Glomerular Filtration (B2: W4)

Question 1

What factors are involved in Starling’s equation?

Answer 1

K = filtration coefficient

P_c = hydrostatic pressure in the capillary

π_t = colloid osmotic pressure in the tubule

P_t = hydrostatic pressure in the tubule

π_c = colloid osmotic pressure in the capillary

Question 2

What is the Starling equation?

Answer 2

Filtration = K [(P_c + π_t) - (P_t + π_c)]

Question 3

Which factors involved in the Starling equation favor filtration?

Answer 3

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

Question 4

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

Answer 4

20%

80% continues on to peritubular capillaries

Question 5

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

Answer 5

• 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

Question 6

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

Answer 6

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

Question 7

What is the filtration coefficient (K) dependent upon?

Answer 7

Capillary surface area and the permeability of this surface

Question 8

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

Answer 8

GFR = 172 liters/day of plasma

Question 9

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

Answer 9

Hydrostatic pressure of the tubule would increase

Filtration would decrease

Question 10

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

Answer 10

Osmotic pressure in the capillary would decrease

Filtration would increase

Question 11

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

Answer 11

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

Question 12

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

Answer 12

Filtration

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

Question 13

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

Answer 13

Reabsorption

• High oncotic pressure

Question 14

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

Answer 14

Lamina rara interna

• Second barrier
• Tends to reject negative charges

Question 15

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

Answer 15

Tight junctions in PCT between epithelial cells

• Water and ions can go between cells

Question 16

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

Answer 16

• 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

Question 17

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

Answer 17

Increases reabsorption

Question 18

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

Answer 18

Decreases reabsorption

Question 19

What happens to reabsorption with contraction of the efferent arteriole?

Answer 19

Increases reabsorption

• Hydrostatic pressure goes down
• Oncotic pressure goes up

Question 20

What are the size and charge contstraints for filtration?

Answer 20

• 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

Question 21

How do we measure GFR?

Answer 21

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

Question 22

What is the equation for clearance?

Answer 22

Ux = concentration in urine

V = volume

Px = concentration in plasma

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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
• U_Cr overestimated because of secretion
• P_Cr overestimated because of lab chemistry

Question 25

How do we calculate filtered load?

Answer 25

Filtered Load = GFR x P_x

Question 26

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

Answer 26

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

Question 27

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

Answer 27

Fractional Excretion