Chapter 4 The kidneys

Introduction

• maintenance of extracellular fluid (ECF) volume and composition, including acid-base balance

They also contribute to glucose supply in the fasting state through gluconeogenesis. Each kidney comprises approximately one million functional units, called nephrons.

The kidneys have a rich blood supply and normally receive about 25% of the cardiac output. Most of this is distributed initially to the capillary tufts of the glomeruli, which act as high pressure filters. Blood is separated from the lumen of the nephron by three layers: the capillary endothelial cells, the basement membrane and the epithelial cells of the nephron (Fig. 4.1). The endothelial and epithelial cells are in intimate contact with the basement membrane; the endothelial cells are fenestrated, and contact between the epithelial cells and the membrane is discontinuous so that the membrane is exposed to blood on one side and to the lumen of the nephron on the other.

Figure 4.1 Diagram of a glomerular capillary, showing fenestrations in endothelial cells, basement membrane and epithelial cells (podocytes) with slit pores between interdigitating foot processes.

The glomerular filtrate is an ultrafiltrate of plasma; that is, it has a similar composition to plasma except that it is almost free of large proteins. This is because the endothelium provides a barrier to red and white blood cells, and the basement membrane, although permeable to water and low molecular weight substances, is largely impermeable to macromolecules. This impermeability is related to both molecular size and electrical charge. Proteins with molecular weights lower than that of albumin (68 kDa) are filterable; negatively charged molecules are less easily filtered than those bearing a positive charge. Almost all the protein in the glomerular filtrate is reabsorbed and catabolized by proximal convoluted tubular cells, with the result that normal urinary protein excretion is <150 mg/24 h.

Filtration is a passive process. The total filtration rate of the kidneys is mainly determined by the difference between the blood pressure in the glomerular capillaries and the hydrostatic pressure in the lumen of the nephron, the nature of the glomerular basement membrane and the number of glomeruli. The difference in the osmotic pressures of the plasma and the ultrafiltrate provides a small force that opposes filtration. The amount of filtrate formed decreases along the length of the glomerular capillaries as the difference in hydrostatic pressures falls and that in osmotic pressures rises. The normal glomerular filtration rate (GFR) is approximately 120 mL/min, equivalent to a volume of about 170 L/24 h. However, urine production is only 1–2 L/24 h, depending on fluid intake; the bulk of the filtrate is reabsorbed further along the nephron.

The glomerular filtrate passes into the proximal convoluted tubules, where much of it is reabsorbed. Under normal circumstances, all the glucose, amino acids, potassium and bicarbonate, and about 75% of the sodium, is reabsorbed isotonically here by energy-dependent mechanisms.

Medullary hyperosmolality, which is vital for the further reabsorption of water, is generated by the counter-current system, summarized in Figure 4.2. Chloride ions, accompanied by sodium, are pumped out of the ascending limbs of the loops of Henle into the surrounding interstitial fluid, and thence diffuse into the descending limbs. As the ascending limbs of the loops of Henle are impermeable to water, the net effect is an exchange of sodium and chloride ions between the ascending and descending limbs. This alters the osmolality of both the fluid within the nephrons and the surrounding interstitial fluid. A gradient of osmolality is set up between the isotonic corticomedullary junction and the extremely hypertonic (approximately 1200 mmol/L) deep medulla. Diffusion of urea from the collecting ducts into the interstitium and thence into the loops of Henle also makes an important contribution to medullary hypertonicity. It is noteworthy that urinary concentrating ability is impaired in malnourished children but can be restored by increasing their dietary protein intake or (experimentally) by adding urea to their diets.

Figure 4.2 Movements of major ions, passive movement of water and changes in osmolality in the nephron. In the ascending loop of Henle, chloride ions are actively transported and sodium ions accompany them to maintain electrochemical neutrality.

The tubular fluid becomes increasingly dilute as it passes up the ascending limbs of the loops of Henle, as a result of the continued removal of chloride and sodium ions. Fluid entering the distal convoluted tubules is hypotonic (approximately 150 mmol/L) with respect to the glomerular filtrate. Further dilution takes place in the early part of the distal convoluted tubules.

Approximately 90% of the filtered sodium and 80% of the filtered water has been reabsorbed from the glomerular filtrate by the time it reaches the beginning of the distal convoluted tubules. In the distal tubules, further sodium reabsorption takes place, in part controlled by aldosterone; this generates an electrochemical gradient that is balanced by the secretion of potassium and hydrogen ions. Ammonia is also secreted in the distal tubule and buffers hydrogen ions, being excreted as ammonium ions (see p. 43).

Whereas the proximal tubules are responsible for bulk reabsorption of the glomerular filtrate, the distal tubules exert fine control over the composition of the tubular fluid, depending on the requirements of the body.

Tubular fluid then passes into the collecting ducts, which extend through the hypertonic renal medulla and discharge urine into the renal pelvices. The cells lining the collecting ducts are normally impermeable to water. Vasopressin (antidiuretic hormone, ADH) renders them permeable by stimulating the incorporation of aquaporins (water channels) into the cell membranes and allows water to be reabsorbed passively in response to the osmotic gradient between the duct lumen and the interstitial fluid. Thus, in the absence of vasopressin, a dilute urine is produced; in its presence, the urine is concentrated. Some reabsorption of sodium also occurs in the collecting ducts under the stimulus of aldosterone. The collecting ducts drain into the renal pelvices, from which urine passes through the ureters to the bladder.

As the normal GFR is approximately 120 mL/min, a volume of fluid equivalent to the entire ECF is filtered every two hours. Disease processes affecting the kidney therefore have a considerable potential for affecting water, salt and hydrogen ion homoeostasis and the excretion of waste products.

The kidneys are also important endocrine organs, producing renin, erythropoietin and calcitriol. The secretion of these hormones may be altered in renal disease. In addition, several other hormones are either inactivated or excreted by the kidneys and hence their concentrations in the blood can also be affected by renal disease.

The Biochemical Investigation of Renal Function

Diseases affecting the kidneys can selectively damage glomerular or tubular function, but isolated disorders of tubular function are relatively uncommon. In acute and chronic renal failure, there is effectively a loss of function of whole nephrons and, as the process of filtration is essential to the formation of urine, tests of glomerular function are almost invariably required in the investigation and management of any patient with renal disease. The principal function of the glomeruli is to filter water and low molecular weight components of the blood while retaining cells and high molecular weight components. The most frequently used tests are those that assess either the GFR or the integrity of the glomerular filtration barrier.

It should be noted that the GFR declines with age (to a greater extent in males than in females) and this must be taken into account when interpreting results.

Measurement of glomerular filtration rate

Clearance

An estimate of the GFR can be made by measuring the urinary excretion of a substance that is completely filtered from the blood by the glomeruli and is not secreted, reabsorbed or metabolized by the renal tubules. Experimentally, inulin (a plant polysaccharide) has been found to meet these requirements. The volume of blood from which inulin is cleared or completely removed in 1 min is known as the inulin clearance, and is equal to the GFR.

Measurement of inulin clearance requires the infusion of inulin into the blood and is not suitable for routine clinical use. The most widely used biochemical clearance test is based on measurements of creatinine in plasma and urine. This endogenous substance is derived mainly from the turnover of creatine in muscle and daily production is relatively constant, being a function of total muscle mass. A small amount of creatinine is derived from meat in the diet. Creatinine clearance is calculated using the formula:

(4.1)

U = urinary creatinine concentration (µmol/L)

= urine flow rate (mL/min or (L/24 h)/1.44)

P = plasma creatinine concentration (µmol/L)

Creatinine clearance in adults is normally of the order of 120 mL/min, corrected to a standard body surface area of 1.73 m². It should be noted that the clearance formula is only valid for a steady state, that is, when renal function is not changing rapidly.

The accurate measurement of creatinine clearance is difficult, especially in outpatients, as it is necessary to obtain a complete and accurately timed sample of urine. The usual collection time is 24 h, but patients may forget the time or forget to include some urine in the collection. Incontinent patients may find it impossible to make a urine collection. Patients have been known to add water or some other person’s urine to their own collection, hoping to gain the doctor’s approval for having been so prolific.

It may be more convenient and reliable to base the collection period on a patient’s normal habits (e.g. overnight). The time at which the bladder is emptied before retiring to bed is noted; any urine passed during the night is collected, as is the urine voided when the patient rises. The time is noted and a blood sample is taken that morning for the measurement of plasma creatinine. As long as the time over which the urine collection is made is known, and the collection is complete, any suitable time period can be used.

Creatinine is actively secreted by the renal tubules and, as a result, the creatinine clearance is higher than the true GFR. The difference is of little significance when the GFR is normal, but when the GFR is low (<10 mL/min), tubular secretion makes a major contribution to creatinine excretion and creatinine clearance significantly overestimates the GFR. The effect of creatinine breakdown in the gut also becomes significant when the GFR is very low. Certain drugs, including spironolactone, cimetidine, fenofibrate, trimethoprim and amiloride, decrease creatinine secretion and thus can reduce creatinine clearance. Lastly, in the calculation of creatinine clearance, two measurements of creatinine concentration and one of urine volume are required. Each of these has an inherent imprecision that can affect the accuracy of the overall result. Even in well-motivated subjects, studied under ideal conditions, the coefficient of variation of measurements of creatinine clearance can be as high as 10%, and it can be two or three times greater than this in ordinary patients.

Thus, although hitherto widely used, measurements of creatinine clearance are potentially unreliable and no longer recommended in routine practice. Alternative methods should be used if a reliable calculation of GFR is required, for example in the assessment of potential kidney donors, investigation of patients with minor abnormalities of renal function and calculation of the initial doses of potentially toxic drugs that are eliminated from the body by renal excretion.

There are two main alternative approaches to determining the GFR in clinical practice. These are to use exogenous markers of clearance or to derive an estimated GFR (eGFR) from the plasma creatinine concentration (see p. 67). GFR can be measured by measuring the disappearance from the blood of a test substance that is completely filtered by the glomeruli and neither secreted nor reabsorbed by the tubules, following a single injection. This approach has the advantage that a urine collection is not required. Suitable substances for this purpose include ⁵¹Cr-labelled EDTA (ethylenediaminetetra-acetic acid), ¹²⁵I-iothalamate (for which the decline in plasma radioactivity is monitored) and iohexol, a non-radioactive X-ray contrast medium that is simple to measure using high performance liquid chromatography. Typically, blood samples are taken at 2, 3 and 4 h after injection, although samples taken over a longer period may be required in renal impairment.

Plasma creatinine

Plasma creatinine concentration is the most reliable simple biochemical test of glomerular function. Ingestion of a meat-rich meal can increase plasma creatinine concentration by as much as 20 µmol/L for up to 10 h afterwards, so ideally blood samples should be collected after an overnight fast. Strenuous exercise also causes a transient, slight increase in plasma creatinine concentrations. Plasma creatinine concentration is related to muscle bulk and therefore a value of 120 µmol/L could be normal for an athletic young man but would suggest renal impairment in a thin, 70-year-old woman. Although muscle bulk tends to decline with age, so too does the GFR, and hence plasma creatinine concentrations remain fairly constant.

Some commonly used laboratory methods for the measurement of creatinine can suffer from interference, for example from bilirubin and ketones. The laboratory should be able to advise on whether this may be a problem in individual cases.

The reference range for plasma creatinine in the adult population is 60–120 µmol/L, but the day-to-day variation in an individual is much less than this range. Equation 4.1 indicates that plasma creatinine concentration is inversely related to the GFR. GFR can decrease by 50% before plasma creatinine concentration rises beyond the normal range; plasma creatinine concentration doubles for each further 50% fall in GFR. Consequently, a normal plasma creatinine does not necessarily imply normal renal function, although a raised creatinine does usually indicate impaired renal function (Fig. 4.3). Furthermore, a change in creatinine concentration, provided that it is outside the limits of normal biological and analytical variation, does suggest a change in GFR, even if both values are within the population reference range (see Case history 1.2).

Figure 4.3 Relationship between creatinine clearance and plasma creatinine concentration.

Changes in plasma creatinine concentration can occur, independently of renal function, owing to changes in muscle mass. Thus a decrease can occur as a result of starvation and in wasting diseases, immediately after surgery and in patients treated with corticosteroids; an increase can occur during refeeding. However, changes in creatinine concentration for these reasons rarely lead to diagnostic confusion.

In pregnancy, the GFR increases. This usually more than balances the effect of increased creatinine synthesis during pregnancy, and results in a decrease in plasma creatinine concentration.

Estimated GFR

An alternative to a formal measurement of creatinine clearance is to calculate an estimate of the clearance from the serum creatinine concentration. Various formulae have been derived for this purpose, including factors such as age, body weight, sex (creatinine production tends to be lower in women than men with the same body weight, because of their relatively smaller muscle mass) and racial origin. Several formulae have been derived from the Modification of Diet in Renal Disease (MDRD) study. The ‘four-variable’ formula is:

where [sCr] = serum creatinine concentration (µmol/L) and age is measured in years. This formula is for white males: the result should be multiplied by 0.742 for females and by 1.21 for African Caribbean people. (A calculator is available at: www.renal.org.)

A six-variable formula includes serum urea and albumin concentrations in addition. Because different laboratories may measure creatinine using different techniques, correction factors can be used to relate the calculated eGFR to the reference method for creatinine measurement, thus supposedly ensuring that all eGFR values are comparable. However, although the recommended formula for routine reporting in UK laboratories is the four-variable MDRD formula, it should be appreciated that it was derived from studies on patients with chronic kidney disease and may not be applicable to subjects with normal or near-normal renal function. Indeed, it is recommended that values of eGFR greater than 60 mL/min should be reported as ‘>60 mL/min’ and regarded as normal in the absence of clinical or laboratory evidence of renal disease (e.g. abnormalities on imaging, proteinuria or haematuria). Neither is it applicable in acute kidney disease, pregnancy, in conditions in which there is severe muscle wasting, oedematous conditions, amputees or in children. A major use of the MDRD eGFR is as a tool for screening for chronic kidney disease (CKD). UK guidelines recommend annual screening using eGFR and urinary protein in patients at risk of developing CKD, as summarized in Fig. 4.4. A more recent formula, the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration), is based on pooled data from several studies and correlates better with measured GFR than the original MDRD formula, especially at values above 60 mL/min.

Figure 4.4 Early identification of chronic kidney disease.

Although now less widely used and not validated for use in screening for CKD, the Cockcroft–Gault formula, which is applicable over a wider range of GFRs, also provides an estimate of the creatinine clearance and hence GFR. Many drug dosing regimens, such as some of those used in cancer chemotherapy, are based on this, despite lack of adjustment for local creatinine calibration. The formula takes into account body weight in addition to sex, age and serum creatinine concentration:

Plasma urea

Urea is synthesized in the liver, primarily as a by-product of the deamination of amino acids. Its elimination in the urine represents the major route for nitrogen excretion. It is filtered from the blood by the glomeruli but significant tubular reabsorption occurs through passive diffusion.

Plasma urea concentration is a less reliable indicator of renal glomerular function than creatinine. Urea production is increased by a high protein intake, in catabolic states, and by the absorption of amino acids and peptides after gastrointestinal haemorrhage. Conversely, production is decreased in patients with a low protein intake and sometimes in patients with liver disease. Tubular reabsorption increases at low rates of urine flow (e.g. in fluid depletion), and this can cause increased plasma urea concentration even when renal function is normal.

Factors affecting the ratio of plasma urea to creatinine are summarized in Figure 4.5. Changes in plasma urea concentration are a feature of renal impairment, but it is important to consider possible extra-renal influences on urea concentrations before ascribing any changes to an alteration in renal function.