4. BASIC PROCESSES OF KIDNEY FUNCTION

Question 1

contents

Answer 1

1. filtration 2. tubulary reabsorption 3. tunulary secretion 4. excretion (not detailed in this topic)

Question 2

filtration

Answer 2

ultrafiltration

Question 3

effective filtration pressure

Answer 3

Question 4

mechanisms maintaining ultrafiltration

Answer 4

glomerular bloodpressure (GP) = 55-60mmHg

Opposed by:

• Osmotic pressure (GCP: glomerular oncotic pressure: proteins remain in capillaries) gcp = 30 mmHg (concentration of plasma proteins decreased in malnutrition, so GFR is increased)
• Capsular pressure (CP: pressure maintained by bowman’s capsule) cp= 15-20 mmHg (it may be increased by kidney stones, so GFR is decreased)

Driver of filtration:

• Net pressure (EFP) = 55 - 30 - 15 = 10 mmHg filtration is also affected by glomerular permeability, decreased in kidney failure Filtration fraction= GFR/RPF= 120(Cin)/670(Cpah)=0.18
• FF± 17 – 30%

Question 5

tubulary reabsorption

Answer 5

• Certain substances have transport maximum (which can be saturated)! The filtration is free, but there is no excretion in the urine still, because of the intensive reabsorption.
• Tm (Transfer maximum) is the upper limit of reabsorption (can be saturated) .
  
  • (i.e. reabsorption of glucose; ± amino acids; ± vitamins)
  • under normal blood-glucose level (4.5 - 5 mmol/l) the amount of filtered glucose is 100-120 mg/min. At this point reabsorbtion is 100%.
  • glucose Tm=350 mg/min; (plasma Gl=300 mg/100 ml=17mmol/l)
  • If Filtered load >350 mg/min, the result is glucosuria!
  • (slight (±) glucosuria is already present at 9-11 mmol/l, because the Tm threshold of nephrons are slightly different!)

Question 6

tubulary reabsorption

reabsorption maximum

Answer 6

Question 7

tubulary secretion

secretion maximum (Tm)

Answer 7

The excretion is carried out in three phases:

1. Linear phase
2. Self depression limit (SDL) phase (more and more cells become unable to secrete, so the excretion is decreased)
3. Saturation phase, when the excretion curve is paralel with the filtration, since there is no further secretion, so from that point on the excretion only depends upon the filtration.

Question 8

tubulary secretion

secretion maximum tm

Answer 8

• The kidney eliminates a variety of materials by secretion (mostly by active transport). It is important from a veterinary point of view, since most of the medicines administered from outside are eliminated in this way.
• The degree of filtration is growing linearly with the increase of plasma concentration.
• Above certain plasma concentration, the secretion becomes constant, since all of the nephrons participating in secretion reached the secreting maximum!
• -> This also proves that the secretion is a result of active cellular work.
• -> Elimination of certain substances (PAH, penicillin,steriods) happens also with secretion also characterized by Tm.
• Therefore Tm(PAH)secr.= 80 mg/min at 1.7 mmol/l plasma concentration, and remains constant above that!
• Excretion is the sum of filtration and secretion.