The Kidney in Glucose, Nitrogenous Waste Products and Drugs

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 T_MG, 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