Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 38Renal Function & Micturition 655

leading to the normal equilibrium condition, although the steps
do not occur in vivo. It is also important to remember that the
equilibrium is maintained unless the osmotic gradient is washed
out. These steps are summarized in Figure 38–16 for a cortical
nephron with no thin ascending limb. Assume first a condition
in which osmolality is 300 mOsm/kg of H 2 O throughout the
descending and ascending limbs and the medullary interstitium
(Figure 38–16A). Assume in addition that the pumps in the
thick ascending limb can pump 100 mOsm/kg of Na+ and Cl–
from the tubular fluid to the interstitium, increasing interstitial
osmolality to 400 mOsm/kg of H 2 O. Water then moves out of
the thin descending limb, and its contents equilibrate with the
interstitium (Figure 38–16B). However, fluid containing 300
mOsm/kg of H 2 O is continuously entering this limb from the
proximal tubule (Figure 38–16C), so the gradient against which
the Na+ and Cl– are pumped is reduced and more enters the
interstitium (Figure 38–16D). Meanwhile, hypotonic fluid flows
into the distal tubule, and isotonic and subsequently hypertonic
fluid flows into the ascending thick limb. The process keeps
repeating, and the final result is a gradient of osmolality from
the top to the bottom of the loop.
In juxtamedullary nephrons with longer loops and thin
ascending limbs, the osmotic gradient is spread over a greater

distance and the osmolality at the tip of the loop is greater. This is because the thin ascending limb is relatively impermeable to water but permeable to Na+ and Cl–. Therefore, Na+ and Cl– move down their concentration gradients into the interstitium, and there is additional passive countercurrent multiplication. The greater the length of the loop of Henle, the greater the osmolality that can be reached at the tip of the medulla. The osmotic gradient in the medullary pyramids would not last long if the Na+ and urea in the interstitial spaces were removed by the circulation. These solutes remain in the pyramids primarily because the vasa recta operate as countercurrent exchangers (Figure 38–17). The solutes diffuse out of the vessels conducting blood toward the cortex and into the vessels descending into the pyramid. Conversely, water diffuses out of the descending vessels and into the fenestrated ascending vessels. Therefore, the solutes tend to recirculate in the medulla and water tends to bypass it, so that hypertonicity is maintained. The water removed from the collecting ducts in the pyramids is also removed by the vasa recta and enters the general circulation. Countercurrent exchange is a passive process; it depends on movement of water and could not maintain the osmotic gradient along the pyramids if the process of countercurrent multiplication in the loops of Henle were to cease.

FIGURE 38–16 Operation of the loop of Henle as a countercurrent multiplier producing a gradient of hyperosmolarity in the
medullary interstitium (MI). TDL, thin descending limb; TAL, thick ascending limb. The process of generation of the gradient is illustrated as oc-
curring in hypothetical steps, starting at A, where osmolality in both limbs and the interstitium is 300 mOsm/kg of water. The pumps in the thick
ascending limb move Na+ and Cl– into the interstitium, increasing its osmolality to 400 mOsm/kg, and this equilibrates with the fluid in the thin
descending limb. However, isotonic fluid continues to flow into the thin descending limb and hypotonic fluid out of the thick ascending limb. Con-
tinued operation of the pumps makes the fluid leaving the thick ascending limb even more hypotonic, while hypertonicity accumulates at the
apex of the loop. (Modified and reproduced with permission from Johnson LR [editor]: Essential Medical Physiology, Raven Press, 1992.)

312 375 375 425 425 513 513 700

300 325 325 425 425 425 425 600

300 300 300 300 300 300 300 300

A 400 400 400 400 400 400 400 400

400 400 400 400 400 400 400 400

200 200 200 200 200 200 200 200

B

TDL MI TAL

300 300 300 300 400 400 400 400

200 200 200 200 400 400 400 400

C 350 350 350 350 500 500 500 500

350 350 350 350 500 500 500 500

150 150 150 150 300 300 300 300

D

300 300 350 350 350 350 500 500

150 150 300 300 300 300 500 500

E 325 325 425 425 425 425 600 600

125 125 225 225 225 225 400 400

F 300 325 325 425 425 425 425 600

125 225 225 225 225 400 400 600

G 312 375 375 425 425 513 513 700

112 175 175 225 225 313 313 500

H 325 325 425 425 425 425 600 600

Ganong's Review of Medical Physiology, 23rd Edition

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