The Loop of Henle

The loop of Henle (LOH) is continuous with the proximal tubule and originates in the renal cortex. It consists of a descending limb that passes into the medulla and loops round to become the ascending limb which passes back into the cortex. This limb then continues as the distal tubule (Figure RE.9).

The fluid entering the LOH is initially isotonic compared with plasma, but after traversing the loop the fluid entering the distal tubule is hypotonic. Thus, the tubular fluid is diluted during its passage through LOH. However, LOH plays a crucial role in the concentration of urine by functioning as a countercurrent multiplier. The salient process is emphasized below:

The loops of Henle do not concentrate the tubular fluid within them, but manufacture a hypertonic interstitial fluid in the renal medulla. Urine is then concentrated by osmosis from collecting ducts as they pass through the medulla.

Figure RE.9

Sodium and water movement in the loop of Henle

Figure RE.9

Sodium and water movement in the loop of Henle

The Countercurrent Mechanism

A proposed mechanism for countercurrent multiplication was described by Wirz, Hargitay and Kuhn in 1951. The hypertonic interstitium is produced by a small osmotic pressure difference between the ascending and descending limbs of the loops (i.e. a small transverse gradient). This small difference is then multiplied into a large longitudinal gradient by the countercurrent arrangement (i.e. flow in opposite directions) in the two adjacent limbs of the loop.

The ascending limb is not uniform in structure but possesses both a thin and a thick segment. This limb produces an increase in the osmolality of the surrounding interstitium by the extrusion of sodium and accompanying ions. Only the thick segment actively extrudes ions. Both thin and thick segments of the ascending limb are impermeable to water so that water is unable osmotically to follow the extruded ions. Consequently, the osmolality of the medullary interstitium is increased and the osmolality of the fluid in the ascending limb is decreased.

The extrusion process is performed by the tubular cells that take up ions from the tubular lumen through their apical membrane and extrude ions through their basal membrane. The entry of solutes into the cells across the apical membrane involves co-transport of sodium, chloride and potassium, with the stoichiometry of one Na+, two Cl- and one K+, so the process is electrically neutral. The primary active transport on the basal cell membrane is performed by Na+K+ATPase. This basal membrane transport is also electrically neutral. The thin ascending limb has little Na+K+ATPase activity, thus the thick segment is primarily responsible for the extrusion of sodium. Much of the K+ leaks back into the tubular lumen, so that it is predominantly NaCl which accumulates in the medullary interstitium. This transport can be inhibited by loop diuretics, such as furosemide and bumetanide.

The descending limb is permeable to water and, to a lesser extent, is also permeable to NaCl. The fluid within the descending limb will, therefore, come to osmotic equilibrium with the interstitium. In effect, then, one can consider the transport of NaCl out of the ascending limb as being directed into the descending limb.

The sodium extrusion mechanism or sodium pump' action in the ascending limb can maintain an osmolality gradient of 200 mosmol/kgH2O between the ascending tubular lumen and the interstitium. In effect, since the descending limb is permeable and assumes the same osmolality as the interstitium, the "sodium pump' can be thought of as maintaining a 200 mosmol/kgH2O gradient between the tubular fluid in the adjacent sections of the ascending and descending limbs. The countercurrent arrangement of the limbs will then result in a gradually increasing interstitial (and descending limb) osmolality, as the LOH descends into the renal medulla. In man the osmolality at the papillary tips may reach 1400 mosmol/kgH2O (Figure RE.10).

The fluid in the ascending tubule leaving the medulla and entering the cortex is hypotonic to plasma, with an osmolality of about 100 mosmol/kgH2O. Thus, the ascending limb of the loop of Henle (and its continuation in the cortex as the distal tubule) can be called the "diluting segment' of the nephron. However, some nephrons possess short loops of Henle, and these are unlikely to lower the osmolality of the ascending limb fluid to 100 mosmol/kgH2O.

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