Topic 40 - Basic physiological functions of the kidney: filtration, reabsorption, secretion, excretion

Question 1

Words to include in filtration

Answer 1

• Permability of the barriers
• Capillary Filtration Coefficient (CFOC)
  
  • Ultrafiltration
  • Renal glomerulus
  • Lamina densa of the basal membrane
    
    • Negatively charged
  • Mesangial cells containing myogen elements
    
    • Size of filtrating surface
• Effective filtration pressure (EFP)
  
  • Ultrafiltration
  • Pressure gradient
  • Ratio:
    
    • Hydrostatic blood pressure
    • Tissue pressure
    • Colloid osmotic pressure
  • Components of EFP:
    
    • Hydrostatic pressure of the glomerulus (GP)
    • Bowman-sheath’s pressure (CP)
    • Colloid-osmotic pressure of the plasma (GCP)
  • EPF = GP - (CP + GCP)
  • Oncotic pressure
  • Vas eferent
  • Vas afferent
  • Protein concentration

Anatomic basis of glomerular filtration

• Filtration barrier layers
  
  • Fenestrated endothelium
  • Glomerular basement membrane (GBM)
  • Podocyte foot processes (outermost)
• Size-selective filter
• Plasma macromolecules
  
  • Negatively charge molecules (filtered in smaller amounts)
  • Negative < neutral < positive
• Type IV collagen
• Nidogen
• Laminin
• Proteoglycan (glycosaminoglycans)
  
  • Negative charge
  • Anion filtration barrier

Question 2

Words to include in reabsorption

Answer 2

• 90% of filtrated amount is reabsorbed
• Pathways of the reabsorption of the ultrafiltrate
  
  • Paracellular
  • Transcellular
• Reabsorption maximum (Tm)
  
  • Tm (Transfer Maximum)
    
    • Upper limit of reabsorption

Question 3

Words to include in secretion

Answer 3

• Directly from the blood vessel
• Plasma
• Glomerulus
• Efferent arteriole
• Tubular lumen
• Peritubular capillaries
• Can occur:
  
  • Transcellular
  • Paracellular
• Nephrons
• Secreting maximum
• Active cellular work

Question 4

Words to include in excretion

Answer 4

• Result of:
  
  • Filtration
  • Reabsorption
  • Secretion
• End of tubular system
• Secondary filtrate of urine
• Rate of excretion = urine of seleced material excreted
• Filtration + secretion = excretion

Autoregulation of renal circulation

• Independent of blood pressure (midpressure)
  
  • Autoregulation of renal perfusion

Question 5

Filtration

Answer 5

• The rate of filtration is determined by:
  
  • Effective Filtration Pressure (EFP)
  • Permability of the barriers
• Proteins are not filtrated

1. Capillary filtration coefficient
   
   • ​​The degree of ultrafiltration taking place in the renal glomerulus is a hundred times higher than in any other capillary areas
   • Most important factor: lamina densa of the basal membrane
     
     • ​Negatively charged
   • The size of the filtrating surface is regulated by the mesangial cells containing myogen elements
   • Total filtration: 180-200 liter/100 kg bwt/day
2. Effective Filtration Pressure (EFP)
   
   • ​​The extent of ultrafiltration depends on the pressure gradient
   • Depends on the ratio of:
     
     • Hydrostatic blood pressure
     • Tissue pressure
     • Colloid osmotic pressure
   • Components of EFP:
     
     • Hydrostatic pressure of the glomerulus (GP)
     • Bowman-Sheath’s pressure (CP)
     • Colloid-osmotic pressure of the plasma (GCP)
   • EFP = GP - (CP + GCP)
   • The oncotic pressure increases in the direction of the vas efferent, as the protein content increases due to ultrafiltration

Anatomic basis of glomerular filtration

• The filtration barrier consists of three layers:
  
  1. Fenestrated epithelium
  2. Glomerular basement membrane (GBM)
  3. Outermost epithelial podocyte foot processes
• Behaves as a size-selective filter that restricts the passage of plasma macromolecules based on their size, shape, and charge
• Negatively charged molecules are filtered in smaller amounts than neutral molecules of comparable size, and the traversal of positively charged molecules is actually facilitated.
  
  • Negative < neutral < positive
• Molecular scaffold:
  
  • Independent structural networks of type IV collagen and laminin
  • Linked through nidogen
  • The role of negatively charged proteoglycans (glycosaminoglycans) is to serve as an anionic filtration barrier
  • The GBM therefore can restrict the traversal of large plasma proteins

Question 6

Give the valuse for the vas afferent and vas efferent occuring in effective filtration pressure

Answer 6

• Vas afferent:
  
  • GP: 60 mmHg
  • CP: 20 mmHg
  • GCP: 28 mmHg
  • EFP: 12 mmHg
• Vas efferent:
  
  • GP: 60 mmHg
  • CP: 20 mmHg
  • GCP: 36 mmHg
  • EFP: 4 mmHg

Question 7

Reabsorption

Answer 7

• The extremely large filtration rate could mean a fatal loss of fluid, if it appeared in full amount in the urine
  
  • Only a small fraction of it (0,5-1%) becomes urine, made possible by reabsorption
• More than 90% of the filtrated amount is reabsorbed
• Two main pathways for the reabsorption of the ultrafiltrate:
  
  • Paracellular
  • Transcellular
• Reabsorption maximum (Tm):
  
  • ​Tm (Transfer maximum)is theupper limit of reabsorption

Question 8

Secretion

Answer 8

• Some substances are either secreted:
  
  • after filtration and reabsorption
  • Directly from the blood vessel
• Secretion: From the plasma leaving the glomerulus through the efferent arteriole further substances can get into the tubular lumen at the site of the peritubular capillaries
• Can occur:
  
  • Transcellular
  • Paracellular
• Above certain plasma concentration, the secretion becomes constant
• All of the nephrons participating in secretion reaches the secreting maximum
• Proves that the secretion is a active cellular work

Question 9

Excretion

Answer 9

• As a result of the filtration, reabsorption and secretion by the end of the tubular system the secondary filtrate of urine is formed
• Rate of excretion = the urine or selected material excreted
• Average excretion rate of urine is 2-3 ml/min/100 kg bodyweight
• Filtration + secretion = excretion
• Phases of excretion:
  
  1. ​Linear phase
  2. SDL phase (Self depression limit)
  3. Saturation phase

Question 10

Figure to draw in the essay

Answer 10