Renal Tubular Function 1 L05 Flashcards
What percentage of the proximal tubule is convoluted?
60% convoluted *convoluted becomes straight for the other 40%
How is the surface area of the proximal tubule increased?
presence of microvilli brush border on apical membrane.
What is the difference between primary and secondary active transport?
primary - relies on ATP
secondary - relies on concentration gradient of another ion e.g. Na+
define transcellular and paracellular.
transcellular - across cells
paracellular - between cells
Give examples of secondary active transporters
ion channels, co-transporters, exchangers
give examples of primary active transporters
protein pumps
In which direction is the Na+ gradient in the PCT and what is this important for?
high concentration in filtrate»_space; low concentration in epithelial cells *strong electrochemical gradient
- secondary active transport of glucose, AA, etc.
- creates an osmotic gradient for the movement of water into cells (water follows movement of Na+)
How is a low concentration of Na+ maintained inside epithelial cells in PCT?
Na+/K+ ATPase on BASOLATERAL membrane moves Na+ out of cell and into the interstitial space
What percentage of water is reabsorbed from PCT?
70%
Through which routes is water reabsorbed from the PCT?
transcellular and paracellular routes
describe the transcellular route of water absorption in PT
water moves by osmosis through AQP1 water channels present on both apical and basolateral membrane of epithelial cell
What is solvent drag?
when the paracellular absorption of water (across tight junctions) by osmosis facilitates the absorption of other solutes e.g. K+, Mg2+, Ca2+. These solutes are carried across the membrane with the movement of water.
Is Inulin absorbed from the PT? How is its concentration at the end of the tubule compared to at the start?
- not absorbed (remember L03)
- concentration higher at the end of the tubule *due to LESS WATER
Is urea absorbed from the PT? How is its concentration at the end of the tubule compared to at the start?
- is absorbed weakly *at a slower rate than water
- concentration at the ends of tubule is HIGHER *due to LESS WATER
How quickly is Cl- reabsorbed from the PT in comparison to water? Is the concentration of Cl- higher or lower at the end of the tubule?
- absorbed weakly/at a slower rate than water
- therefore concentration is HIGHER at the end of the tubule *due to LESS WATER
How quickly is Na+ reabsorbed from the PT in comparison to water? Is the concentration of Na+ higher or lower at the end of the tubule?
- absorbed at the same rate as water *remember Na+ movement aids water movement
- concentration of Na+ is the same at the start and end of PT
How quickly is K+ reabsorbed from the PT in comparison to water? Is the concentration of K+ higher or lower at the end of the tubule?
- absorbed at the same rate as water
- concentration of K+ is the same at the start and end of PT
How quickly is HCO3- reabsorbed from the PT in comparison to water? Is the concentration of HCO3- higher or lower at the end of the tubule?
- absorbed at a faster rate than water
- concentration of HCO3- is LOWER at the end of PT *due to MORE WATER than ion
How quickly are amino acids reabsorbed from the PT in comparison to water? Is the concentration of amino acids higher or lower at the end of the tubule?
- absorbed more strongly than water
- concentration of amino acids much LOWER at end of PT
How quickly is glucose reabsorbed from the PT in comparison to water? Is the concentration of glucose higher or lower at the end of the tubule?
- absorbed more strongly than water
- concentration of amino acids much LOWER at end of PT
By what route is glucose reabsorbed from PT?
transcellularly
Describe how glucose is reabsorbed from PT.
- secondary active transport across apical membrane
- facilitated diffusion across basolateral membrane
early PT (90%):
- apical membrane: SGLT2 cotransporter transports glucose and Na+ in 1:1 ratio into cell
- basolateral membrane: GLUT2 channel transports glucose into cortical interstitial space
late PT (10%):
- concentration of glucose in filtrate diminishing, so more energy required to actively transport it into cell against electro-chemical gradient
- apical membrane: SGLT1 cotransporter transports glucose and Na+ in 1:2 ratio into cell
- basolateral membrane: GLUT1 channel transports glucose into cortical interstitial space
what is tubular maximum (Tm) transport and its value for glucose?
The maximum rate of reabsorption (for glucose) . After this limit is reached, rate of glucose excretion begins to rise as not all of the glucose can be reabsorbed > glucosuria. This occurs when plasma concentration/filtrate concentration of glucose is around 12mM.
2.1mmol/min or 380mg/min.
One of the symptoms of diabetes mellitus in glucosuria. Why does this occur?
Occurs as the glucose concentration in the filtrate exceeds Tm.
