Loop of Henle Flashcards
What is the function of the proximal tubule?
Major site of reabsorption, 65-75% of all NaCl and H2O all nutritionally important substances.
Also reabsorbs some protein, particularly albumin (0.5%)
How are drugs and pollutant excreted?
They are lipid soluble and non-polar, so the liver metabolises them to polar compounds to reduce their permeability and facilitate their excretion.
What occurs in the loop of Henle?
Only reabsorption producing concentrated urine
Describe the fluid that enters the loop of Henle from the proximal tubules
Fluid is isosmotic with plasma (300mOsmoles/l) - this is because all solute movements are followed by equivalent H2O movements to maintain osmotic equilibrium
Where are the proximal and distal tubule located in the nephron?
In the cortex
What is the maximum concentration of urine that can be produced by the human kidneys?
1200-1400mOsmoles/L (x4 more concentrated than plasma)
What is the minimum loss of H2O each day and what causes this loss?
The urea, sulphate, phosphate, other waste products and non-waste ions (Na+ and K+) which must be excreted each day amount to 600 mOsmoles. This requires a minimum obligatory H2O loss of 500mls.
How are the kidneys able to produce urine of varying concentration?
As loop of Henle nephrons act as counter-current multipliers
What are the critical characteristic of the loops which make them counter-current multipliers?
- Descending limb in freely permeable to H2O but impermeable to NaCl
- Ascending limb actively co-transports Na+ and CL- ions out of the tubule lumen into the interstitium (in-between tubule and peritubular capillary) and is impeccable to H2O
Describe the process of reabsorption of H2O and electrolytes throughout the loop of Henle
- Initial isosmotic fluid (300mOsmol/L)
- NaCl is pumped out of the ASCENDING limb, its conc. and osmolarity inside tubule falls and that of the interstitium rises. Establishing gradient of 200mOsmol.
- Descending limb now exposed to greater osmolarity in interstitium, causing H2O to diffuse out , and then is reabsorbed by high oncotic pressure into the vasa recta
- Concentrated fluid in descending limb, delivering [high] to ascending limb, causing NaCl removal and therefore further concentrating the interstitium
- Greater conc. of descending limb (due to H2O removal) means greater conc. of interstitium by addition of salt from ascending limb. Fluid is concentrated down the descending limb and progressively diluted up the ascending limb (by removal of salt)
What is the gradient between the ascending limb and interstitium?
200mOsmol
What is the gradient between the descending limb and the interstitium?
Isosmotic
How do diuretics work?
Inhibits active transport of NaCl out of the ascending limb, causing all concentration differences to be lost, producing isotonic urine.
This is due to deactivation of active transport, so NaCl is not reabsorbed (and thus H2O) and so they are both excreted
What is the importance of the counter-current multiplier?
- Concentrates fluid on the way down and redilutes in the ascending limb (reabsorption of NaCl)
- One consequence of this is that 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 (hypotonic)
What is the vasa recta?
The specialised arrangement of the peritubular capillaries of the juxtamedullary nephrons also participate in the countercurrent mechanism by acting as countercurrent exchangers.
Freely permeable to H2O and solutes. At equilibrium with medullary interstitial gradient)
