Renal Principles of Tubular Transport - Lecture 4

Question 1

What are the 3 steps of urine formation?

Answer 1

1. Ultrafiltration of plasma by glomerulus
2. Reabsorption of water and solutes from the ultra filtrate
3. Secretion of selected solutes into tubular flow

Question 2

What modulates the volume and composition of urine? How does this occur?

Answer 2

Renal Tubules

• via rebasorption & secretion
• monitors the pH, volume, osmolality, composition
• transport proteins

Question 3

What are the primary mechanisms of transport?

Answer 3

1. Passive Transport
   - facilitated diffusion (glucose uniporter)
   - transport with electrochemical gradient
2. Primary Active Transport
   - against electrochemical gradient, needs energy
   ACTIVE SODIUM rebasorption
3. Secondary Active Transport
   - against electrochemical gradient
   - needs energy source
   Tm limited
   –> symporter for Na & glucose, antiporter for Na-H exchange

Question 4

What limites Secondary Active Transport?

Answer 4

Tm

Question 5

What is the equation for the general Fick Principle?

Answer 5

CaO2 - CvO2 = Vo2/Q = O2 demand/ O2 supply

Question 6

When does the CaO2 - CvO2 difference increase?

Answer 6

SKELETAL MUSCLE!
- due to decrease in flow with no change in Vo2

(in heart there is no change)

Question 7

What is the fick principle in the kidney?

Answer 7

Decrease in the O2 supply decreases the O2 demand with no change in Cao2 - Cvo2 (kidney)

• if blood flow is restricted to the kidney, the kidney requires less oxygen!

Renal O2 consumption = Na reabsorbed = Na filtered = GFR = RPF = RBF

Question 8

Reduction of arterial blood perfusion in the kidney results in what?

Answer 8

Increased Oxygen extraction in skeletal muscle but NOT RENAL TISSUE

• resting skeletal muscle has a constant oxygen demand, the kidney reduces its oxygen demand with restricted perfusion

Question 9

What is special about glucose?

Answer 9

It is 100% reabsorbed

- only appears in the urine under abnormal conditions

Question 10

What is the R,F,S,E equation for glucose?

Answer 10

R = F-E

S = 0

Question 11

What is Tm? Under what conditions can it be calculated? What is the equation for GLUCOSE?

Answer 11

Tubular Transport Maximum
- if Tm is exceeded then and only then can it be calculated

Tm = Pa * GFR - U * V (GLUCOSE)

Question 12

What is RPT? What is it defined as?

Answer 12

Renal Plasma THreshold

• defines at a point where glucose first appears in the urine

Question 13

What does spay allow for?

Answer 13

Direct correspondence between Tm and RPT

• if splay is present, for increasing plasma concentrations of substance x,
  RPTx is reached before Tmx is maximized

Question 14

At low doses, PAH is normally what? What is the equation for PAH?

Answer 14

90% over-secreted

S= E-F (R=0)

Question 15

How is the tubular transport maximum calculated for PAH?

Answer 15

Tm = U * V - Pa * GFR

Question 16

When is the Renal Plasma Threshold (RPT) for PAH?

Answer 16

RPT is defined at point where secretion slope of PAH first decreases

Question 17

What is osmotic Diuresis?

Answer 17

Diuresis is defined clinically as urine flow > 1ml/min

Question 18

What functions as an osmotic Diuretic? How does it do this?

Answer 18

Mannitol

• is filtered, but not reabsorbed and holds excess water in the tubules by osmotic forces
• excess water in the tubules increases urine flow (diuresis)

Question 19

What can glucose function as when there is excess glucose in the tubules? What is a clinical sign of this?

Answer 19

Osmotic Diuretic!

• under abnormal conditions, excess glucose in the tubules can hold water by osmotic forces
• causes dehydration and thirst

Question 20

What type of passive transport is limited?

Answer 20

Facilitated Diffusion

Question 21

What type of transport is the Uniporter for glucose?

Answer 21

Facilitated Diffusion - passive transport

Question 22

What limits the secondary active transporters like the Na - Glucose Symporter or the Na-H Antiporter?

Answer 22

Tm!

• once all transporters are saturated with substance x (due to high solute concentrations) the rate at which the substance is transported cannot be increased (over-saturated)

Question 23

By what Mechanism does the Na/K AtpASE pump function?

The Ca, Mg, K transfer between cells by solvent drag?

Answer 23

1. Transcellular Pathway

2. Paracellular Pathway

Question 24

Which of the following types of transport are saturable & non-saturable

1. Secondary Active Transport (Carrier-Mediated)
2. Passive Transport (Simple Diffusion)

Answer 24

1. Saturable
   - Active Transport can have substance limited diffusion, but not rate??
2. Non-Saturable

Question 25

Across which membrane are solute and water reabsorbed?

Answer 25

APICAL

Question 26

What forces determine whether fluid re-enters the tubule fluid or enter the interstitial space & thus the capillary?

Answer 26

SATRLING FORCES

Question 27

Where does the CaO2-CvO2 difference not change? Why?

Where does this change? Does it increase or decrease?

Answer 27

1. HEART - no change due to simultaneous increase in V and Q

and KIDNEY!!!

2. Increases in SKELETAL muscle due to the decrease in perfusion (Q) and no change in Vo2

Question 28

Why does the CaO2 and CvO2 difference not change in the kidney?

Answer 28

If flow is restricted to the kidney, the kidney requires les Oxygen!

• decreases the demand due to the decrease in O2 supply

Question 29

Reducing the arterial blood perfusion results in increased oxygen extraction in what system? Why?

Where does this oxygen extraction stay constant?

Answer 29

SKELETAL MUSCLE
- resting skeletal muscle has a constant oxygen demand

Oxygen extraction stays constant in the KIDNEY (with a reduction in perfusion) due to the fact that it REDUCES its oxygen demand with decreased perfusion

Question 30

What is the Tm equation for Glucose?

For PAH?

Answer 30

1. Tm glucose = (Pa*GFR) - (U * V)

2. Tm PAH = (U * V) - (Pa * GFR)

Question 31

What is the equation for Glucose Reabsorption?

For PAH Secretion?

Answer 31

R = F - E

S = E - F