LOOP OF HENLE TRANSPORT
A. Roles
1. Establish the appropriate osmotic environment in the renal
medulla (hyper-osmotic, hypertonic) for the adjustment of fluid balance
2. Provide very dilute (hypo-osmotic, hypotonic) fluid
to the distal tubule
B. Solute and Water Transport
1. descending limb: freely permeable to
water, less permeable to Na+, Cl-, no active transport
2. ascending thin limb: freely permeable:
Na+, Cl-, impermeable to water, no active transport
3. ascending thick limb (& first part
of distal tubule): active reabsorb (transcellular): Na+, Cl-
(also some K+), passive reabsorb (paracellular): Na+,
K+, Ca2+, Mg2+,
impermeable to water
C. Countercurrent Multiplication
1. Process
a. The active reabsorption
of Na+ and Cl- in the ascending thick limb of juxtamedullary
nephrons combined with the thick limb impermeability to water results in
an increased osmolality in the renal medulla.
b. The increased
medullary interstitial osmolality draws water from the descending thin limb,
progressively concentrating the fluid remaining in the tubular lumen.
c. As this fluid
passes around the hairpin turn and flows into the ascending limb, it loses
NaCl into the interstitium by passive diffusion in the ascending thin limb
and by active transport in the ascending thick limb.
d. As a result,
interstitial fluid throughout the whole medulla becomes hyperosmotic and
the fluid leaving the ascending thick limb and entering the distal tubule
is hypoosmotic.
2. Because this process occurs as the fluid
is flowing along the loop of Henle and exchange takes place between the interstitium
and fluid streams moving in opposite directions (descending and ascending)
and because osmolality increases progressively with depth in the medulla,
the mechanism is termed countercurrent multiplication.
3. The interstitial osmolality is further
increased by the accumulation of urea reabsorbed from the distal tubule and collecting duct and trapped in the medulla).
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LEGEND
PT = Proximal Tubule
TAL = Thick Ascending Limb
DT = Distal Tubule
CnT = Connecting tubule
CCT = Cortical Collecting Tubule (collecting duct)
MCT = Medullary Collecting Tubule (collecting duct)
VR = Vasa Recta
Note: Numbers represent total
osmolal concentration in mOsm/kg |
D. Consequences of Countercurrent Multiplication
1. Water is reabsorbed from the loop of
Henle at the rate of about 6 ml/min (8 L/day); this represents 20% of the
water entering the loop of Henle
2. About two-thirds of the Na+
and Cl- entering the loop of Henle are reabsorbed (some
of the NaCl enters the vasa recta capillaries and is returned to the circulation
but some remains trapped in the renal medulla, raising the medullary osmolality)
3. The fluid leaving the loop of Henle has
an osmolality of 100 mOsm/kg (one-third of blood plasma)
4. The osmotic concentration at the tip
of renal papillae can become as high as 1200-1400 mOsm/kg, about half from
NaCl and half from urea (this is about the same osmolality
as sea water)
E. Role of the Vasa Recta (capillaries paralleling
the loop of Henle)
1. Vasa recta vessels resemble other capillaries
in their high permeability to sodium, chloride, urea, and water
2. Blood in the vasa recta capillaries entering
the medulla loses water and gains NaCl and urea progressively as it descends
3. When blood flow in the vasa recta capillaries
reverses direction and ascends from the medulla to the cortex, it progressively
gains water and loses NaCl and urea, resulting in countercurrent exchange
4. This anatomical arrangement, combined
with the relatively low blood flow to the medulla, permits sufficient perfusion
for nutrition without destroying the high osmolality of the medulla