: NaCl (medullary collecting duct is slightly permiable to urea and water even in absence of ADH)
if ADH present
: permiable to water
impermiable to
: water (unless ADH present), urea
transported
: NaCl (out of tubule)
tonicity of tubule
: hypotonic (100 mOsm by end of distal tubule, about 150 mOsm by end of medullary collecting duct)
if ADH present
: starts hypotonic but increases in tonicity, becoming isotonic through the cortical collecting duct and markedly hypertonic (up to 1200 mOsm) in the medullary collecting duct
tonicicty of interstitium
: isotonic (300 mOsm) through distal tubule and cortical collecting duct, becoming hypertonic through the medullary collecting duct: hypertonic (increasing to 600 mOsm or even 1200 mOsm if ADH is present)
flow
: NaCl actively transported out of tubule throughout; in medullary collecting duct, there is also slight passive movement of urea into tubule and water out of tubule
H2O and urea permeable, Na impermeable; No active transport.
Ascending Limb:
Thin -
H2O impermeable, Solute permeable. Passive NaCl and perhaps urea reabsorption.
Thick -
H2O impermeable, Solute permeable. Active NaCl reabsorption
Early Distal Tubule:
the diluting segment; Minimal to No H2O reabsorption. Active Na reabsorption.
Late distal Tubule and Collecting Ducts
: H2O Permeability is ADH Dependent.
ADH Ý H2O permeability.
Driving force is the interstitial gradient
Urea reabsorption occurs in the outward direction in the inner medullary collecting tubule. (ADH Dependent)
Electrogenic Na+ reabsorption (prevented by amiloride/ enhanced by aldosterone)
ADH absent
: Minimum osm = 50-100 mOsm/kg H2O
ADH present:
Maximum osm = 1100-1200 mOsm/kg
Countercurrent Multiplier
Mechanism by which the Ascending Limb of LoH actively creates the hypertonic interstitium which produces a hypertonic end urine
Isotonic fluid enters from the proximal tubule
Ultrafiltrate becomes more concentrated as it flows down Descending limb and loses H2O to the interstitium
This action is a consequence of the hypertonic interstitium created by the the tranport of solute out of the ascending Limb
Fluid moving up the Ascending Limb eventually becomes hypotonic due to the step wise removal of Solute
The horizontal gradient at any point is small 200 mOsm; the vertical gradient is significant 300
Þ 1200 mOsm.
The small horizontal gradient is Multiplied into a large vertical gradient by Countercurrent flow.
This vertical medullary gradient is necessary to concentrate the tubule fluid flowing through the Collecting Ducts (when ADH is present)
Urea Handling/Recycling
A passive process that contributes to and maintains the inner medullary hypertonic interstitium necessary for maximal [urine].
50% of the 1200 mOsm/kg solute in the papillary tip is urea
50% of filtered urea is reabsorbed in the proximal tubule
Urea enters the TAL and to a lesser extent the TDL of LoH
Net effect is that the quantity of urea in the early distal tubule is > filtered amount.
ADH enhances the Medullary collecting duct’s permeability to urea.
Cortical collecting duct is impermeable to urea.
ADH markedly enhances H2O reabsorption out of the cortical collecting duct thus concentrating urea in the tubular fluid.
Urea reabsorption is indirectly enhanced by active Na reabsorption in the Ascending Limb which creates [NaCl] that drives H2O reabsorption
Urea does not drive H2O reabsorption from the medullary CD;
Highly concentrated Urea in the tubular fluid and medullary interstitium equilibrate to generate both urine and inner medulla with
Ý Osmolality.
The
Ý in medullary interstitial gradient from urea increases H2O reabsorption out of the Descending Limb and passive reabsorption of Na (down its concentration gradient) in the Thin Ascending Limb of the loop of Henle
Vasa Recta/Countercurrent Exchange
A passive mechanism which protects the medullary osmolar gradient while permitting the removal of large quantities of solute and H2O reabsorption from the LoH and CDs. Requires the Following:
(1) Maintenance of medullary mass balance
(Starling forces) by returning reabsorbed NaCl/H2O to the systemic circulation.
Bulk Flow occurs because of favorable Starling Forces (
ß P and Ý p pressures in the Vasa Recta)
Net effect is that Ascending VR flow rate = 2X descending VR (12% of RBF vs 6%)
(2) Maintenance of medullary osmotic gradient
(osmolar forces) because the VR equilibrates with interstitium
Counter Current exchange architecture minimizes loss of solute by diffusion; blood returning to cortex 325 mOsm/kg
Disruption of the mechanism results in impaired ability to maintain the interstitial osmolar gradient; occurs with:
Ý
medullary blood flow Þ washout because the Ascending VR contain more solute and H2O
ß
medullary blood flow Þ ischemia with resulting cell injury and inability to maintain the energy requiring processes