Home PHGY 210 Monday, April 2, 2012

Monday, April 2, 2012 CC-BY-NC
Renal: Filtration, reabsorption, clearance

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1glomerular filtration

  • stuff that DON't get filtered: cells, proteins, protein-bound substances (1/2 of calcium, fatty acid)
  • forces involved:
    • glomerular capillary blod pressure (more filtration)
    • fluid pressure in Bowman's space
    • osmotic force due to protein in plasma
    • (overall there's filtration)
  • filtered load:
    • total amount of any freely filtered subsance
    • equals GFR X plasma concentration of the substance
    • filtered load > amount secreted in the urine : net reabsorption
    • filtered load < amount secreted : net excretion.
      • meaning that besides the substance in circulation, body also secrete it and excrete it.
    • filtered loads are enormous and exceed the amounts of the substances in the body

1.1filtration rate

  • volume of the fluid filtered from the glomeruli into Bowman's space per unit time.
  • normal GFR (glomeruluar filtration rate): 180L/day
    • which means plasma is filtered about 51 times a day.
  • regulated by
    • net filtration pressure (as discussed above)
      • decrease GFR:
        • constriction of afferent arteriole (decrease in glomerulus capillary pressure)
        • dilation of efferent arteriole (same)
      • increase GFR:
        • dilation fo afferent arteriole
        • constriction of efferent arteriole
    • membrane permeability
    • surface area

2Reabsorption and Secretion#

  • pathways
    • paracellular (between cell)
    • transcellular (through cell)
  • some substances :
    • water: 99% reabsorbed
    • sodium: 99.5%
    • glucose: 100%
    • urea : 44%
    • potassium : 86%
    • basically reabsorption of useful plasma components is relatively complete
    • reabsorption of waste product (urea) is relatively incomplete
    • some are highly regulated (water, inorganic ions), and some aren't (gluocse, amino acids)
  • Two types of absorption:
    • diffusion (passive)
      • urea reabsorption in proximal tubule:
        • reabsorption of Na (mediate transport)
        • water follows Na --> then urea concentration becomes higher in tubular fluid
        • urea diffuse into the intersittial fluid and peritubualar capillaries
    • mediated transport
      • facilitated (passive)
        • solvent drag: requires tranport proteins in plama membrane of tubular cells. usually coupled to the reabsorption of sodium. (just like urea)
        • channels (uniport or coupled transport) : can become saturated (transport maximum)
        • people with diabete mellitus has high concentration of glucose in the blood --> receptors are all occupied -->exceed capacity of tubules to reabsorb glucose entirely --> presence of glucose in urine.
        • glucose and amino acid diffuse into the cell from tubular lumen with sodium. Na then get pumped out by Na+/K+ ATPase, which keeps intracellular [Na] low.
      • active : against gradient
  • Secretion
    • substances moving from peritubular capillaires into the tubules.
    • most important ions: hydrogen and potassium
    • usually coupled with reabsorption of sodium (just like absorption, but opposite direction)
  • division
    • proximal tubule:
      • major site for secreting K+
      • reabsorbs most of filtered water and solutes
    • Henle's loop:
      • reabsorb lots of ions and less water.
    • distal convoluted tubule (DCT) and collecting duct (CD) then do the fine-tuning.
      • dtermine the final amounts excreted in the urine by adjusting reabsorption.
      • target of homeostatic controls

3Clearance

  • "volume of plasma from which that substance is completely removed by kidneys per unit time"
  • the basic concept is finding the volume of plasma within which there's this amount of substance that's equals to how much kidney excrete per unit time.
  • Clearance = (mass of excretion per unit time)/(plasma concentration of the substance)
  • insulin clearance:
    • freely filtered by glomerulus and is not absorbed
    • when administered intravenously over a period of time, could serve a indicator of clearance.
    • basically, clearance of insulin equals to plasma being filtered.
      • a good marker for GFR because its clearance = GFR. about 180L/day
  • creatinine clearance
    • even a better indicator than insulin because it's produced by muscle (endogenous)
    • also fitlered freely and not reabsorbed.
    • secreted at tubule(but very small) plus it's not metabolized by the tubule
    • thus a good clinical indicator for GFR.