Loop of Henle Flashcards
What % of nutritionally important substances are reabsorbed?
100%.
The fluid that leaves the proximal tubule is isosmotic with plasma i.e. 200mOsm/L. How?
This is because all solute movements are accompanied by equivalent water movements, so that osmotic equilibrium is maintained.
What is the maximum conc. of urine that can be produced by the kidney (most/L)?
1200-1400mOsm/L i.e. 4x more concentrated than plasma.
What amount of waste products need to be excreted every day (mOsm)?
600mOsm.
How much water (ml) is required to excrete 600mOsm of water?
500ml.
What is the minimum conc. of urine that can be produced by the kidney (most/L)?
30-50mOsm/L i.e. 10x more diluted than plasma.
What are the critical characteristics that allow the loops of Henle to act as counter-current multipliers?
- The ascending limb of the loop of actively co-transports Na+ and Cl- ions out of the tubule lumen into the interstitium but is impermeable to water.
- The descending limb is freely permeable to water but relatively impermeable to NaCl.
Describe (in steps) the loops of Henle acting as counter-current multipliers?
- Loop of Henle is filled with 300mOsm/L of static fluid, i.e. isosmotic with plasma.
- As NaCl is pumped out of the ascending limb, conc. of the limn falls while conc. of the interstitium rises. This occurs until a limiting gradient of 200mOsm is established.
- Descending limb is now exposed to greater osmolarity in the interstitium, so water will move out to equate it (water follows NaCl).
Water is immediately absorbed into the vasa recta (due to increased pressure caused by presence of the water) so the osmolarity is not diluted in the interstitium.
What does the counter-current multiplier achieve?
- Concentrates fluid on the way down and promptly re-dilutes it on the way back up,
- 15-20% of the initial filtrate (up to 36L) is removed from the loop of Henle.
- Fluid which enters the distal tubule is more dilute than plasma.
- Delivers hypotonic fluid to the distal tubule.
Explain how the specialised arrangement of the vasa recta benefits the counter-current mechanism.
If capillaries drained straight through, they would carry away the NaCl removed from the loop of Henle and abolish the gradient.
This does NOT happen because they are arranged as hairpin loops.
(Vasa recta, like all capillaries, are freely permeable to water and solutes.)
Name the functions of the vasa recta.
- Provide O2 for medulla.
- In providing O2, they must not disturb the gradient.
- Remove volume from the interstitium.
How do the vasa recta equilibrate with the osmolarity of the interstitium?
The flow rate is very low so there is plenty of time to equilibrate and make sure the medullary gradient is not disturbed.
What causes fluid to be driven into the vasa recta?
Because of high oncotic pressure, and high pressure due to the tight renal capsule, Starling’s forces favour reabsorption.
What controls the permeability of the collecting duct?
Anti-diuretic hormone (ADH) AKA vasopressin = posterior pituitary hormone.
The collecting duct is the site of water regulation. Urine arrives at 100mOsm/L. Whether the urine is then concentrated more/less depends on the presence or absence of ADH.
