Functions of the Kidneys and Functional Anatomy
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal in Principles of Physiology for the Anaesthetist, 2020
The loop of Henle consists of a thin limb, which descends into the medulla, followed by a hairpin bend and an ascending limb, which returns to the cortex (Figure 40.1). In nephrons with short loops, the ascending limb is thick. In nephrons with long loops that course through the medulla to the tips of papillae, the ascending limb is at first thin and becomes thickened as it passes through the outer medulla back to the cortex. The purpose of the loop of Henle is to create an increasing interstitial osmotic gradient in the medulla, permitting the reabsorption of water from the collecting ducts and the production of concentrated urine (up to 1400 mOsm/kg) in the presence of antidiuretic hormone (ADH). The descending limb of the loop of Henle reabsorbs water. The thick, ascending limb reabsorbs sodium, potassium, chloride and bicarbonate and secretes hydrogen ions.
SBA Answers and Explanations
Vivian A. Elwell, Jonathan M. Fishman, Rajat Chowdhury in SBAs for the MRCS Part A, 2018
The maximum concentration of urine that can be excreted by the human kidney is 1200 m0sm/L, four times the osmolality of plasma. This is primarily a function of the length of the loop of Henle, the hyperosmotic medullary interstitial gradient, and the concentration of ADH. A counter-current multiplication system sets up an osmotic gradient in the renal medulla which allows an efficient way for urine to be concentrated over a relatively short distance along the nephron with minimal energy expenditure. The descending limb of the loop of Henle is permeable to water (but only slightly permeable to salt and urea). Therefore, water is progressively absorbed down the limb, becoming more concentrated (up to 1200 m0sm/L). The ascending limb is impermeable to water but permeable to sodium chloride. The tubular fluid is therefore hypotonic by the time it reaches the distal convoluted tubule and collecting ducts. In the presence of a high concentration of ADH, by the time the urine is excreted it has a high osmolality (up to 1200 m0sm/L).
The Nature of Renal Function
Anthony R. Mundy, John M. Fitzpatrick, David E. Neal, Nicholas J. R. George in The Scientific Basis of Urology, 2010
The processes of countercurrent multiplication and exchange convert the hyperosmolality of the interstitium surrounding the TAL, and resulting from NaCl reabsorption, into an axial concentration gradient maximal at the papillary tip. The elements of the countercurrent system are the loops of Henle (multipliers) and the vasa recta (exchangers), the collecting duct being the site of final osmotic water reabsorption. The mechanism was described in principle many years ago, but a more detailed understanding of the process required methods of investigation not available until much later. A general description of the medullary countercurrent system is beyond the scope of this chapter: the reader is referred to an authoritative symposium published in 1983 (82), and to a lucid account of the properties of such systems in biology (83). A recurring difficulty has been to account for concentration of solute in the inner medullary and papillary interstitium (where the tonicity is highest) in the apparent absence of active solute reabsorption in the thin (inner medullary) segment of the ascending limb (Fig. 2). This probably depends on passive recycling of urea through facilitated diffusion in the medulla. Various models have been constructed to account for the observed hypertonicity in the inner medulla without the need to postulate active solute reabsorption in the thin ascending limb, but none has been unequivocally validated to date.
Current concepts and advances in biomarkers of acute kidney injury
Published in Critical Reviews in Clinical Laboratory Sciences, 2021
Osteopontin (OPN) is an extracellular matrix protein that serves as a transient proinflammatory cytokine for monocyte/macrophage recruitment. It is located mostly in the loop of Henle and the distal tubule in normal kidney tissue, but can be upregulated in all tubular and glomerular segments following injury [68]. It is therefore considered a more general marker of tubular injury [69]. OPN also has protective roles in tubular injury via decreasing cell apoptosis, participating in adaptive repair, decreasing nitric oxide synthase, and promoting cell survival during hypoxia [68]. It has also been demonstrated to predict AKI in neonates with low birthweight, in adult patients with transplant rejection or drug-induced nephrotoxicity, and in critically ill patients on KRT [69–72].
The furosemide stress test: current use and future potential
Published in Renal Failure, 2021
Blaithin A. McMahon, Lakhmir S. Chawla
Once a loop diuretic is secreted into the tubular fluid it then reaches its site of activity at the NKCC-2 at the thick ascending limb of the loop of Henle. The tubular concentration of furosemide determines its natriuretic effect, and the urine concentration of furosemide has been used as a surrogate for the tubular concentration [10]. Once at the NKCC-2 site, furosemide binds within the translocation ion pocket embedded in the chloride-binding site (portions of the transmembrane domains 11 and 12) resulting in obstruction and subsequent inhibition of the NKCC-2 transporter (Figure 1). Expression of Na-K-2Cl is also evident in cytoplasmic vesicles, suggesting a reservoir of transporters for insertion into the membrane [14], a process that can be modified by vasopressin and PGE2 [18,19]. At least half of an administered furosemide dose is excreted unchanged into the urine, a process that is prolonged in kidney failure (half-life of furosemide increases) [5] and the other half undergoes renal conjugation to glucuronic acid.
Advances in understanding vertebrate nephrogenesis
Published in Tissue Barriers, 2020
Joseph M. Chambers, Rebecca A. Wingert
By the time the nephron is fully developed, it will contain a number of unique cell-types that each need to have the appropriate gene expression to complete their vital functions (Figure 1). The nephron begins with the blood filter, or renal corpuscle encompassing the glomerulus and Bowman’s capsule. This contains a number of cell types including capillaries, mesangium, podocytes, and parietal cells. Next, the tubule contains the proximal convoluted tubule, proximal straight tubule, the Loop of Henle (including descending limb, thin ascending limb, thick ascending limb), distal convoluted tubule, and connecting tubule. The proximal tubule functions in absorption and secretion in an effort to regulate pH of the filtrate. Largely, the Loop of Henle functions to concentrate the filtrate by reabsorbing water. The distal tubule ensures proper ion transport occurs to fine-tune the filtrate by regulating potassium, sodium, and calcium levels. Each unique segment is needed to maintain blood homeostasis by completing these functions. Nephron cells must acquire a number of features to be generally considered terminally differentiated, including proper epithelization, cilia formation, and expression of functional proteins such as tight junctions and solute transporters.
Related Knowledge Centers
- Anatomy
- Aquaporin
- Collecting Duct System
- Distal Convoluted Tubule
- Molecular Diffusion
- Nephron
- Proximal Tubule
- Renal Medulla
- Kidney
- Countercurrent Multiplication