Physiological Effects and Clinical Assessment of Renal Dysfunction

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Functions of the Kidney[edit]

  • Water balance
  • Electrolyte balance
  • Acid-base balance
  • Excretion of wastes
  • Endocrine functions

Loss of Concentrating and Diluting Ability[edit]

  • In early renal disease, urine becomes less concentrated, and urine volume is often increased causing polyuria
  • Ability to form dilute urine is retained, but as the disease advances the osmolality of the urine becomes fixed to that of plasma as diluting/concentrating function is lost
  • This occurs due to loss of functional nephrons, as well as loss of the countercurrent mechanism
  • As nephrons are lost, the remaining nephrons are forced to filter more osmotically active substances, causing an osmotic diuresis, and the osmolality of the urine approaches that of plasma
  • Increased filtration in remaining nephrons occurs, damaging them, and eventually they are lost
  • Eventually oliguria occurs, followed by anuria

Uraemia[edit]

  • Uraemia occurs when waste products of protein metabolism accumulate in the blood
  • Symptoms: lethargy, nausea and vomiting, anorexia, mental deterioration, muscle twitching, convulsions and coma
  • Urea and creatinine levels are used as an index of severity of uraemia, although it is the accumulation of other toxic substances which cause the symptoms
  • Dialysis is used to remove these substances when the kidneys fail to do it

Abnormal Na+ Handling[edit]

  • Many renal patients retain excessive amounts of Na+ and become oedematous
  • 3 causes for Na+ retention in renal disease:
  • Acute glomerulonephritis - amount of Na+ filtered is markedly decreased
  • Nephrotic syndrome - increased aldosterone secretion causes salt retention
  • Plasma protein levels are reduced, therefore fluid moves from plasma into interstitial spaces and plasma volume falls, triggering the secretion of aldosterone
  • Heart failure resulting from hypertension causes Na+ retention and oedema

Other features of chronic renal failure[edit]

  • Hypertension due to fluid overload and production of vasoactive hormones created by the kidney via the RAS (renin-angiotensin system)
  • Hyperkalaemia due to decreased excretion
  • Metabolic acidosis due to accumulation of sulfates, phosphates, uric acid etc. This acidosis disrupts enzymes, and can cause hyperkalaemia.
  • Anaemia - due to failure to produce erythropoietin
  • Secondary hyperparathyroidism - due to 1,25 dihydroxycholecalciferol deficiency
  • Hyperphosphataemia occurs almost always in renal failure due to loss of nephrons to excrete phosphate
  • A common treatment for this is oral calcium, which binds dietary phosphate in the gut to prevent its absorption

Clinical Assessment of Renal Dysfunction[edit]

  • Common preoperative tests of renal function: Plasma urea and creatinine
  • These provide a measure of the kidney's ability to excrete nitrogenous waste products of metabolism
  • All of the functions of the kidney, except for endocrine functions involve changes in GFR
  • Total GFR is a measure of the total amount of filtration in all the nephrons, and gives an indirect index of the function of all nephrons
  • Creatinine clearance is used as an estimate of GFR
  • It is not as accurate as inulin clearance, but doesn't need to be administered like inulin because it is an endogenous substance
  • A 12 or 24 hour urine collection is required, which means it isn't frequently performed
  • Plasma creatinine levels are inversely related to creatinine clearance/GFR, and therefore are used as an index of GFR