Loop of Henle Flashcards
What occurs at the proximal tubule?
- major site of reabsorption
- 65-75% of all NaCl and H2O
(all nutritionally important substances)
What protein in particular can get filtered?
Albumin
What happens to the albumin that is filtered?
Completely reabsorbed by a Tm carrier mechanism in the proximal tubule.
How are many drugs which are nonpolar and highly lipid soluble removed?
- removal of H2O in the proximal tubule would establish concentration gradients for their reabsorption
- thus the liver metabolises them to polar compounds, reducing their permeability and facilitating their excretion.
What type of fluid leaves the proximal tubule?
The fluid is isoosmotic with the plasma (300mOmles/l)
Why is the fluid leaving the proximal tubule isomotic with the plasma?
- all solute movements are accompanied by equivalent H2O movements
- osmotic equilibrium is maintained
Where do all the nephrons have their proximal and distal tubules?
In the cortex of the kidney
What do all nephrons share?
Common processes for the reabsorption and secretion of solutes of the filtrate
What is a special system essential for water balance attributable to?
The loops of Henle of juxtamedullary nephrons
What is the maximum concentration of urine the kidney can produce?
- 1200-1400mOsmoles/l
- 4x more concentrated than plasma (excess of solute over water)
What amounts to 600mOsmoles daily?
The urea, sulphate, phosphate and other waste products and non waste ions (Na and K)
What does the ‘waste’ that must be excreted require?
A minimum obligatory H2O loss of 500mls
How can one urinate to death?
If the kidneys are functioning but there is no H2O intake
What is the minimum [urine]?
30-50mOsmoles/l
When will the minimum [urine] be produced?
In conditions of excess H2O intake
Why is the kidney able to produce urine of varying concentration?
The loops of Henle of juxtamedullary nephrons act as counter-current multipliers
What is counter current?
Fluid flows down the descending limb and up the ascending limb
What are the critical characteristics of the loops which make them current multipliers are:
- The ascending limb of the loop of Henle actively co-transports Na+ and Cl- ions out of the tubule lumen into the interstitium. The ascending limb is impermeable to H2O.
- The descending limb is freely permeable to H2O but relatively impermeable to NaCl
What does the countercurrent multiplier achieve?
concentrates fluid on the way down and promptly re-dilutes it on the way back up by removing NaCl not by adding H2O
What is the overwhelming significance of the countercurrent multiplier?
It creates an increasingly concentrated gradient in the interstitium
What does the countercurrent multiplier deliver to the distal tubule?
Hypotonic fluid
What is the osmolarity of fluid that enters the countercurrent multiplier?
300mOsm
What is the osmolarity of the fluid that leaves the countercurrent multiplier?
100mOsm
What gradient exist within the countercurrent multiplier?
- 200 mOsm gradient exists at any horizontal level
- its effect is multiplied by the countercurrent flow
What is the countercurrent multiplier all about?
- concentrating medullary interstitium
- delivering hypotonic fluid to distal tubule
What is the vasa recta?
The specialized arrangement of the peritubular capillaries of the juxtamedullary nephrons
What does the vasa recta participate in?
The countercurrent mechanism by acting as countercurrent exchangers
Why do the medullary capillaries not drain straight through abolishing the gradient?
They are arranged as hairpin loops and therefore do not interfere with the gradient
Why do the vasa recta equilibrate with the medullary interstitial gradient?
The vasa recta are freely permeable to H2O and solutes
What are the functions of the vasa recta?
- Provide O2 for medulla.
- In providing O2 must not disturb gradient.
- Removes volume from the interstitium, up to 36l/day.
What is the balance of Starling’s forces in favour of?
reabsorption because of high osmotic pressure, and high tissue pressure due to tight renal capsule which drives fluid into capillaries.
Why is the flow rate through the vasa recta very low?
So that there is plenty of time for equilibration to occur with the interstitium, further ensuring that the medullary gradient is not disturbed.
Where is the site of water regulation?
The collecting duct
What is the permeability of the collecting duct under control of?
Anti-diuretic hormone (ADH aka. vasopressin)
What produced ADH?
Posterior pituitary
Whether or not the dilute urine delivered to the distal tubule is concentrate and to what extent depends on…
The presence or absence of ADH
What is the key step in the counter current multiplier system?
Active removal of NaCl from ascending limb
What happens in the counter current multiplier system as NaCl is removed from the ascending limb?
The osmolarity decreases and increases in the interstirium
What does removal of NaCl from the ascending limb mean for the descending limb?
- It is now exposed to a greater osmolality in the interstirum
- H2O will move out of the descending limb to equate osmolality
What happens to the H2O what moves into the interstitium in the counter current mechanism?
It does not stay in the interstitium but is reabsorbed by the high osmotic pressure and tissue pressure into the vasa recta (Starling forces)
Where does fluid enter and exit the counter current system?
Enters at the proximal tubule and exits via the distal tubule
What does the concentrated fluid in the descending limb of the counter current mechanism do?
Rounds the bend and delivers a [high] to the ascending limb
What does increasing concentration of the descending limb in the counter current system mean?
Greater concentration of interstitium by addition of salt from the ascending limb
How is the fluid in the counter current system progressively concentrated?
The fluid in the tubule is progressively concentrated as it moves down the descending limb and progressively diluted as it moves up the ascending limb.
What does the vertical gradient in the interstitium go from?
300 to 1200 mOsmol
What happens to the counter current system when frusemide is used?
- active transport of NaCl out of the ascending limb is abolished
- all concentration differences are lost
- kidney can only produce isotonic urine
