Unit 2, L14 Principles of Tubular Transport Flashcards
Simple Diffusion
Move from high to low concentration, passive transport
Channel-mediated transport
Molecules that are specifically selected for those channels will move through when the channels are open, passive transport
Transporter-mediated transport
Requires a transporter to move, but is still passive transport as no energy is needed, the molecule is moving down its concentration gradient
Active transport
Moving against the concentration gradient of the molecule, requires energy in the form of ATP
Transcellular pathways
Moving through the cell itself, by simple diffusion so passive transport
Paracellular pathway
Bypassing the cell, doesn’t enter into the cell. There are adhesion junctions that provide some specificity that prevents everything from getting through, but some of the solutes get through
If moving using a paracellular pathway, is that against or with its concentration gradient?
With its concentration gradient
Primary active transport
Hydrolysis of ATP produced by mitochondria, gives energy that pushes the chemical out, against its concentration gradient
Secondary active transport
Movement of the molecule against its concentration gradient, coupled with the movement of a second molecule. Can move in an antiport fashion, which is going against its concentration gradient AND opposite of the second molecule, or in symport fashion, which is going against its concentration gradient and the same direction of the second molecule
What is the primary active transporter and what is it transporting
Na/K/ATPase, and moving Na out and K in, both of which is moving against its concentration gradient
What functions first, primary or secondary active transport?
Primary
Active or passive, carrier mediated, uses metabolic energy, and dependent on Na+ gradient:
Simple Diffusion
Passive, downhill
Not carrier mediated
Does not use metabolic energy
Is not dependent on Na+ gradient
Active or passive, carrier mediated, uses metabolic energy, and dependent on Na+ gradient:
Facilitated Diffusion
Passive and downhill
Yes, carrier mediated
No, does not use metabolic energy
No, is not dependent on Na+ gradient
Active or passive, carrier mediated, uses metabolic energy, and dependent on Na+ gradient:
Primary active transport
Active and uphill
Yes, is carrier mediated
Yes, directly uses metabolic energy
No, not dependent on Na+ gradient
Active or passive, carrier mediated, uses metabolic energy, and dependent on Na+ gradient:
Cotransport
Secondary active (Na+ is transported down hill and one or more solutes are transported uphill)
Yes, carrier mediated
Yes, indirectly uses metabolic energy
Yes, dependent on Na gradient as solutes move in the same direction as Na+ across cell membrane
Active or passive, carrier mediated, uses metabolic energy, and dependent on Na+ gradient:
Countertransport
Secondary active (Na+ is transported downhill and one or more solutes are transported uphill)
Yes, carrier mediated
Yes, indirectly uses metabolic energy
Yes, dependent on Na+ gradient as solutes move in opposite direction as Na+ across cell membrane
If something is moving by simple diffusion, what is the one thing that can limit its movement
Its concentration
At low solute concentrations, what is happening with binding sites and rate of transport
Many binding sites are available and the rate of transport increases steeply as the concentration increases
At high solute concentrations, what happens with binding sites and transport
At high solute concentrations, the available binding sites become less available and when all the binding sites are occupied, saturation is achieved
What is the transport maximum
When all binding sites are occupied and saturation is achieved
What percentage of free filtered glucose is reabsorbed back into the blood?
100%
Na-glucose co-transporter (SGLT)
Secondary active transport, it brings Na and glucose into the cell
GLUT 1 and GLUT 2 transporters
Enable glucose to move across the basal membrane, only moves glucose
With normal diets and being healthy, what is the normal level of glucose
200 mg/dL of glucose
At lower concentrations of glucose, what does the reabsorption and filtration amounts look lie?
Reabsorbed parallels filtered, so all filtered glucose is reabsorbed
When glucose concentration reaches renal plasma threshold, what happens to the filtered, reabsorbed, and excreted amounts
Filtered still continues in a linear line
Reabsorbed starts to plateau, as we hit the RPT
Excreted begins here, starts to increase and spill over into the urine
At the highest plasma glucose concentration, what happens to the excreted line, reabsorbed line, and filtered line
Filtered line continues linearally
Reabsorbed line continues with its plateau
Excreted line continues and becomes parallel with the filtered line, as all the excess is being excreted rather than reabsorbed
What is the tubular maximum of glucose transport
Tm = 375 mg/min
What is the renal plasma threshold value
RPT = 220 mg/dL
When RPT is reached, what type of transporters are saturated
The lowest capacity transporters
What is a region of splay (think glucose titration curve_)
Region of splay is defined from the RPT to the plasma concentration when Tm is reached
What are the transporters that are the last to saturate and have the highest capacity?
SGLT-1
Glucosuria
Whenver the renal plasma threshold for glucose is exceeded, glucose will be found in the urine. This leads to excretion or spilling of glucose in the urine
For PAH titration curve, what do the filtered, excreted, and secreted lines look like?
Filtered line is linear
Excreted line is above the filtered line because PAH appearing in urine comes from both filtration and secretion
Secreted line raises linearly until Tm, where it plateaus out
Pi for starling forces across peritubular capillary wall
Interstitial hydrostatic pressure
P pc for peritubular capillary wall (starling forces)
Peritubular capillary hydrostatic pressure
Pi i, starling forces for peritubular capillary wall
Interstitial fluid oncotic pressure
Pi PC, starling forces for peritubular capillary wall
Peritubular capillary oncotic pressure
In the peritubular capillaries, what happens with the starling forces?
First, solute and water are reabsorbed across apical membrane into the epithelial cell
Second, some solute and water reenter the tubule fluid
Third, most of the solute and water will move into the interstitium space, where it will go into the capillary
What is the main driving force for uptake to the peritubular capillaries
oncotic pressure in the peritubular capillaries
Major force opposing uptake to the peritubular capilarries
Hydrostatic pressure in peritubular capillaries, its 20
Overall, in the peritubular capillaries, what is the direction of fluid movement (starling forces)
Favors driving fluids into the capillaries
These starling forces only apply to the _______ and not the rest of the tubule system due to the permeability of water
Proximal tubule
Reabsorption of water and salt in the proximal tubule is __________
Isosmotic
Glomerulotubular balance
Where Na reabsorption in the proximal tubule varies in parallel with the filtered load, so 2/3 of the filtered Na is reabsorbed in the PT, even if GFR varies
What is the pathway for glomerulotubular balance
Increase GFR, leads to increase filtration fraction, leads to increased filtered load, leads to increased peritubular capillary reabsorption
Decrease GFR leads to decrease filtration fration, leading to decreased peritubular capillary reabsorption
