Kidney and acid base physiology:part 2

Transport of ions at the loop of Henle

Water is absorbed passively at the thin descending limb of the loop of Henle and is (?) accompanied by diffusion of sodium ions in the loop of Henle.

Limited absorption of NaCl can occur through the thin ascending limb passively.

Key players at The Thick Ascending limb

As in the previous post, the accompanying diagram is self explanatory and the steps involved in the H⁺/ HCO₃^- transport and Na⁺/K⁺ movement are more or less same barring a few exceptions which can be summarized as follows:

 i.                    About 15% of the filtered HCO₃^- can be reabsorbed here at the basolateral  (BL) surface.

ii.                  The Na⁺-HCO₃- cotransporter at the BL surface is NBCn1.

iii.            The HCO₃- can exit the cell in exchange for Cl^- (Anion Exchanger2, AE2) or by a K⁺-HCO₃ symporter.

iv.                There is no Carbonic anhydrase in the brush border (luminal side).

v.        The most remarkable difference is the presence of Na⁺-K⁺-2Cl^- cotransporter here which is responsible for the downhill transport of Na⁺ and Cl^- and uphill transport of K⁺ inside the cell.

vi.                K⁺ and Cl^- ions exit at the basolateral surface through separate channels whereas Na⁺ leaves the cell primarily through the Na⁺/K⁺ ATPase.

vii.              Some K+ is able to leak through the apical surface into the lumen and results in a slightly positive (+6 mV) inside the lumen. This acts as a driving force for cations like Na⁺, Mg⁺⁺, Ca⁺⁺ and even K⁺ to pass through a paracellular route to capillaries.

viii.            This segment is totally impermeable to water so known as the ‘diluting segment’.

ix.           Thus 20% Na⁺ and K⁺ are reabsorbed in this segment where half of that Na⁺ absorption is passive (paracellular) and the rest is active.

x.                  The site of action of Furosemide is the Na⁺-K⁺-2Cl⁺ cotransporter to which it reaches through the luminal side after being secreted in the PT by organic anion transport (shown in the diagram in the previous post).