Loop of Henle Flashcards
1
Q
general considerations 1
A
- loop of Henle is shaped in U and begins at end of PT in outer medullar and descends as thin descending limb (tDLH) toward inner medulla, where it turns 180 degrees and ascends as thin Al (tAL) and then thick (TAL)
- the tALH and TAL are relatively impermeable to water and the TAL reabsorbs approx 25% of filtered Na. TAL dilutes the tubular fluid be reabsorbing Na and Cl without water. decreases tubular osm below 100
- active Na and Cl reabsorption occurring the in the TAL is the solute transport engine, driving and maintaining a countercurrent multiplication of solute concentration difference (gradient) in intersitium surrounding LH and collecting duct, extending from cortex to inner medulla
- there is also urea recycling in the inner medulla that contributes to gradient, especially in dehydration/ hyperosm
- osm of interstitium progressively increases from 600-1200 depending on needs
2
Q
general considerations 2
A
- reabsorptive solute transport in the TAL is essential for:
- diluting the tubular fluid osm to values less than plasma and this excreting a dilute urine when volume is expanded or hypoosm
- concentrating the tubular fluid osm in the collecting duct by maintaining a gradient of interstitial osm driving reabsorption of water from collecting duct back into circulation and concentrating the urine when the plasma volume is contracted/hyperosm
3
Q
general considerations 3
A
- the osm of the tubular fluid in the diluting segment of the nephron (tALH, TAL, DCT) is always less than the osm of the plasma, no matter what. the segment is insensitive to ADH and remains water impermeable
- gradient of osm extending from cortex to inner medulla in the medullary interstitium is always present, but gradient is less steep in diuresis (hypervolemic, hypoosm) than antidiuresis (hypovolemic, hyperosm)
- the controlling variable determining excretion of a dilute or concentrated urine is the circulating level of ADH, which modulates the water permeability of the collecting duct and the reabsorption of water from the collecting duct back into circulation when the plasma volume is contracted and/or hyperosm and less water is returned to the circulation when the plasma volume is expanded and/or hypoosm
4
Q
ADH
A
- secreted by post pit in response to changes in plasma osm
- increased osm increases ADH
- ADH increases water perm in collecting duct
- allows the osm equilibration of tubular fluid with the interstitium and the peritubular vasculature surrounding the collecting duct
- in presence of ADH, more water back into circulation, and concentrated uring
- no ADH, duct is impermeable to water, preventing reabsorption
5
Q
thin descending limb
A
- low perm to solutes
- high perm to water
- passive tubular fluid- interstitium equilibration of solute and water concentrates tubular fluid as it descends
6
Q
thin ascending limb
A
- passive NaCl reabsorption
- relatively impermeable to water
- passive urea secretion
7
Q
thick ascending limb
A
- active NaCl reabsorption via Na/K/2Cl symporter
- relatively water impermeable
- generates and maintains a 200 osm/l difference in osm between tubular fluid in lumen (low) and interstitium (high)
8
Q
transport mechanisms mediating NaCl reabsorption in the TAL
A
- 25% of Na and 20% of K
- Na/K/2Cl transporter at luminal membrane
- at basolateral membrane-ion specifiv channels mediate efflux of Cl and K and Na/K ATPase mediates efflux of Na
- K channels in luminal membrane as well
- lumen positive potential (from K out and Cl out basolateral)
- potential is a DF for paracellular Na reabsorption
- Na/K/2Cl inhibited by loop diuretics
- solute transport across the TAL generates the 200 mosm/L gradient of osm from lumen to interstitium
- small amounts of HCO3 reabsorbed too
9
Q
countercurrent multiplication of solute concentration difference- role of active Na reabsorption
A
- active reabsorption of Na and Cl is the solute transport engine maintaining countercurrent gradient
- TAL reabsorbs or pushes solute out of the tubular fluid against a gradient of 200 mosm/L, creating a higher osm outside the TAL in the interstitium
- Where does the huge gradient come from? - loop of Henle generates and maintains a large osm gradient by countercurrent amplifying the transport capacity of the TAL to pump solute against a 200 mosm/L gradient
10
Q
generation of high interstitial osm by countercurrent multiplication
A
- stepwise process for understanding countercurrent multiplication begins with equivalent interstitial and tubular fluid osm in the descending and ascending limbs. 5 cycles with 2 steps per cycle
1. a pumping (reabsorption) of solute out of the ascending limb into the interstitium to a limiting osm gradient of 200 mosm/L and instant osmotic equilibration with the tubular fluid in the opposing descending limb
2. an axial shift of fluid forward along the tubule and instantaneous equilibration of tubular fluid in the descending limb and interstitium (because 300 mosm comes from PT) - then depending on fluid status, the collecting duct is equilibrated with the interstitium, which can drive water out if gradient is very large (volume contracted) or a little water out if not too large
- see notes/picture
11
Q
urea recycling
A
- tubule to interstitium to tubule
- contributes to generation and maintenance of interstitial gradient
- 1200 during antidiuresis to 500 during diuresis
- difference is due to difference in urea present in the interstitium (600 during anti and less than 100 during diuresis)
- in outer medulla, interstitial osm due to NaCl
- inner medulla during antidiuresis, 50% solute is Nacl (300mM), and 50% is urea (600mM) (both 600 mOsm/L)
12
Q
urea recycling steps
A
- reabsorption of urea from tubular fluid of inner medulla by collecting duct into inner medullary interstitium
- secretion of urea from interstitium to tubular fluid of the tDLH and tALH
- delivery of urea in the flow of tubular fluid from the tDLH and the tALG to the inner medullary collecting duct, where it is reabsorbed again
- this occurs in the presence of ADH/ volume contraction/ hyperosm
- when ADH, water out of collecting duct and concentrates urea, which creates and outward gradient that urea can then flow down and be reabsorbed, then recycled again (600 mM in inner medulla)
- no ADH- no water or urea, stays in tubule for excretion (less than 100 mM in inner medulla)
13
Q
concentrations
A
- during antidiuresis is calculated by Na concentration (300), Cl concentration (300), and urea (600)= 1200
- during diuresis is Na plus Cl- 600 total
14
Q
vasa recta
A
- blood supply in the renal cortex and medulla is segregated, which allows solutes reabsorbed from the PT in the cortex to rapidly re enter circulation and exit the kidney in the renal vein
- the blood supply surrounding loop in medulla is vasa recta
- specialized vascular anatomy facilitating the countercurrent exchange of solutes between vasa recta and interstitium, preserving the cortico-medullary gradient and preventing wash out of solute from medulla
- slower rate of blood flow through the vasa recta
- peritubular and surrounds loop of henle
15
Q
vasa recta 2
A
- curved to get both types of exchange
- maintains gradient created by loop
- permeable to water and small solutes
- PASSIVE exchange
- as blood comes down (iso-osmotic), solutes move in and water moves out
- as blood goes back up (hyperosmotic), solutes move out and water moves in
- allows blood osm to get back to normal
