Glomerular Filtration (B2: W4) Flashcards Preview

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Flashcards in Glomerular Filtration (B2: W4) Deck (27):

What factors are involved in Starling's equation?

K = filtration coefficient

Pc = hydrostatic pressure in the capillary

πt = colloid osmotic pressure in the tubule

Pt = hydrostatic pressure in the tubule

πc = colloid osmotic pressure in the capillary


What is the Starling equation?

Filtration = K [(Pc + πt) - (Pt + πc)]


Which factors involved in the Starling equation favor filtration?

Hydrostatic pressure of the glomerulus and colloid osmotic pressure of the tubule


What is the typical percentage of plasma that gets filtered into the tubule initially?


80% continues on to peritubular capillaries 


What happens to the various pressures along the length of the glomerulus?

  • Capillary hydrostatic pressure is high and does not change
  • Oncotic pressure is the least at the beginning, and then goes up
    • Water has left and proteins are left behind
    • Large effect on net filtration
  • Tubular hydrostatic pressure stays at a constant level


What happens to the hydrostatic and oncotic pressures in a skeletal muscle capillary along the length?

  • Hydrostatic pressure goes from high to low
  • Filtration to reabsorption
  • Net oncotic stays the same


What is the filtration coefficient (K) dependent upon?

Capillary surface area and the permeability of this surface


What is the average glomerular filtration rate (GFR) per day?

GFR = 172 liters/day of plasma


What would happen to filtration in the event of a urinary tract obstruction?

Hydrostatic pressure of the tubule would increase

Filtration would decrease


What would happen to filtration in the event of hypoalbuminemia (lack of albumin in the plasma)?

Osmotic pressure in the capillary would decrease

Filtration would increase


What would happen to filtration in the event of diabetic nephropathy (loss of nephrons)?

  • Increase K
    • Breakdown in the filtration barrier
    • Permeability goes up
  • Increases osmotic pressure in the tubule
    • Proteins enter nephron
  • Filtration increases


What is favored at the glomerular capillaries by net Starling forces?


  • High hydrostatic pressure
  • Oncotic pressure increases along the length of the capillary


What is favored at the peritubular capillaries due to net Starling forces?


  • High oncotic pressure


Where is the major site of negative charge for the basement membrane?

Lamina rara interna

  • Second barrier
  • Tends to reject negative charges 


Between the PCT and the peritubular capillaries, where does bulk flow occur?

Tight junctions in PCT between epithelial cells

  • Water and ions can go between cells


How are particles and water moved from PCT to peritubular capillaries?

  • Bulk flow
    • Water and ions
  • Movement of particles from apical membrane to basal side
    • Na and K, not protein
    • Water follows particles
  • Water goes through via aquaporins 
    • Not regulated by ADH


What happens to reabsorption with a decrease in peritubular capillary hydrostaic pressure?

Increases reabsorption


What happens to reabsorption with a decrease in peritubular capillary colloid osmotic pressure?

Decreases reabsorption


What happens to reabsorption with contraction of the efferent arteriole?

Increases reabsorption

  • Hydrostatic pressure goes down 
  • Oncotic pressure goes up


What are the size and charge contstraints for filtration?

  • The larger the molecule, the harder it is to get through
    • H2O, glucose, and NaCl move through easily 
    • Concentration in tubule is the same as the concentration in capillary
    • Inulin is the largest molecule that can get through
  • Easier for a positively charged molecule to get through
    • Negatively charged molecules are stopped by lamina rara interna


How do we measure GFR?

Clearance of inulin

Clearance: the volume of plasma per minute from which all of a substance is removed

Clearance is a flow; GFR is a flow


What is the equation for clearance?

Ux = concentration in urine

V = volume

Px = concentration in plasma


What are the special properties of inulin that allow us to use it to determine GFR?

  1. Freely filtered 
  2. Not reabsorbed 
  3. Not screted 


What can be used instead of inulin to determine GFR that is made by the body?

Creatinine clearance

  • Behaves similarly to inulin
    • Difference is that there is some secretion
  • Body makes it, so there is no need to put it in the body
  • UCr overestimated because of secretion
  • PCr overestimated because of lab chemistry


How do we calculate filtered load?

Filtered Load = GFR x Px



What is the average amount of sodium that we filter per day?

3.3 lbs NaCl/day

We don't have this much salt in our body...

Sodium repeatedly goes into the tubule and back to the body


How do we figure out how much of a substance is excreted per day? Reabsorbed?

Fractional Excretion