Renal Principles of Tubular Transport - Lecture 4 Flashcards
What are the 3 steps of urine formation?
- Ultrafiltration of plasma by glomerulus
- Reabsorption of water and solutes from the ultra filtrate
- Secretion of selected solutes into tubular flow
What modulates the volume and composition of urine? How does this occur?
Renal Tubules
- via rebasorption & secretion
- monitors the pH, volume, osmolality, composition
- transport proteins
What are the primary mechanisms of transport?
- Passive Transport
- facilitated diffusion (glucose uniporter)
- transport with electrochemical gradient - Primary Active Transport
- against electrochemical gradient, needs energy
ACTIVE SODIUM rebasorption - Secondary Active Transport
- against electrochemical gradient
- needs energy source
Tm limited
–> symporter for Na & glucose, antiporter for Na-H exchange
What limites Secondary Active Transport?
Tm
What is the equation for the general Fick Principle?
CaO2 - CvO2 = Vo2/Q = O2 demand/ O2 supply
When does the CaO2 - CvO2 difference increase?
SKELETAL MUSCLE!
- due to decrease in flow with no change in Vo2
(in heart there is no change)
What is the fick principle in the kidney?
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
Reduction of arterial blood perfusion in the kidney results in what?
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
What is special about glucose?
It is 100% reabsorbed
- only appears in the urine under abnormal conditions
What is the R,F,S,E equation for glucose?
R = F-E
S = 0
What is Tm? Under what conditions can it be calculated? What is the equation for GLUCOSE?
Tubular Transport Maximum
- if Tm is exceeded then and only then can it be calculated
Tm = Pa * GFR - U * V (GLUCOSE)
What is RPT? What is it defined as?
Renal Plasma THreshold
- defines at a point where glucose first appears in the urine
What does spay allow for?
Direct correspondence between Tm and RPT
- if splay is present, for increasing plasma concentrations of substance x,
RPTx is reached before Tmx is maximized
At low doses, PAH is normally what? What is the equation for PAH?
90% over-secreted
S= E-F (R=0)
How is the tubular transport maximum calculated for PAH?
Tm = U * V - Pa * GFR
When is the Renal Plasma Threshold (RPT) for PAH?
RPT is defined at point where secretion slope of PAH first decreases
What is osmotic Diuresis?
Diuresis is defined clinically as urine flow > 1ml/min
What functions as an osmotic Diuretic? How does it do this?
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)
What can glucose function as when there is excess glucose in the tubules? What is a clinical sign of this?
Osmotic Diuretic!
- under abnormal conditions, excess glucose in the tubules can hold water by osmotic forces
- causes dehydration and thirst
What type of passive transport is limited?
Facilitated Diffusion
What type of transport is the Uniporter for glucose?
Facilitated Diffusion - passive transport
What limits the secondary active transporters like the Na - Glucose Symporter or the Na-H Antiporter?
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)
By what Mechanism does the Na/K AtpASE pump function?
The Ca, Mg, K transfer between cells by solvent drag?
- Transcellular Pathway
2. Paracellular Pathway
Which of the following types of transport are saturable & non-saturable
- Secondary Active Transport (Carrier-Mediated)
- Passive Transport (Simple Diffusion)
- Saturable
- Active Transport can have substance limited diffusion, but not rate?? - Non-Saturable
Across which membrane are solute and water reabsorbed?
APICAL
What forces determine whether fluid re-enters the tubule fluid or enter the interstitial space & thus the capillary?
SATRLING FORCES
Where does the CaO2-CvO2 difference not change? Why?
Where does this change? Does it increase or decrease?
- HEART - no change due to simultaneous increase in V and Q
and KIDNEY!!!
- Increases in SKELETAL muscle due to the decrease in perfusion (Q) and no change in Vo2
Why does the CaO2 and CvO2 difference not change in the kidney?
If flow is restricted to the kidney, the kidney requires les Oxygen!
- decreases the demand due to the decrease in O2 supply
Reducing the arterial blood perfusion results in increased oxygen extraction in what system? Why?
Where does this oxygen extraction stay constant?
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
What is the Tm equation for Glucose?
For PAH?
- Tm glucose = (Pa*GFR) - (U * V)
2. Tm PAH = (U * V) - (Pa * GFR)
What is the equation for Glucose Reabsorption?
For PAH Secretion?
R = F - E
S = E - F
