diseases

Chapter 21 Kidney diseases





NORMAL STRUCTURE AND FUNCTION OF THE KIDNEYS


The kidneys contribute to the body’s biochemical homeostasis by:





The kidneys also produce:






The kidneys have a large functional reserve; the loss of one kidney produces no ill-effects. However, in renal disease waste products can accumulate, causing a condition known as uraemia. If the glomerular filters become excessively leaky, large protein molecules are lost in the urine—proteinuria. If the glomeruli are severely damaged, erythrocytes pass through causing haematuria.


The basic unit of the kidney is the nephron; each nephron comprises a glomerulus connected to a tubule. Each kidney contains approximately 1 million nephrons. These form in the embryonic metanephros, after the physiological involution of the pronephros and mesonephros. The ureter, calyceal system and collecting ducts form from the ureteric bud arising from the original duct of the pronephros—the Wolffian duct.



Glomerular structure and function


The formation of urine begins in the glomeruli, where the filtration of approximately 800 litres of plasma each day results in 140–180 litres of filtrate, most of which is reabsorbed by the tubules. Each glomerulus comprises a tuft of capillaries projecting into Bowman’s space (Fig. 21.1A).



Blood enters and leaves the glomerular capillaries by arterioles. In contrast to all other systemic capillaries in which there is a fall in pressure towards the venous end, the hydrostatic pressure within the glomerular capillary remains high throughout its length, thus enabling efficient filtration.


The glomerular capillary comprises:





All components of the capillary wall contribute to the filtration barrier, which is entirely extracellular and has two complementary aspects:




The integrity of the filtration barrier is disturbed in glomerular disease.


The attenuated endothelial cytoplasm lining the luminal aspect of the glomerular capillary has numerous holes or fenestrae, 70–100 nm diameter. Functionally the endothelial cells:






The basement membrane is a mesh of filamentous matrix proteins, including collagen IV, laminin and fibronectin, many of which are anionic, through which the ultrafiltate must pass.


The external aspect of the basement membrane bears epithelial cells with complex interdigitating cellular processes, termed foot processes, enveloping the capillary loops. Modified adherens-type junctions (filtration slit diaphragms) occur where the foot processes meet and are essential to the function of the epithelial cell (Fig. 21.1C). The integrity of the slit diaphragm is maintained by the complex inter-relationship of numerous proteins including nephrin, podocin and CD2-associated protein (CD2AP). Other proteins, such as integrins, span the membrane and anchor the actin cytoskeleton to the collagen IV in the lamina rara externa of the basement membrane. Therefore, changes in these proteins modify the configuration of the foot process, and defects in the genes encoding these proteins result in simplification of the foot processes.


The glomerular capillary tufts are supported centrally by mesangial cells, which proliferate and become more prominent in some diseases. Surrounded by a loose network of fibrillary material — the mesangial matrix — the mesangial cells comprise two types. One type of mesangial cell contains actin filaments and is contractile. They attach to the capillary basement membrane at the point where it is reflected over the mesangial matrix, thus anchoring the capillary to the central structure. Contraction of these cells therefore pulls on the glomerular basement membrane and will alter the shape and calibre of the capillary. Damage to this area reduces the strength of the capillary with the formation of a micro-aneurysm. The other type of mesangial cell resembles a monocyte and is analogous to a tissue macrophage. Both types of cell are involved in the mesangial reaction in glomerular disease by synthesising new matrix material and secreting cytokines responsible for cell proliferation and attraction of inflammatory cells.


The basement membrane is reflected over the mesangial area to extend onto the adjacent capillary, which means endothelium is attached to the mesangial matrix in the central core area (Fig. 21.1B). This allows access of immune complexes to the mesangium and the ability of mesangial cells to probe the capillary lumen.



Glomerular filtration rate


Blood flow through the kidneys produces 130–180 l/day of ultrafiltrate which is termed the glomerular filtration rate (GFR). The GFR reflects the permeability of the capillary wall, together with the hydrostatic and osmotic gradients between the capillary lumen and the Bowman’s capsular fluid. The hydrostatic pressure within the glomerular capillary is determined by the calibres of the feeder afferent and draining efferent arterioles.


The GFR is modified by three important mechanisms, all of which are closely inter-related and involve the juxtaglomerular apparatus (JGA):






Autoregulation and tubuloglomerular feedback


The JGA, situated at the hilum of the glomerulus, comprises the afferent and efferent arterioles and the modified tubular cells of the thick loop of Henle, the macula densa (Fig. 21.2), and enables autoregulation and tubuloglomerular feedback. The specialised cells of the macula densa monitor the level of chloride in the tubular luminal fluid, reflecting the amount of chloride reabsorbed by the tubule. A reduced GFR leads to a fall in the luminal chloride level. This results in dilatation of the afferent arteriole, together with constriction of the efferent arteriole, resulting from the release of renin. These two changes increase the hydrostatic pressure within the glomerular capillary and restore the GFR. Autoregulation and tubuloglomerular feedback are important for normal renal function, and are disturbed in patients with systemic hypertension due to renal artery stenosis.





Tubular structure and function


Epithelial cells modify the filtrate by transferring electrolytes and solutes aided by a series of carrier proteins or transporters within the apical (luminal) cell membrane. Transfer from the cytoplasm to the interstitial and peritubular fluid is performed by an energy-dependent ATPase pump situated on the basolateral membrane of the cell. The epithelial cells are separated from each other by tight junctions that contain claudins, membrane proteins that prevent the unregulated passage of electrolytes, water and solutes through the epithelial layer between the cells.


In the proximal tubule approximately 50–55% of the sodium in the filtrate is reabsorbed through selective sodium transporters, together with specific transmembrane co-transporters linked separately to glucose, phosphate or amino acids. In this way nearly all of the glucose, phosphate and amino acids are reabsorbed by the proximal tubule, thus altering the osmolality of the tubular fluid and causing water to flow into the cytoplasm through specialised water channels termed aquaporins. Some of the sodium transporters are linked with hydrogen exchange, whereby sodium is reabsorbed and hydrogen is excreted. Consequently, c. 80% of all the bicarbonate filtered is reabsorbed by the proximal tubules.


The loop of Henle, situated in the medulla and doubling back on itself, is the next part of the nephron through which the now reduced volume of the filtrate must pass. The two limbs have quite different physiological properties. The descending loop is permeable to water but not to ions, whereas the ascending limb is permeable to ions but, lacking aquaporins, is impermeable to water. Thus, the interstitium of the medulla becomes hypertonic. The filtrate in the loop lumen equilibrates with this, because of the permeability to water in the descending limb.


The distal tubule is continuous with the ascending limb of the loop of Henle. The epithelial cells of this segment lack aquaporins, making this segment impermeable to water. Sodium and chloride are reabsorbed by a co-transporter, the activity of which is governed by the concentration of chloride in the luminal fluid. Transport of sodium and chloride in the loop of Henle and distal convoluted tubule is flow-dependent, an important concept in the context of understanding the action of loop diuretics which tend to increase the rate of flow. Calcium transport, under the influence of parathyroid hormone and 1,25-dihydroxycholecalciferol (vitamin D3), occurs in the distal convoluted tubule and adjacent segments.


The distal convoluted tubule continues into the collecting duct. Two main cell types are present:




Aldosterone increases the number of open sodium channels, thus increasing the reabsorption of sodium in the event of volume depletion. The principal cells of the collecting ducts are relatively impermeable to water due to the paucity of aquaporins on the apical membrane. However, under the influence of antidiuretic hormone (ADH) produced by the pituitary, a complex sequence of changes occurs within the cell. This culminates in the fusion of intracytoplasmic vesicles containing preformed aquaporins with the apical membrane so that water can be cleared into the circulation.


The intercalated cells are concerned with hydrogen ion excretion. The excreted hydrogen combines with ammonia in the lumen to form ammonium. Ammonia, formed in the proximal tubule by the metabolism of glutamine and by diffusion from the interstitial fluid, is freely diffusible in contrast to ammonium which is lipid insoluble and cannot pass back into the tubular cytoplasm.


The vasa recta is the delicate meshwork of capillaries that invests the tubules and is derived from the efferent glomerular arteriole. The configuration of the vascular network complements that of the tubule and plays an integral role in the functioning of the countercurrent mechanism.






RENAL DISEASE



Clinicopathological features


Diseases of the kidney can present with a variety of features, alone or in combination. As the kidneys are so often affected by a primary disease elsewhere in the body, a simple urine examination (e.g. colour, glucose, protein, haemoglobin) is routine practice in patients being investigated for a variety of disorders.



Investigation


The investigation of patients with renal disease is multidisciplinary (Table 21.1). Urine and blood analyses are essential; imaging, biopsies and cystoscopy are optional depending on the nature of the clinical problem. Tests with the greatest general clinical utility are urine testing for glucose (to exclude uncontrolled diabetes mellitus), protein (to determine the permeability characteristics of the glomerular basement membrane), and determination of blood concentrations of urea and/or creatinine, the latter being the more reliable indicator of renal function. The GFR is an important expression of renal function and is a useful parameter for monitoring the severity and progress of renal disease.


Table 21.1 Investigation of patients with urinary tract disease





















Investigation Diagnostic utility








Determination of urine production rate and concentrating power of the kidneys; investigation of urinary tract infections; urinary protein indicates integrity of glomerular filter; exclusion of diabetes mellitus; investigation of glomerular or tubular lesions




Determination of integrity of renal function; glomerular filtration rate can be calculated from urinary and plasma creatinine concentration and urine flow rate





Determination of kidney size and symmetry; investigation of suspected tumours, cysts, etc.; detection of calculi; position and integrity of ureters
Cystoscopy Investigation of haematuria and other symptoms; biopsy of bladder lesions




Diagnosis of glomerular, tubular and interstitial renal diseases

Renal biopsy is performed only when clinically justified, because of the risk of haemorrhage. The biopsy is examined by light microscopy with additional information revealed by immunofluorescence and electron microscopy in some cases.


Accurate information about the incidence of diseases of the urinary tract is available from transplant and dialysis registries. However, two important factors conspire to make the true incidence of renal disease almost impossible to ascertain. First, not all countries have registries for the accurate recording of cases. Second, transplantation and dialysis registries record severe and end-stage disease only, making no allowance for mild and subliminal disease. Clinical experience suggests that the prevalence of post-infectious glomerulonephritis is much higher in Africa and India than in Europe and North America.



Pathophysiological basis of renal disease


Two main clinical syndromes occur in renal disease: nephritis (also referred to as the nephritic syndrome) and the nephrotic syndrome. As part of these, patients present with either acute or chronic renal failure.




Nephrotic syndrome


The nephrotic syndrome is due to excessive leakiness of the glomerular filter and comprises:







The unifying abnormality in patients with the nephrotic syndrome is damage to the epithelial cells resulting in ‘effacement’, ‘fusion’ or ‘simplification’ of the foot processes. It results from molecular alterations at the base of the cells and the filtration slit diaphragms. Nephrin and podocin, amongst other proteins, span the cell membrane and effectively hold the cells together in the region of the filtration slit diaphragm. Similarly, integrins anchor the cell surface to the underlying lamina rara externa of the basement membrane, and these proteins in turn connect to the actin-based cytoskeleton of the cell. Thus, when these proteins are altered, through either mutation or damage, the cell cytoplasm retracts, resulting in simplification or effacement, and leading in some cases to focal epithelial separation from the basement membrane.


Diseases causing the nephrotic syndrome fall into four broad groups:







Proteinuria and oedema



Proteinuria


Three types of proteinuria occur in patients with renal disease emanating from:





Proteinuria arising from the glomeruli implies severe damage leading to loss of the slit diaphragms and effectively reducing the filtration surface. This results in a major change in the character of the resistance to macromolecules, with ensuing substantial leakage of proteins. A degreeof vasoconstriction of the efferent arteriole occurs in response to proteinuria and this serves to increase the intraglomerular hydrostatic pressure which exacerbates the passage of protein through the wall. Therefore, reducing the hydrostatic pressure in the capillary will have a beneficial effect and rationalises the use of antihypertensive therapy in chronic renal disease. The terms selective and non-selective proteinuria refer to the degree and composition of the proteinuria which corresponds to the degree of damage to the filter. Selective proteinuria is seen mostly in minimal change disease.


The tubular epithelial cells reabsorb most of the smaller low molecular weight proteins normally escaping from the glomeruli. Tubulo-interstitial diseases therefore will impede this reabsorption and result in loss of small proteins. In patients with multiple myeloma and other plasma cell dyscrasias, excessive quantities of immunoglobulin light chains are produced and appear in the urine.


Detection of microalbuminuria is particularly important in the management of diabetes and indicates very early increases in permeability. The urinary loss of albumin in these cases amounts to 30–300 mg/day. Routine ‘dipstick’ testing of urine will not detect microalbuminaemia and is therefore unsuitable for managing diabetics.






Chronic renal failure


Chronic renal failure (CRF) leads to clinical features often referred to as uraemia, which reflect a severe reduction in functioning renal mass. The development of the clinical changes in uraemia results from:





Alterations in electrolyte excretion are minimal initially because of adaptive mechanisms involving reducing tubular reabsorption and do not become evident until the GFR reduces by c. 80%. Conversely, organic solutes are excreted primarily by glomerular filtration and will accumulate when the GFR falls. Although individual proteins and metabolites cannot be linked to specific uraemic symptoms, they are known collectively as uraemic toxins.


Hormone production by the severely damaged kidney is impaired. The consequences include renal osteodystrophy and secondary hyperparathyroidism, hypertension and anaemia.



Renal osteodystrophy and secondary hyperparathyroidism


Phosphate retention occurs with mild reductions of GFR and is important in CRF. Whilst calcium and phosphate are inversely related, the role of phosphate retention in CRF is pivotal. By attaching to specific receptors, calcitriol (vitamin D3), produced predominantly by the proximal tubular epithelial cells, appears to be the main regulator of phosphate and exerts a negative feedback on the parathyroids, reducing both production and release of parathyroid hormone (PTH). In CRF, a rise in phosphate inhibiting the secretion of calcitriol may in turn reduce the inhibitory effect on PTH release. The reduced level of calcitriol continues as renal damage proceeds and GFR falls. Eventually the inhibitory effect by PTH on phosphate reabsorption by the proximal tubule will be saturated and hyperphosphataemia and hyperparathyroidism will persist, with two important clinical consequences:




The effects of hyperparathyroidism in patients with CRF can be minimised by a low phosphate diet or the administration of a phosphate-binding agent. Calcitriol is also used but is associated with hypercalcaemia due to the effect on bone. Calcimimetics, which bind to the receptors on the surface of the parathyroid cell, reduce the synthesis and release of PTH. However, oversuppression of PTH may result in adynamic bone disease.





MECHANISMS OF GLOMERULAR DAMAGE


Glomeruli can be damaged by immunological or non-immunological mechanisms.



Immunological mechanisms



Immune glomerular injury


Immunological damage causes most human glomerular disease. There are two mechanisms:




The resulting disease is called glomerulonephritis (in some cases, glomerulopathy). Genetic factors influence susceptibility and prognosis.




Immune complex deposition/activation


An immune complex develops when an antibody binds to its soluble specific antigen. The antigen may be extrinsic, e.g. derived from an infective agent, or endogenous, e.g. DNA in lupus; the latter is said to be autoimmune. Some immune complexes form large lattice structures within the circulation and are eliminated by the reticuloendothelial system; others are smaller and initiate glomerular damage by either deposition or in-situ formation (Fig. 21.3). The complexes seen in the glomeruli are termed ‘deposits’. The glomeruli are vulnerable because the kidneys filter large volumes of blood. Immune complex damage occurs by:




The interaction of antigen and antibody within the deposit activates the complement cascade with the production of the C5b-9 membrane attack complex. In addition, C3a and C5a are chemotactic and recruit polymorphonuclear leukocytes, macrophages and monocytes.


The site of the interaction determines the type of glomerular lesion and the clinical features. Thus, deposits within the mesangium or subendothelial lamina rara interna have access to the circulating blood, and complement activation elicits a proliferative reaction and an active nephritis with haematuria. Both membrano-proliferative glomerulonephritis and IgA disease have this pattern.


In contrast, deposits within the subepithelial lamina rara externa will also activate complement, but the reactants are sequestered from the circulation by the basement membrane. There is therefore no evidence of inflammatory reaction; an example of this pattern is seen in membranous glomerulonephritis (glomerulopathy).




Mediators of glomerular damage


Glomerular cells are affected by a wide variety of substances:


Complement activation (Ch. 9) has a major role in glomerulonephritis of both anti-GBM and immune complex types. All complement pathways activate C3 and yield C5b-9, the membrane attack complex.





The contribution of the individual mediators varies in each case.




CONGENITAL DISEASES


Approximately 10% of individuals have a congenital abnormality of the urinary tract, some of which are hereditary:







Developmental abnormalities




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Jun 16, 2017 | Posted by in GENERAL SURGERY | Comments Off on diseases

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