CPR 67-69 - Renal Glomerular Filtration and Tubular Mechanisms

Question 1

What is the average amount of urine production per day?

Answer 1

1-1.5 liters

Question 2

Which enzyme does the kidney use to convert 25-OH Vitamin D to 1,25-(OH)₂ Vitamin D and where is it located within the kidney?

Answer 2

The kidney uses 1-α-hydroxylase which is located in the proximal tubule

Question 3

What percentage of CO does the kidney receive? How much is that? How much of that is filtered? How much of that filtrate is reabsorbed?

Answer 3

The kidney receives ~25% of CO which is equivalent to 1800L of blood per day. Only about 180L of that is filtered per day and 99% of that volume gets reabsorbed.

Question 4

Give the glomerular filtration equation and define each variable. What is the normal value for each variable?

Answer 4

GFR = K_fΔP

GFR = glomerular filtration rate (flux across glomerulus) = 90-140 mL/min

K_f = permeability constant = 10-15 mL/min/mmHg

ΔP = difference in glomerular filtration pressures = 10 mmHg

Question 5

Describe how a molecule’s size affects it glomerular filterability.

Answer 5

Larger than 35Å is not filterable

15-35Å more filterable as molecule decreases in size

Smaller than 15Å is freely filterable

Question 6

Describe how a molecule’s charge affects its glomerular filterability.

Answer 6

Cations are most filterable

Neutral molecules have intermediate filterability

Anions are least filterable

Question 7

What are the four glomerular filtration pressures and their abbreviations? What are these values normally?

Answer 7

Capillary hydrostatic pressure (P_G) = 45 mmHg

Bowman’s Space hydrostatic pressure (P_BS) = 10 mmHg

Blood oncotic pressure (π_G) = 25 mmHg

Bowman’s Space oncotic pressure (π_BS) = 0 mmHg

Question 8

How does constriction or dilation of the afferent and efferent arterioles affect renal plasma flow (RPF) and GFR?

Answer 8

• Afferent constriction decreases RPF and GFR
• Afferent dilation increases RPF and GFR
• Efferent constriction decreases RPF while increasing GFR
• Efferent dilation increases RPF while decreasing GFR

Question 9

What does the term “filtration fraction” refer to?

Answer 9

GFR/RPF

Question 10

What does the term “renal autoregulation” refer to? When does this not occur?

Answer 10

Autoregulation refers to the fact that the kidneys will keep GFR constant as long as the MAP is between 80 and 180 mmHg. Above or below that range and the kidneys will still attempt to maintain GFR but will not be able to.

Question 11

What are the primary mechansisms by which the kidneys autoregulate GFR?

Answer 11

1. Myogenic response - when MAP increases, the increased afferent arteriole stretch induces constriction
2. Tubuloglomerular feedback - a sensed decrease in [Na+] by the macula densa induces the afferent arteriole to dilate.

Question 12

What is filtered load? How is it calculated? What are some other names/abbreviations for it?

Answer 12

Filtered load (aka - rate of filtration or FL) is the rate at which a solute is filtered per unit time.

FL = GFR * P_x * filterability quotient

Question 13

What is filtration fraction? How is it calculated? What is it in a healthy person? What are some other names/abbreviations for it?

Answer 13

Filtered fraction (FF) is the percentage of plasma that has reached the glomerulus that gets filtered.

FF = GFR/RPF

RPF = renal plasma flow

RPF = RBF * (1 - Hct)

Question 14

What is a solute’s “rate of net transport?” How is it calculated? What does this value mean?

Answer 14

The rate of net transport (TR) is a number that quantifies how much reabsorption or secretion that solute experiences.

TR = FL - ER

Excretion Rate (ER) = U_x x V

U_x = urine solute concentration; V = rate of urine flow

If TR is positive then net reabsorption is experienced

If TR is negative then net secretion is experienced

Question 15

How does the osmolarity of the PCT compare to plasma osmolarity? Explain

Answer 15

It is isoosmotic because as solute is reabsorbed so is H2O

Question 16

What is the primary solute reabsorbed in the early proximal tubule and how is it reabsorbed?

Answer 16

NaHCO₃

Na+ is symported in with glucose, amino acids, and phosphate. It is also exchanged for H+ by the Na+/H+ antiporter. Na+ is then pumped out the basal surface of the cell by the Na/K ATPase.

Carbonic anhydrase in the lumenal cell converts CO2 to bicarb and H+. The H+ is then secreted via the Na+/H+ antiporter and the bicarb is reabsorbed.

Question 17

What is the primary solute reabsorbed in the late proximal tubule and how is it reabsorbed?

Answer 17

NaCl

Na+ is reabsorbed via the Na+/H+ antiporter and Na+/K+ ATPase

Cl- is reabsorbed via the Cl-/formate antiporter into the lumenal cell and then passively diffuses into the interstitium.

NaCl is also passively reabsorbed paracellularly

Question 18

How much of filtered Na+ is reabsorbed by the PCT and how much is reabsorbed in each region of the PCT and by which pumps?

Answer 18

67% of filtered Na+ is reabsorbed by the PCT

10% by Na+ symporters in the early PCT

20-25% by Na+/H+ antiporters in the early PCT

~35% by Na+/H+ antiporters in the late PCT

Question 19

What is the largest driving force for reabsorption in the PCT? How can changes in GFR affect the fraction of solute absorption in the PCT?

Answer 19

The large colloid pressure difference between the PCT (essentially zero) and the peritubular capillaries (fairly high) is what drives reabsorption in the PCT.

An increase in GFR will not decrease the fraction of solute reabsorbed in the PCT because it will also increase the colloid pressure in the peritubular capillaries.