Renal Physiology Lecture 1: Renal Filtration

Question 1

How much plasma is filtered out of glomerular capillaries?

Answer 1

20% at any given time

Question 2

Plasma that is not filtered the glomerular capillaries goes where?

Answer 2

• Remaining plasma + blood cells & protein enters peritubular capillaries
  
  • Re-enters systemic circulation

Question 3

What is the fitration fraction?

Answer 3

Proportion of plasma volume that filters into the tubule

Question 4

What are the three filtration barriers in the Renal Corpuscle?

Answer 4

• Glomerular capillary endothelium
• Basal Lamina (basement membrane)
• Epithelium of Bowman’s capsule

Question 5

Glomerular Capillary Endothelium

Answer 5

• Fenestrated capillaries
  
  • Facilitates filtration → features large gaps which facilitates filtration by allowing fluid and solutes to pass through but not large enough for proteins or blood cells
• Pore size permits most solutes but not plasma proteins or blood cells (RBCs)
• Pore surface negatively charged
  
  • Repels plasma proteins

Question 6

Glomerular Basement Membrane (GBM, basal lamina)

Answer 6

• Extracellular matrix
  
  • Acellular → not separated into cells
• Separates glomerular capillary endothelium from Bowman’s epithelium
• Negatively charged glycoproteins & collagen

Question 7

Bowman’s Capsule Epithelium

Answer 7

• Podocytes feature foot processes
  
  • Wrap around glomerular capillaries
  • Slits present for filtration
• Contractile mesangial cells contribute to capillary blood flow control

Question 8

What are the mesangial cells?

Answer 8

Mesangial cells are contractile cells that constitute the central stalk of the glomerulus. On the capillary lumen side, mesangial cells are in direct contact with the glomerular endothelium without an intervening basement membrane

Question 9

What dictates glomerular filtration?

Answer 9

Starling Forces

Question 10

What are the Starling Forces for glomerular filtration?

Answer 10

• Glomerular capillary hydrostatic pressure favours filtration
  
  • Bowman’s capsule hydrostatic pressure normally lower and net filtration occurs
• Protein remaining in the capillaries opposes filtration (osmotic)
  
  • Usually lower than glomerular hydrostatic pressure and net filtration occurs

Question 11

hydrostatic vs. osmotic pressure

Answer 11

hydrostatic: force of fluid volume against a membrane

osmotic: protein concentration on either side of the membrane pulling water toward the region of great concentration

Question 12

What are the three main forces of glomerular filtration?

Answer 12

• capillary hydrostatic pressure
• capillary colloid osmotic pressure
• Bowman’s capsule hydrostatic pressure

Question 13

What is GFR?

Answer 13

Glomerular filtration rate

• volume of fluid entering Bowman’s capsule/unit time

Question 14

What is GFR influenced by?

Answer 14

• Net filtration pressure → based on the three forces
  
  • Altered by renal blood flow (RBF) and blood pressure → can drive further GFR and further filtration
• Filtration Coefficient (Kf)
  
  • Total surface area for filtration → surface area of the glomerular capillaries
  • Permeability of barrier between capillaries & Bowman’s capsule

Question 15

What results in reduced GFR?

Answer 15

reduced filtration pressure or filtration coefficient