3. Loop of Henle Flashcards
what is the main function of the proximal tubule
major site of reabsorption - 65-75% of all NaCl and H20
what happens to proteins that get through the glomerulus
Completely reabsorbed by a Tm carrier mechanism in the proximal tubule
why would some pollutants/drugs not naturally be removed
lipid soluble - would just pass through lipid membrane - could never be removed
removal of H20 in the proximal tubule would establish concentration gradients for their reabsorption
how are pollutants/drugs ultimately able to be removed
the liver metabolises them to polar compounds - reduces permeability - facilitates their excretion - will stay where kidneys put them
what occurs at the loop of henle
reabsorption
NO excretion
what are the components of the loop of henle
a descending limb and an ascending limb
what is the osmolarity of the fluid that leaves the proximal tubule
300 mOmoles/L = isosmotic to plasma
why is the fluid isosmotic as it laves the proximal tubule
because all the solute movement are accompanied by equivalent H20 - i.e. osmotic equilibrium has been maintained
what does the mechanism in the loop of henle allow with regards to urine concentration
enables kidney to produce concentrated urine in times of H20 deficit - i.e. can reabsorb more water to protect the body - excreting less water per molecule of substance excreted
what is the maximum concentration of urine that can be produced by the human kidney
1200-1400mOsmoles/l
ie 4x more concentrated than plasma = excess of solute over water
what is the minimum obligatory H20 loss per day
500mls
why is there a minimum obligatory H20 loss per day
certain substances MUST be excreted each day - urea, sulphate, phosphate, other waste products, non-waste ions (Na+ and K+)
what would happen to the minimum obligatory H20 loss if there was no H2o intake
NO CHANGE - as long as kidneys functioning, volume will be excreted even if no H2O intake
what would happen in conditions of excess H2) intake
H2O is excreted in excess of solute -
minimum [urine] in man is 30-50 mOsmoles/l ie 10 fold dilution compared with plasma.
what enables the kidneys to produce urine of varying concentration
the loops of henle act as COUNTER-CURRENT MULTIPLIERS
what are the permeabilities of the DESCENDING limb of the loop of henle
freely permeable to H20
impermeable to NaCl
what are the permeabilities/actions of the ASCENDING limb of the loop of henle
actively co-transprot Na+ and Cl- ions out of the tubule lumen
impermeable to H2O - no aquaporins
what does the impermeability to water of the ascending limb mean for osmolarity
creates a large osmotic effect
if starting with 300mOsm/l of stationary fluid in loop, what is the first thing that occurs to affect osmolarity
NaCl actively pumped out of the ascending limb - causes concentration to fall in tubule - concentration in interstitium rises - occurs until limiting gradient of 200mOsm is established
what happens to the osmolarity of the tubule as NaCl is removed
as concentration decreases - osmolarity decreases
what happen to the osmolarity of the interstitium as NaCl is added
as concentration increases - osmolarity increases
what is the effect on the descending limb of the increased osmotic effect in the interstitium
H20 moves out to equate osmolarity
what happens to H2O once in the interstitium
reabsorbed by the vasa recta (veins following the loop) by the high osmotic pressure and tissue permeability
what does this reabsorption of water into the vasa recta enable
osmotic gradients to be maintained to further remove NaCl and Water out of their respective parts of the loop
fluid is constantly moving through the loop - what is the concentration of the fluid arriving from the descending limb
high concentration fluid arrives at the ascending limb - due to water removal at the descending limb
what does this high concentration fluid arriving at the ascending limb promote
active removal of NaCl
what does the active removal of NaCl at the ascending limb promote
further concentration of interstitium - increasing osmolarity as body tries to maintain 200mOsm gradient
what happens to the fluid concentration as it moves through the tube
progressively concentrated as it moves down the descending limb
progressively diluted as it moves up ascending limb
what is the overall affect on interstitium concentration as concentrated fluid is delivered to the ascending limb
interstitium become more and more concentrated
what is the gradient at any horizontal level between the ascending limb and interstitium
200mOsmol
what does the vertical gradient in the interstitium go from
300-1200mOsmol
summary of counter current system and osmolarities along the loop - start with fluid at 300 as it exits proximal tubule
start with 300
- starts to concentrate up to 1200 down descending tubule (by removing water)
- then goes down from 1200 to 100 in ascending tubule (by removing salt)
- ends up with fluid that contains less salt
what happens if the active transport of NaCl out of the ascending limb is stopped e.g. by frusemide
all concentration differences are lost and the kidney can only produce isotonic urine:
can’t transport Na out in AL - can’t concentrate interstitium - can’t make osmotic effect to draw water out DL - produces isotonic (300) urine
what percentage of the initial filtrate is removed from the loop of henle
15-20%
what is the osmolarity of the fluid that enters the distal tubule from the loop of henle
hypotonic fluid - ie more dilute than plasma
fluid enters loop at 300 and leaves at 100 = hypotonic
what are the vasa recta
specialised arrangement of peritubular capillaries - veins that follow the loop of henle
what is the importance of the vasa recta following the loop
means gradient is maintained as water is reabsorbed by veins as it goes along
how do the vasa recta maintain/mediate the interstitial gradient
are freely permeable to both H20 and solutes - equilibrate at any given level with osmolarities
what moves in and out if the vasa recta at the DL
Na in
H2O out
what moves in and out of the vasa recta at the AL
H20 in
Na out
what are the 3 main functions of the vas recta
- provide O2 for the medulla
- does not disturb/maintains gradient
- removes volume from the interstitium
why is the flow rate through the vasa recta very low
to allow plenty of time for equilibration to occur with the interstitium
what is the site of water regulation in the nephron and what regulates it
the collection duct - regulated by ADH (vasopressin)
more ADH = less fluid lost
