The Kidney in Glucose, Nitrogenous Waste Products and Drugs

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Glucose Reabsorption

  • Glucose - removed from the urine by secondary active transport
  • Filtered at a rate of 100mg/min, but almost all glucose is reabsorbed, therefore no more than a few milligrams appear in the urine per 24h
  • Amount absorbed is proportional to the amount filtered up unto the transport maximum TMG, above which glucose is lose in urine
  • Renal threshold is the plasma level of glucose at which significant amounts begin to appear in the urine
  • Some glucose begins to appear in urine at levels much below the transport maximum due to splay, which occurs due to variability in tubules
  • Glucose reabsorption occurs via a sodium-dependent glucose transporter SGLT 2 in the apical membrane, along with Na+ moving down its concentration gradient
  • Glucose then exits the cell via facilitated diffusion via the GLUT 2 glucose transporter

Urea Excretion

  • Urea is the major nitrogenous waste product requiring excretion in the body. It is produced in liver and excreted by the kidneys.
  • Deamination of glutamate in the liver produces ammonium, which is toxic in the CNS. Ammonium enters the urea cycle and is converted to the much less toxic urea.
  • A high protein diet causes excessive amounts of amino acids, meaning increased urea production and increased requirement for renal excretion.
  • Can be calculated by
  •  % of cardiac output going to the kidneys (25%)
  •  % of RBF going to glomeruli (93% but assume all)
  • Filtration fraction (0.17 - 0.20) - given by GFR divided by RBF
  • Percentage of filtered load of urea that is excreted (20 to 50% depending on hydration state)

Renal Drug Clearance

  • The kidneys clear drug from plasma by filtration and direct transport into the tubules
  • Renal blood flow and creatinine clearance are inversely correlated with age
  • Creatinine clearance can be predicted using the Cockroft Galt formula:
Creatinine clearance (ml/min) = (140 - age x wt)/(72 x serum creat) (and multiply by 85% for women)
  • Elderly patients have decreased creatinine clearance, even in the presence of normal serum creatinine