Thin ascending limb of the nephron loop
Tubulus attenuatus, pars ascendens
Also known as: tAL
A thin, water-impermeable segment present only in the long loops of juxtamedullary nephrons, in which NaCl leaves the concentrated tubular fluid passively into the inner medullary interstitium.
Overview
After the hairpin bend in the inner medulla, the tubule continues as the thin ascending limb, which rises to the boundary between inner and outer medulla and then becomes the thick ascending limb. Its flat epithelium is impermeable to water but highly permeable to NaCl (ClC-K1 chloride channels, paracellular Na⁺). Because the fluid arriving from the descending limb is NaCl-rich, NaCl diffuses out passively, contributing to the inner medullary gradient without energy expenditure, while urea enters from the interstitium (urea recycling).
Short-looped cortical nephrons lack a thin ascending limb.
- Location
Inner medulla (long loops only).
Anatomy
- Histology
Simple squamous epithelium; distinguishable from the descending limb mainly by ultrastructure and transporter expression.
Function
- Passive NaCl reabsorption into the inner medulla
- Begin dilution of tubular fluid
- Participate in urea recycling
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.