Loop of Henle Flashcards
What is the function of the proximal tubule?
Major site of reabsorption
65-75% of NaCl and H2O
How much protein (particularly albumin) gets though tubule?
20g/day so 0.5% of total amount presented at glomerulus
Completely reabsorbed by a Tm carrier mechanism in proximal tubule
Describe drugs and pollutants and tubular function
Many are non polar and high lipid soluble so removal of H2O in proximal tubule establishes conc. gradients for their reabsorption
Liver metabolises them polar so reduces their permeability and facilitates the excretion
What is the fluid described as when it leaves the proximal tubule?
Isosmotic with plasma - 300mOmoles/l
Why is the fluid leaving the proximal tubule isosmotic with plasma?
All solute movements are accompanied by equivalent H2O movements so osmotic equilibrium is maintained
What process is attributable to the loops of Henle of juxtamedullary nephrons?
Essential for water balance
Where are the nephrons distal and proximal tubules?
In cortex
All nephrons have common processes for reabsorption and secretion of solutes of filtrate
What does the mechanism of the loop of Henle ensure?
The kidney can produce concentrated urine in times of H2O deficit
What is the max. conc. of urine that can be produced by the human kidney?
1200-1400mOsmoles/l so 4 x more concentrated than plasma as excess of solute over water
What is the minimum obligatory H2O loss and why?
500mls
As this is required to excrete 600mOsmoles of waste products (urea, sulphate and phosphate) and non-waste ions which must be excreted
What happens if no H2O intake?
Functioning kidneys still excrete 500mls so urinate to death
What is the min. conc. of urine?
30-50mOsmoles/l so 10 fold dilution compared with plasma
In conditions of excess H2O
How are kidneys able to produce urine of varying conc.?
Loops of Henle of juxtamedullary nephrons act as counter-current multipliers
What is counter current?
Fluid flows down descending limb and up the ascending limb
What are the critical characteristics that make loops counter-current multipliers?
Ascending limb actively co-transports Na and Cl ions out of tubule lumen into interstitium
Ascending limb impermeable to H2O
Descending limb is freely permeable to H2O but relatively impermeable to NaCl
What happens after loop of Henle is filled with stationary isosmotic fluid (300mOsmoles/l)?
Active removal of NaCl from ascending limb - is put into interstitium so osmolarity in tubule decreases and increases in interstitium
What happens when the descending limb is exposed to greater osmolarity in interstitium?
H2O will move out to equate the osmolarity
H2O does not stay in the interstitium but is reabsorbed by high osmotic and tissue pressure into vasa recta
What happens when fluid is moving in the loop of Henle?
Fluid is entering at proximal and leaving at distal
Concentrated fluid in descending limb rounds the bend and delivers a high conc. to ascending limb - active NaCl removal which further concentrates the interstitium
What does greater conc. of descending (by removal of water) limb mean?
Greater conc. of interstitium by addition of salt from ascending limb
Where in the tubule is the conc. of fluid more and progressively diluted?
More concentrated - as moves down descending limb
Diluted - moves up ascending limb
At horizontal level what is the mOsmol gradient between ascending limb and interstitium?
200mOsmol
As 100 in ascending and 300 in interstitium
What happens when more and more concentrated fluid is delivered to ascending limb?
The interstitium becomes more and more concentrated
What is the vertical gradient in the interstitium?
300 to 1200mOsmol
What is the key step in the loop of Henle?
Active transport of NaCl out of ascending limb
If abolished then (ex. by use of diuretic frusemide) all conc. differences are lost and kidney produces isotonic urine
What has the counter-current multiplier achieved?
Concentrates fluid on way down and dilutes on way up, by removing NaCl
Consequence - 15-20% of initial filtrate is removed from loop of Henle
Fluid which enters tubule is more dilute than plasma
What is the significance of the counter-current multiplier?
Creates an increasingly concentrated gradient in the interstitium
What does the counter-current multiplier deliver to the distal tubule?
hypotonic fluid
What is the main importance of the counter-current multiplier?
All about concentrating the medullary interstitium
And delivering hypotonic fluid to the distal tubule
What is the vasa recta?
Specialized arrangement of peritubular capillaries of juxtamedullary nephrons
How does the vasa recta participate in the counter-current mechanism?
By acting as counter-current exchanges
They are arranged as hairpin loops and therefore don’t interfere with gradient
Vasa recta are freely permeable to H2O and solutes so equilibrate the medullary interstitial gradient
What are the functions of the vasa recta?
Provide O2 for medulla
In providing O2 must not disrupt gradient
Removes volume from interstitium - up to 36l/day
How are the Starling’s forces in favour of reabsorption in vasa recta?
High osmotic pressure and high tissue pressure due to tight renal capsule which drives fluid into capillaries
What is the flow rate of vasa recta?
Very low so there is plenty of time for equilibration to occur with interstitium and does not disturb medullary gradient
Where is the site of water regulation?
Collecting duct - whose permeability is under control of ADH
What does the presence of ADH determine?
Whether or not the dilute urine delivered to the distal tubule is concentrated and to what extent
