree glucose and urea.** Ionic components make up over 95% total osmotic concentration, while the remainder are contributed by unionized solutes. **Serum sodium represent nearly half of the total osmolality because over 90% of cation fraction is sodium.** Clinical Significance Simultaneously **measuring serum and urine osmolality is a more accurate and clinically more useful way of determining the concentrating ability of the tubules.** In this way, the ratio of the urine osmolality to serum osmolality could be calculated, as well as calculating osmotic clearance. Analogous to other clearance tests, osmotic clearance measures the ratio of the concentration of the **osmotically active particles in urine to that in serum.** This ratio (or clearance) expresses the actual degree to which the kidney has concentrated the glomerular filtrate which in respect to osmolality is very close to serum. **The ratio is greatly affected by the volume of fluid intake, therefore fluid intake restriction is advocated for more meaningful data.** **Acidification of Urine** Strong acids such as sulfuric, hydrochloric and many organic acids are not excreted by the kidneys in their free form. The hydrogen ions derived from these acids react with a buffer base (such as HPO~4~^2-^ or HCO~3~^-^) or NH~3~ before they are excreted. In order to provide electrical balance, excretion of the acid anions is accompanied by the removal of equal number of cations. Weak acids (acetoacetic acid and β-hydroxybutyric acid) present in the blood, may be partially excreted in the form of free acids (because at an acid pH they in part undissociated). H^+^ ions can be excreted into the tubular urine in exchange for Na^+^ ions which returns into the tubular cell. The phosphate ions in plasma and in glomerular filtrate of pH 7.4 are present as HPO~4~^2-^ (hydrogen phosphate) and H~2~PO~4~^-^ (dihydrogen phosphate) in a ratio of approximately 80/20 (4/1). With increase in acidity, the ratio decreases gradually to 1/99 at pH 4.8; and at a urine pH of 4.5 all phosphate is present as H~2~PO~4~^-^ in a ratio of 1/100. Each HPO~4~^2-^ ion (an anion) can accept one H^+^ ion (a cation), and can make one cation (mainly Na^+^) available for reabsorption. The more H^+^ ions are exchanged for Na^+^ ions, the urine becomes more acid, and the body fluids more alkaline. +-----------------------+-----------------------+-----------------------+ | Glomerular filtrate | Tubular cell | Plasma | | | | | | (Tubular lumen) | (Tubular membrane) | (Peritubular | | | | capillaries) | +=======================+=======================+=======================+ | Na^+^ + | 2CO~2~ + 2H~2~O | Na^+^HCO~3~^-^ | | Na^+^HPO~4~^2-^ | | | | | H^+^ + HCO~3~ | Na^+^HCO~3~^-^ | | Na^+^A^-^ | | | | | H^+^ + HCO~3~ | | | H^+^A^-^ | | | | | Glutamine | | | NH~3~ | | | | | NH~3~ + Glutamic acid | | | NH~4~^+^A^-^ | | | +-----------------------+-----------------------+-----------------------+ The place where the Na^+^-H^+^ exchange takes place is in a larger portion of the tubule (collecting ducts). It was formerly thought to take place at the distal portion of the tubule (DCT). The various function of the renal mechanism responds to specific requirements: in case of acidosis, there is an increased excretion of acids and a conservation of base; in case of alkalosis, there is an increased excretion of base and conservation of acids. In renal tubular acidosis (when the kidneys do not remove acids from the blood into the urine, resulting in elevated acid level in the blood), the Na^+^-H^+^ exchange does not take place or is reduced, thus acids formed during metabolic processes are not effectively removed, and can disturb many bodily functions. Accumulation of these acids in the blood results in metabolic acidosis. The various acids produced during metabolic processes are buffered in the extracellular fluid, although at the expense of HCO~3~^-^, therefore, the supply of HCO~3~^-^ would be exhausted if the kidneys did not excrete the acids and restore the HCO~3~ ion. The ability to excrete variable amounts of acid and base is of utmost importance and makes the kidney the final defense mechanism against drastic changes in body pH and cation-anion composition. Excretion of Ammonia The renal tubular cells have the ability to form ammonia from amides (mainly glutamine) and some amino acids. The ammonia (NH~3~) produced diffuses into the tubular urine and combines with H^+^ to form NH~4~^+^. The process is greatly enhanced in acidosis and reduced in alkalosis. The formation of ammonia in the tubular cells permits increased excretion of H^+^ ions and increased conservation of cations mainly Na^+^ ions. Increased Na^+^-H^+^ exchange also increases HCO~3~^-^ absorption. **Test of Urinary Acidification** **Ammonium Chloride loading test** The ammonium ion (NH~4~^+^) is acidic because it can dissociate to ammonia and H^+^. If ammonium chloride is ingested the kidneys should normally secrete the excess of hydrogen ion. Procedure - No food or fluid is taken after midnight. - At 8:00am, ammonium chloride (0.1g/kg body weight) is administered orally. - Hourly urine specimens are collected between 10 a.m. and 4 p.m., and the pH of each specimen measured **immediately** with a pH meter (pH paper is not very accurate). - If pH of any specimen falls to 5.2 or below, the test can be stopped. Interpretation **In normal subjects the pH of the urine falls to 5.2 or below between 2 and 8 hours** after the dose. In renal tubular acidosis (RTA): condition in which the kidneys are unable to remove acid from the blood into the urine resulting in too much acid in the blood, **this degree of acidification fails to occur.** **Interferences to Normal Kidney Function** Normal kidney function depends upon an **adequate blood flow** to the organ. Any interference with renal blood supply will result in: i. ***diminished kidney function,*** ii. ***alteration in the amount and composition of the urine that is produced, and*** iii. ***an accumulation of metabolic products in the blood***. - In the case of cardiac failure, diminished kidney function is not as a result of ***intrinsic lesion*** in the kidney, but its due to an inadequate flow of blood to the kidney. - Also, intravascular changes that interfere with the flow of blood to the kidney may produce increase in blood pressure, a condition known as renal hypertension. - Reduction of kidney function may develop as a consequence of damage to the epithelium of the capillaries which make up the glomerular tuft. This is the case in glomerulonephritis where the endothelium is damaged to the extent that not only water and other small molecules are able to pass through the glomerular membrane but red cells and larger molecules (proteins) which are not normally filtered also pass through and are excreted with the urine, since they are not reabsorbed by the tubules. In nephrotic syndrome, as well as in chronic glomerulonephritis large amounts of proteins are lost in urine. - Note also that damage to the tubules and the glomerular tuft occurs following a massive hemolytic reaction after transfusion with incompatible blood resulting from overloading the system with a toxic substance. Massive amounts of hemoglobin released into the plasma exerts a toxic effect on the tubules. Agglutination of red cells may occur resulting in anoxia (absence or deficiency of oxygen supply/lack of oxygen) and consequent damage to the nephron. All of these could result in complete renal failure. - Administering various poison to the body may selectively interfere with certain functions of the tubules. - Other factors that interfere with the normal function of the kidney include: actual mechanical trauma to the kidney, and development of neoplastic growth. These would either destroy the tissues directly or produce pressure on kidney tissues, thus disrupting normal functions. - Solutes in urine could be present in such concentrations that precipitation of these substances occur within the pelvis of the kidney, or in the ureter or bladder. These precipitates are referred to as ***calculi***. They are similar to natural stone in appearance. They may cause infections, and produce injury by causing obstruction and subsequent back pressure. This would reduce filtration rate and decrease urinary excretion. Sometimes the stones cause severe unimaginable (excruciating) pain in the ureter due to reflex spasms. - Certain inborn error of metabolism involving the renal tubules occasionally occur. An example is cystinuria, a condition where the tubules are unable to reabsorb cystine and several other amino acids resulting in their presence in the urine in increased amount.