Topic 41 - Glomerular filtration rate (GFR), extraction, clearance

Question 1

Words to include in glomerular filtration rate (GFR)

Answer 1

• Amount of filtrate produced per unit time
  
  • By all nephrons
  • Two kidneys
• Filter load
  
  • Mg/min
  • Amount ultrafiltrated per unit time
• Filtered only
  
  • Secreted (ø)
  • Reabsorbed (ø)
• Filter load = rate of filtration
  
  • Inulin (polyfructose)
  • Endogenous creatinine
• Clerance value
  
  • 120 ml/min/100 kg bwt (of inulin & endogenous creatinine)
• Autoregulation of GFR
  
  • Constant
  • Periopheral arterial midpressure changes
  • Myogenic mechanism
  • Tubulo-glomerular feedback
    
    • Juxtaglomerular apparatus
      
      • Macula dens
        
        • Senses volume changes of the distal tubule

Question 2

Words to include in extraction

Answer 2

• Eliminate substance from organism
  
  • Remove substance from plasma
• A. renalis
  
  • Arterial concentration (P_art)
• V. renalis
  
  • Venous concentration (P_vein)
• Whole kidney
• One glomerulus
• Maximal
  
  • Substance enterin arterial side, does not appear on venous side
  • E=1
• Minimal
  
  • Entire amount of substance appears on venous side
  • Nothing in urine
  • E=0
• E = (P_art - P_vein) / P_art

Question 3

Words to include in clerance

Answer 3

• Volume from which all test substances are removed/min
  
  • Cleared
  • ml/min
• Clerance value
  
  • All filtered substances
• Exclusively filtered
  
  • Absorbed, secreted (ø)
  • Inulin
• Clerance = rate fo Glomerular Filtration Rate (GFR)
• Renal plasma flow (RPF)
• (U x V)P ml/min
  
  • U: substance concentration in urine
  • P: substance concentration in plasma
  • V: produced urine per minute
• Clerance of para-amino-hyppuric-acid (PAH)
  
  • Constant low plasma content
• Clerance of inulin
  
  • Only filtered (reabsorbed, secreted ø)
• Clerance of urea
  
  • ​Freeliy filtered
  • Interstitium
    
    • A portion of the filtered amount
  • ↓ than inulin
• Clerance of glucose
  
  • ​Freely filtered
  • Proximal tubular activity
    
    • Descending loop of Henle (don’t get in)
  • Proximal tubule (entirely reabsorbed)
  • Clerance = 0
  • Diabetes
    
    • Glucose in urine
      
      • Tubular cells not able to reabsorb glucose

Question 4

Glomerular filtration rate (GFR)

Answer 4

• GFR: The amount of filtrate produced per unit time by all of the nephrons of the two kidneys
• GFR is a test used to check how well the kidneys are working
• GFR estimates how much blood passes through the glomeruli each minute
• Filter load (FL, mg/min) of a give substance is the amount of that is ultrafiltrated per unit time
• To measure GFR, we have to use a material which is only filtered, not secreted or reabsorbed in the tubules
• The filtered load and the rate of filtration has to be equal to the amount of this matter appearing in the urine within a unit of time.
  
  • Such a material is either:
    
    • Inulin (polyfructose)
    • Endogenous creatinine
• Clerance value of inulin and creatinine: 120 ml/min/100 kg bwt

Autoregulation of GFR

• Regulation of renal blood flow is important to maintain a stable GFR despite changes in systemic blood pressure
• Tubuloglimerular feedback: the kidney changes its own blood flow in response to changes in sodium concentration. The macula densa of the juxtaglomerular apparatus senses the volume changes of the distal tubule and it secrets signals to adjust the GFR

Question 5

Extraction

Answer 5

• Extraction: The ability of the kidney to elilimate a substance (remove substances from the plasma)
• Extraction can refer to the whole kidney, or to one glomerulus
• In a. renalis, a certain amount of substance is a certain concentration arrives at the kidney, and the same or reduced amount leaves through v. renalis​
• Maximal extraction:
  
  • ​The substance entering in a. renalis does not appear at all in v. renalis
  • E=1
• Minimal extraction:
  
  • ​The entire amount of the substance appears in v. renalis, and nothing gets into the urine
  • E=0

E = (P_arterial - P_venous) / P_arterial

Question 6

Clearance

Answer 6

• Clearance: Volume (of plasma) from which all test substances are removen/min (cleared)
• Clearance: The ability of the kidney to remove a substance from the blood plasma and to forward it to the urine
• All filtered substance has a clearance value
• Certain substances are exclusively filtered
  
  • ​Not absorbed or secreted (eg. inulin)
  • Their cleranceis equal to rate ofGFR
• Certain substances are etirely secreted
  
  • ​Eg. PAH (para-amino-hyppuric-acid)
  • Clerance of these gives the renal plasam flow (RPF)
• (U x V)P ml/min
  
  • U: substance concentration in urine
  • P: substance concentration in plasma
  • V: produced urine per minute

Question 7

Clearance

Clearance of para-amino-hippuricacid (PAH)

Answer 7

• The clearance of PH is constant at low plasma concentrations
• At higher concentrations the secretory capacity of the tubules decreases, and the tubular cells become unable to secrete more PAH its clearance decreases

Question 8

Clearance

Clearance of inulin

Answer 8

• Inulin is only filtered (not reabsorbed or secreted)
• Its concentration in the plasma does not influence its clearance even under extermely high values

Question 9

Clearance

Clearance of urea

Answer 9

• Urea is freely filtered,and then passively moves among tubules sections which are not permable for urea​
• A portion of the filtered amount of this substance always remains in the intersititum, but the capacity of filtration is unlimited for this substance as well
• Clearance is smaller than that of inulin, but doesn’t depend on plasma concentration

Question 10

Clearance

Clearance of glucose

Answer 10

• Glucose is freely filtered
• Under normal proximal tubular activity, glucose does not get into the descending limb of Henle-loop
  
  • ​Entirely reabsorbed in the proximal tubule
• Under normal glucose plasma concenctration clearance is 0
• Diabetes: glucose appears in the urine
  
  • ​Glucose level becomes so high that the tubular cells are unable to reabsorb it