L4: Tubular Transport Flashcards
What is the equation to find filtered load?
FL= (GFR) (Px)
What is the equation to find excretion rate?
ER= (Ux)(V)
Define transport rate and what is the equation to find it?
The amount of material added to (secreted) or removed (reabsorbed) from glomerular filtrate.
Tx= FL -ER
What does it mean if the transport rate is positive?
Some material was removed from the filtrate by reabsorption
What does it mean if the transport rate is negative?
Some material was added to the filtrate by secretion
What are the two routes of reabsorption?
Paracelluar (between cells in leaky epithelium gap junctions, or transcellular (through the cell)
What are the mechanisms that facilitate transcellular transport?
Simple diffusion, faciliated diffusion, primary active transport, secondary active transport, and endocytosis
The proximal tubule (reabsorbs/secretes) *** of the water.
Reabsorbed 2/3s
***water follows sodium out
Why does the peritubular capillary next to the proximal tubule have high fluid reabsorption?
It has high peritubular capillary oncotic pressure (drives fluid in)
How does Na+ enter the peritubular cells?
Either by cotransport with organics or by Na+/H+ antiport (NHE)
How does Na+ leaves the peritubular cells?
Na+/K+ ATPase or cotransport with bicarbonate
How does angiotensin affect proximal tubule reabsorption?
It stimulates Na+/H+ exchange across apical membrane (NHE transporters) therefore increasing Na+ reabsorption and H+ secretion
How does the sympathetic nervous system affect Na+ reabsorption in the proximal tubule?
SNS activity stimulates Na+ reabsorption
How does the parathyroid hormone affect Na+ reabsorption in the proximal tubule?
PTH is released with low serum calcium and inhibits Na+/phosphate cotransport. This increases urinary excretion of phosphate
What does it mean if the TF/Px ratio is 1?
Reabsorption of the solute has been exactly proportional to the reabsorption of water
What does it mean if the TF/Px ratio is <1?
Reabsorption of the substance has occurred to a greater extent than water
What does it mean if the TF/Px ratio is >1?
Reabsorption of the substance has been less than water OR there has been net secretion o the substance
What is the tubular maximum?
The maximal rate at which the renal transport system can transport a particular solute
What do the portions above and below Tm mean?
Below Tm: all the filtered load is reabsorbed
Above Tm: excreted
What is the threshold for reabsorption?
The point where glucose first appears in the urine
Does decreases GFR increase or decrease the threshold?
Increase
What is osmotic diuresis and what does it do?
Excess unreabsorbed solute inhibits osmotic water flow from lumen to basolateral spaces. This causes Na+ back diffusion into the tubular lumen with increased Na+ and water loss
What is mannitol?
An osmotic diuretic that increases loss of water and electrolytes
-nonreabsorbed carb
What are the two things that osmotic diuresis can accompany?
- high filtered load of urea because urea can act as an osmotic diuretic
- when glucose exceeds Tm in diabetic patients
