Flashcards in Lect 11: Loop of Henle Deck (34)
The LoH is shaped like a U and it begins at the end of the PT in the cotex and descends as the thin descending limb toward the inner medulla
where it turns 180 degrees and ascends as the thin ascending loop (tALH), back toward the outer medulla, where the tubule epithelia thickens and becomes the thick ascending loop
tAL & TAL are IMPERMEABLE TO WATER (but it allows reabsorption of solutes; Na) and the TAL
reabsorbs 25% of the filtered Na. Therefore the TAL dilutes the tubular fluid but reabsorbin Na and Cl w/o reabsorbing water. WATER reabsorption IS NOT FOLLOWING SOLUTE reabsorption HERE. It DILUTES the fluid in the TAL. This decreases the tubular fluid osmolarity below 100mOs/L.
The TAL is called "the diluting segment" for this reason
"solute transport engine" refers to
the active Na and Cl reabsorption occurring in the TAL and maintaining a counter-current multiplication of solute concentration difference or solute conc gradient, in the interstitium (not in TF) surrounding the LH and the collecting duct, extending from the cortex to the inner medulla.
In addition to the active reabsorption of Na and Cl, another process occurs in the TAL,
2. UREA RECYCLING occurs in the inner medulla also contributes to the solute conc gradient in the interstitium, especially when the need arises to defend against plasma volume depletion (dehydration) &/or hyperosmolarity to excrete conc urine.
UREA recycling occurs when you are dehydrated; this process adds an additional osmotic portential or an additional conc of solute as urea in the inner medulla
Thus, osmolaity of the medullary intersititium (outside the tubule!!!!) progressively
incerases upon descending to the inner medulla and can achieve a maximum value in the inner medula varying betw 600-1200mOsm/L which is 2-4 fold the osmolarity of plasma (~300).
600 when you are over-hydrated (bc urea goes away into the interstitium)
1200 when you are under-hydrated
This value depends on the presence or absence of urea
in this unique instance, water reabsorption does not follow solute reabsorption and therefore
the epithelia of the TAL effectively dilutes the tubular fluid by reabsorbing Na and Cl w/o reabsorpbing water. This decreases the tubular fluid osmolarity
increased excretion of urine with a low osmolarity and low volume
increased excretion of urine with a high osmolarity and small volume
Reabsorptive solute transport in the TAL is essential for 1.
diluting the tubular fluid osmolarity to values less than plasma thereby excreting a dilute urine when the plasma is expanded or hypo-osmotic
Reabsorptive SOLUTE transport in the TAL is essential for 2
concentrating the tubular fluid osmolarity in the collecting duct by maintaining a gradient of interstitial osmolarity driving reabsorption of water from the COLLECTING DUCT back into circulation (not into the TAL) and conc the urine when the plasma volume is contracted and/or hyper-osmotic. The (corticomedullary osmotic) gradient is set up by the thick and thin ascending limb. But the collecting tubule is sensitive to ADH which opens to door to water reabsoprtion in response to this osmotic gradient outside the c.d. Sucks water out of the c.d into the interstitium and into the peritubular capillary and c.d. then into the blood. This is how the collecting duct allows you to retain water in excess of solute when you are dehydrated and you need to defend against volume contraction & hyperosmolarity
=works on the collecting duct top increase its water permeability
=is secreted in response to changes in plasma osmolarity and volume (decr volume, incr osmolarity). it opens the door for water permeability
Increased plasma osmolarity
increased ADH secretion and vice versa. ADH incr the water permeability of the collecting duct allowing the osmotic equilibration of tubular fluid with the intersitium and the peritubular vasculature surrounding the collecting duct.
in the presence of ADH,
osmotic equilibration of the tubular fluid permits water reabsorption back into circulation and excretion of a conc urine.
In the absence of ADH
the collecting duct is impermeable to water preventing water reabsorption and therefore osmotic equilibrium of the tubular fluid with the interstitium and the peritubular vasculature surrounding the collecting duct does NOT oocur, permitting excretion of a dilute urine
The osmolarity of the TF in the diluting segment of the nephron (tAL, DCT) is always less than the
that is less than plasma osmolarity in the presence of ADH, regardless of whether volume expanded or contracted (100-120<300). The osmo of the urine that you excrete will depend on the activity of ADH, WHICH MODULATES THE COLLECTING DUCT. (When no ADH present that 100mOsm TF will be excreted as 100mOsm TF urine. When under-hydrated and hv hyperosmotic plasma, ADH is around and that TF changes from 120 to 1200 mOsm due to ADH induced water reabsorption).
More water is returned to the plasma (high ADH)
when the plasma volume is contracted or hyperosmotic and less water is returned to the circulation when the plasma volume is expanded and/or hypoosmotic
The gradient of osmolarity extending from cortex to inner medulla in the medullary intersititium is always present, in the presence or absence of ADH,
regardles of volume contraction or expansion. But the gradient is less steep in diuresis (volume expanded, decr, plasma osmol) than in antidiuresis (volume contracted, decr)
So the controlling variable determining the excretion of a dilute or conc urine is a circulating level of ADH which modulates
the water permeability of the collecting duct and the reabsorption of water from the c.d. back into circulation.
Cell physiology of Thin descending loop
-low permeability to solutes (NaCl, urea)
-HIGH WATER PERMEABILITY, so it is concentrating it
-PASSIVE tubular fluid - interstitium equilibration of solute and water concentrates the tubular fluid as it descends into the tDLH this then ascends into the tALH. So at the bottom of the tDLH you have a high conc of NaCl in the TF. Then it turns to enter the tALH.
Thin ascending limb
1. passive NaCl reabsorption (outward gradient, losinfgsolute)
2. WATER IMPERMEABLE...so water does not follow...so if you are losing solute in excess of water you are DILUTING the TF.
3. passive urea secretion
Thick ascending limb - solute transport engine
-ACTIVE (change from passive) NaCl reabsorption via Na/K/Cl symporter - solute absorbing engine
-relatively water impermeable
-generates and maintains 200mOsm difference in osmolarity between the tubular fluid in the lumen and the interstitium (high) and this transport of solute is multiplies buy a countercurrent process to convert the 200mOs gradient of solute into a 300-1200mOsm gradient in the interstitium.
Background: As the TF descends in the thin descending loop, due to its relatively low permeability to solutes and high permeability to water, osmotic equilibration of the TF
with the increasingly hypertonic interstitium results in water reabsorption with little solute reabsorption. ==>progressive conc of Na and Cl in the TF
As the TF ascends in the thin ascending limb of LH, passive Na and Cl efflex from the TF occurs (NaCl reabsorption) driven by
high TF Na and Cl conc created by the preceding thin descending limb.
==>tAL is water impermeable so passive efflux of Na and Cl from the TF w/o efflux of water effectovely dilutes and decreases the TF osmolarity in the thin ascending limb
In the outer medilla thin becomes thick ascending limb and the transport changes from passive to ACTIVE Na reabsorption. (You reabsorb about 25% of NaCl filterered)
Both thick and thin are impermeable to water and the NaCl reabsorption occurring in the thick dilutes and decreases the TF osmolarity as it advances toward the distal tubule.
IMPORTANT: The TAL generates & maintains a 200mOsm difference in osmolarity between the TF in the lumen and intersititium
This function is multiplied by a counter-current process to convert the 200mOs gradient of solute to a 300-1200 gradient in the interstitium
is essential to creating the "counter-current multiplication" of intersititial solute conc and interstitial solute conc gradient extending from cortex to medulla
Transport of NaCl Reabsorption in the TAL 1.
Transport for reabsorption of K, Na and Cl at the lumen is a Na-K-Cl symporter.
==>At basolateral side is channels that mediate efflux of intracellular K, Cl (Na uses the Na/K ATPase
Transport of NaCl Reabsorption in the TAL 2. Potassium channels
The presence of K channels in the lumenal membrane also mediates efflux of intracellular K back into the TF which together with Cl efflux across the basolateral membrane generates a lumen positive difference across the TAL tubular epithelium.
The lumen positive potential difference serves as a driving force for
paracellular Na reabsorption across the TAL epithelium by paracellular diffusion
inhibits the Na/K/2Cl cotransporter which decreases the reabsorption of Na, k and Cl and increase excretion of Na, k and Cl in the urine and plasma fluid. Solute transport across the TAL generates a 200mOsm gradient across the epithelia from lumen to intersititium