Quick Facts
The ascending limb is the distal part of the loop of Henle (Dorland, 2011).
Structure and/or Key Features
The nephron loop, more widely known as the loop of Henle, describes the recognizable loop that characterizes the nephron. Its length may measure up to 30 mm in juxtamedullary nephrons (Martini, Nath and Bartholomew, 2017).
The nephron loop is comprised of the proximal straight tubule (or thick descending limb), a thin descending limb, a thin ascending limb, and a distal straight tubule (or thick ascending limb).
The ascending limb of the nephron loop arises as the descending thin limb turns back or “loops” back on itself. Measuring approximately 15 µm in diameter, the epithelium of the ascending thin limb is described as simple squamous, continuous with that of the descending thin limb (Kerr, 2007).
The distal straight tubule, or thick ascending limb, is a continuation of the ascending thin limb of the nephron loop. Measuring approximately 30 µm in diameter, the thick ascending limb has a simple cuboidal epithelium with a brush border (Martini, Nath and Bartholomew, 2017; Kerr, 2007).
The nephron loop is supplied by a series of straight arterioles that branch from the cortical efferent arterioles; these capillaries are known as the vasa recta.
Anatomical Relations
The thin ascending limb of the nephron loop is located distal to the thin descending limb and proximal to the thick ascending limb. The thick ascending limb, or distal straight tubule is continuous with the distal convoluted tubule. Their point of union is demarcated by a region of narrowed, clustered cells known as the macula densa.
The ascending limb of the nephron loop of juxtamedullary nephrons travels in an upward trajectory through the inner and outer renal medulla.
Function
The thin ascending limb of the nephron loop is responsible for sodium chloride reabsorption. Sodium is primarily reabsorbed through passive diffusion, while chloride is reabsorbed across chloride channels (Kondo, 1997). As salt (sodium chloride) moves out of the tubules, the osmolarity of the renal filtrate decreases slightly, however, most equilibration of tubular filtrate with the interstitium occurs in the ascending thick limb (Pocock, Richards and Richards, 2013).
The thick ascending limb, or distal straight limb, is responsible for the reabsorption of ions, mainly Na+ (25–30% of total Na+ reabsorption), K+, and Cl- via active transport. Mg+ and Ca2+ are also reabsorbed here. The primary pumps involved in this reabsorption are the Na+/K+ ATPase, Na+/K+/Cl- cotransporter and the NKCC2 transporter. The thick ascending limb is completely impermeable to water.
As a result of the ion reabsorption, the urinary filtrate that reaches the distal convoluted tubule is hypotonic, while the medullary environment becomes more hyperosmotic.
List of Clinical Correlates
—Bartter syndrome
References
Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.
Kerr, J. B. (2007) Atlas of Functional HistologyElsevier Science Health Science Division.
Kondo, Y. (1997) Mechanism of sodium chloride reabsorption in the ascending thin limb of Henle's loop. Clinical and Experimental Nephrology, 1(2), 67-75.
Martini, F. H., Nath, J. L. & Bartholomew, E. F. (2017) Fundamentals of Anatomy & PhysiologyPearson Education.
Pocock, G., Richards, C. D. & Richards, D. A. (2013) Human Physiology, 4 edition. OUP Oxford.
