What is True about ascending limb of loop of Henle ? and explain in detail?
Ans. is 'a' i.e., Recieves hypeonic saline Fluid leaving the descending limb of loop of Henle and entering the ascending limb of loop of Henle is very hypeonic. Renal reabsorption and secretion The Glomerular filtrate enters the renal tubules and flows through the successive pas of the tubule (i.e. the proximal tubule, loop of Henle, distal tubule and finally collecting ducts) before it is excreted as urine. Along its course, the tubular cells may selectively secrete some substances from the blood into the tubular lumen (tubular secretion), or may selectively remove some substances (tubular reabsorption) from the tubular filtrate, or may do both. When the Glomerular filtrate is initially formed, its osmolarity is about the same as that of plasma i.e. 300mOsm/L. Its osmolalilty changes depending on the absorption of water and solutes in the segment. Proximal convoluted tubule The PCT reabsorbs- sodium, chloride, bicarbonate, phosphate, potassium, glucose and amino acids. The PCT secretes- H+, organic acids and bases (such as bile salts, oxalate, urate and catecholamines) The PCT is highly permeable to water. In PCT solutes and water are reabsorbed in equal propoions, thus little change in the osmolarity occurs i.e. the tubular fluid in the PCT is isotonic to the plasma. Loop of Henle: Descending limb The thin descending segment is highly permeable to water and moderately permeable to most solutes, including urea and sodium. No active reabsorption or secretion takes place. The water is reabsorbed as the tubular fluid passes down the descending limb and the filtrate becomes concentrated, in equilibrium with the surrounding interstitial fluid of the renal medulla, which is very hypeonic. Thus fluid leaving Descending limb of loop of Henle is very hypeonic. Loop of Henle: Ascending limb The thick ascending limb reabsorbs - sodium, chloride, and potassium, calcium, bicarbonate, and magnesium. o This segment also secretes- hydrogen ions into the tubular lumen. The thin segment of the ascending limb has a much lower reabsorptive capacity than the thick segment, and the thin descending limb does not reabsorb significant amounts of any of these solutes. The ascending limb, including both the thin and the thick poions, is completely impermeable to water, even in the presence ofADH. Since other solutes are being reabsorbed, the tubular fluid becomes hypotonic. Thus the fluid leaving the ascending limb of loop is hypotonic, with an osmolarity of only about one third the osmolarity of plasma i.e. 100 mOsm/L. Early distal tubule Early distal tubule has propeies similar to those of the thick ascending limb of the loop, that is, it avidly reabsorbs most of the ions, including sodium, potassium, and chloride, but is viually impermeable to water so there is fuher dilution of the tubular fluid. The late distal tubules and coical collecting tubules The late distal tubules and coical collecting tubules are composed of two distinct cell types, the principal cells and the intercalated cells. The principal cells reabsorb sodium from the lumen and secrete potassium ions into the lumen. The intercalated cells reabsorb potassium and bicarbonate ions from the lumen and secrete hydrogenions into the lumen. The reabsorption of water from this tubular segment is controlled by the concentration of antidiuretic hormone. Medullary Collecting Duct The medullary collecting ducts actively reabsorb sodium and secrete hydrogen ions and are permeable to urea, which is reabsorbed in these tubular segments. The reabsorption of water in medullary collecting ducts is controlled by the concentration of antidiuretic hormone. In late distal tubule, coical collecting tubules and medullary collection tubules the tubular fluid osmolarity depends on the presence or absence ofADH. In presence of high levels of ADH these tubules are highly permeable to water and significant amounts of water are reabsorbed and the tubular fluid becomes hypeonic, up to 1200 mOsm/L (equal to osmolarity of the surrounding medullary interstitium). In the absence of ADH, these segments are almost impermeable to water; therefore osmolarity decreases even fuher because of continued active reabsorption of ions from these segments-tubular fluid thus becomes hypotonic with osmolarity as low as 50mOsm/L.