| Ascending limb of loop of Henle | |
|---|---|
 Scheme of renal tubule and its vascular supply. (Labeled at center left.) | |
 Nephron ion flow diagram | |
| Details | |
| Identifiers | |
| Latin | tubulus rectus distalis, pars recta tubuli distalis |
| FMA | 17717 |
| Anatomical terminology | |
Within thenephron of thekidney, theascending limb of the loop of Henle is a segment of the heterogenousloop of Henle downstream of thedescending limb, after the sharp bend of the loop. This part of therenal tubule is divided into athin andthick ascending limb; the thick portion is also known as thedistal straight tubule, in contrast with thedistal convoluted tubule downstream.
The ascending limb of the loop of Henle is a direct continuation from thedescending limb of loop of Henle, and one of the structures in thenephron of the kidney. The ascending limb has a thin and a thick segment. The ascending limb drains urine into thedistal convoluted tubule.
The thin ascending limb is found in themedulla of the kidney, and the thick ascending limb can be divided into a part that is in therenal medulla and a part that is in therenal cortex. The ascending limb is much thicker than thedescending limb.
At the junction of the thick ascending limb and the distal convoluted tubule are a subset of 15–25 cells known as themacula densa that are part ofrenal autoregulation through the mechanism oftubuloglomerular feedback.
As in the descending limb, the epithelium issimple cuboidal epithelium.[1]
The thin ascending limb is impermeable to water; but is permeable to ions allowing for some sodium reabsorption. Na/K-ATPase is expressed at very low levels in this segment and thus this reabsorption is likely through passive diffusion.[2] Salt moves out of the tubule and into the interstitium due to osmotic pressure created by the countercurrent system.
Functionally, the parts of the ascending limb in the medulla and cortex are very similar.[3]
The medullary ascending limb is largely impermeable to water.Sodium (Na+),potassium (K+) andchloride (Cl−) ions are reabsorbed byactive transport. The predominant mechanism of active transport in this segment is through the Na+/K+/Cl− co-transporter NKCC2 as well as the sodium/hydrogen exchangerNHE3.[4] In total this segment accounts for approximately 25–30% of total Na+ reabsorption along the nephron. This is of clinical importance since commonly used "loop diuretics" act by inhibiting the NKCC2.[5] This active transport enables the kidney to establish an osmotic gradient that is essential to the kidneys ability to concentrate the urine pastisotonicity.
K+ is passively transported along its concentration gradient through a K+ leak channel in the apical aspect of the cells, back into the lumen of the ascending limb. This K+ "leak" generates a positiveelectrochemical potential difference in the lumen. This drives more paracellular reabsorption of Na+, as well as othercations such asmagnesium (Mg2+) and importantlycalcium Ca2+ due to charge repulsion.
This is also the part of the tubule that generatesTamm–Horsfall protein. The function of this protein is not well understood, but is responsible for creatingurinary casts.
The thick ascending limb symporter:Na-K-Cl cotransporter.
This article incorporates text in thepublic domain frompage 1223 of the 20th edition ofGray's Anatomy(1918)
