Renal Reabsorption and Secretion

Question 1

Calculation of Reabsorption and Secretion Rates

Answer 1

• Reabsorption/Secretion rate is difference b/t amount filtered across the glomerular capillaries and amount excreted in urine
• Filtered load=GRF X [plasma]
• Excretion Rate= V X [Urine]
• Reabsorption Rate= Filtered load-Excretion rate
• Secretion Rate= Excretion Rate-Filtered load
• If filtered load>excretion rate then net reabsorption occurred
• If filtered load<excretion rate then net secretion occurred

Question 2

Transport maximum (Tm) curve for Glucose

Answer 2

• Reabsorbed substance
• Filtered load: increases in direct proportion to plasma [glucose]
• Reabsorption: Na+/glucose cotransport in proximal tubule
  
  • reabsorbs glucose from tubule into the blood
  • Plasma [glucose]350mg/dL all carriers saturated
    
    • Increasing concentration doesn’t increase reabsorption
    • Tm is the reabsorptive rate where all carriers saturated
• Excretion of Glucose:
  
  • [plasma]350 Saturated (Tm): Increased glucose in urine

Question 3

Splay

Answer 3

• Region on curve between threshold and Tm
• In glucose, b/t 250 and 350mg/dL
• Represents excretion of glucose in urine before saturation of reabsorption (Tm) is fully achieved
• Explained by heterogeneity of nephrons and relatively low affinity of the Na+/glucose carriers

Question 4

Tm Curve for PAH

Answer 4

• Secreted Substance
• Filtered load increases in direct proportion to [PAH]
• Secretion occurs from peritubular capillary blood into urine
  
  • via carriers in proximal tubule
  • At [low] secretion rates increase as [plasma] increases
  • When carriers saturated, more PAH doesn’t up secretion
• Excretion of PAH: sum of filtration plus secretion
  
  • curve steepest at low plasma PAH (Tm excretion curve flattens and parallel to filtration
• RPF is measured by PAH clearance at plasma concentrations less than Tm

Question 5

Nonionic diffusion: weak acids

Answer 5

• HA form and A- form
• HA form is uncharged and lipid soluble
  
  • can “back-diffuse” from urine to blood
• A- form is charged and not lipid soluble
  
  • cannot back diffuse
• Acidic urine: HA predominates & more back diffusion
  
  • Decreased excretion of weak acid
• Alkaline urine: A- predominates & less back diffusion
  
  • Increased excretion of weak acid
  • Ex. salicylic acid excretion increased by alkalinizing urine

Question 6

Nonionic diffusion: weak bases

Answer 6

• BH+ form and B form
• B form is uncharged and lipid soluble
  
  • can back diffuse from urine to blood
• BH+ form is charged and not lipid soluble
  
  • Cannot back diffuse
• Acidic urine: BH+ predominates and less back diffusion
  
  • Increased excretion of weak base
• Alkaline urine: B predominates and more back diffusion
  
  • Decreased eexcretion of weak base