Chapter 21 Kidney diseases
COMMON CLINICAL PROBLEMS FROM KIDNEY DISEASE
Pathological basis of renal symptoms and signs
Symptom or sign | Pathological basis |
---|---|
Proteinuria | Increased permeability of the glomerular basement membrane |
Uraemia | Renal failure |
Haematuria | |
Urinary casts | |
Hypertension | |
Oliguria or anuria | Severe renal failure (acute or chronic), obstruction or dehydration |
Polyuria | |
Renal (ureteric) colic | Calculus, blood clot or tumour in ureter |
Oedema | Primary renal abnormality leading to sodium and fluid retention |
Dysuria | Stimulation of pain receptors in urethra due to inflammation |
NORMAL STRUCTURE AND FUNCTION OF THE KIDNEYS
The kidneys contribute to the body’s biochemical homeostasis by:
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).
The glomerular capillary comprises:
The integrity of the filtration barrier is disturbed in glomerular disease.
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 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
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
The distal convoluted tubule continues into the collecting duct. Two main cell types are present:
Countercurrent mechanism
The countercurrent mechanism ensures urine of variable osmolarity forms in response to a variable water intake. The hairpin configuration of the loop of Henle, the complementary vasa recta, coupled with the selective permeabilities to ions and water of the different segments of the loop, the distal tubule and collecting tubules, are all pivotal to the countercurrent mechanism. The active transport of sodium by the thick ascending limb increases the osmolarity of the interstitium. As a result of this, water diffuses from the filtrate in the lumen of the descending limb, which is permeable to water but not to ions. With progress towards the tip of the loop, osmolarity of the filtrate and interstitium increases, particularly in the longer loops derived from the juxtamedullary glomeruli. The principal cells of the collecting tubules display a variable permeability to water under the influence of ADH, achieving urine of variable osmolarity by passing through this hyperosmolar environment on the way to the papillae.
Renal papillae and urinary reflux
RENAL DISEASE
Clinicopathological features
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 |
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
Nephrotic syndrome
The nephrotic syndrome is due to excessive leakiness of the glomerular filter and comprises:
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:
Oedema
Oedema is common in renal disease, especially in patients with the nephrotic syndrome, and also in some severe forms of nephritis. Sodium retention by the damaged kidney is the fundamental event, but the mechanism is not known. Oedema in renal disease relates therefore to sodium retention and fluid retention rather than alterations in plasma proteins and plasma osmotic pressure.
Chronic renal failure
Renal osteodystrophy and secondary hyperparathyroidism
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.
Nephrotoxic antibody
In anti-GBM disease, an IgG antibody forms against the alpha-3 chain in the collagenase-resistant component of collagen IV within the basement membrane, binds to it and activates the complement cascade. Renal biopsy reveals, in glomeruli, a linear pattern of immunofluorescent staining for IgG and granular staining for C3. Polymorphs are attracted and a florid proliferative glomerulonephritis results (Fig. 21.3).
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:
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:
The contribution of the individual mediators varies in each case.
Non-immunological mechanisms
Non-immunological mechanisms include:
CONGENITAL DISEASES
Developmental abnormalities
Conditions affecting the volume of renal tissue
Renal agenesis (absence of the kidney) may be unilateral or bilateral.
Disorders of differentiation
Renal dysplasia may present in childhood as an abdominal mass simulating a tumour. It is characterised by islands of undifferentiated mesenchyme or cartilage within the parenchyma. The pathogenesis is failure of induction of both the ureteric bud and mesenchymal tissues leading to abnormal development of the collecting ducts and subsequent loss of potential nephrons with the formation of cysts. The prognosis is good if the lesion is unilateral with no obstruction.
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