PASSIVE WATER REABSORPTION BY OSMOSIS COUPLED MAINLY TO SODIUM REABSORPTION When solutes are transported out of the tubule by pri-mary or secondary active transport, their concentrations tend to decrease inside the tubule while increasing in the renal interstitium. This phenomenon creates a con-centration difference that causes osmosissame direction that the solutes are transported, from the tubular lumen to the renal interstitium. Some parts of the renal tubule, especially the proximal tubule, are highly permeable to water, and water reabsorption occurs so rapidly that there is only a small concentration gradient for solutes across the tubular membrane. A large part of the osmotic flow of water in the proxi-mal tubules occurs through water channels (aquaporins) in the cell membranes, as well as through the tight junc-tions between the epithelial cells. As noted previously, the junctions between the cells are not as tight as their name would imply and permit significant diffusion of water and small ions. This condition is especially true in the proxi-mal tubules, which have a high permeability for water and a smaller but significant permeability to most ions, such as sodium, chloride, potassium, calcium, and magnesium. Water moving across the tight junctions by osmosis also carries with it some of the solutes, a process referred to as solvent drag. In addition, because the reabsorption of water, organic solutes, and ions is coupled to sodium reabsorp-tion, changes in sodium reabsorption significantly influ-ence the reabsorption of water and many other solutes. In the more distal parts of the nephron, beginning in the loop of Henle and extending through the collecting tubule, the tight junctions become far less permeable to water and solutes, and the epithelial cells also have a greatly decreased membrane surface area. Therefore, water cannot move easily across the tight junctions of the tubular membrane by osmosis. However, antidiuretic hormone (ADH) greatly increases the water permeability in the distal and collecting tubules. Thus, water movement across the tubular epithelium can occur only if the membrane is permeable to water, no matter how large the osmotic gradient. In the proximal tubule and descending loop of Henle, water permeability is always high, and water is rapidly reabsorbed to reach osmotic equilibrium with the surrounding interstitial fluid. This high permeability is due to abundant expres-sion of the water channel aquaporin-1 (AQP-1) in the luminal and basolateral membranes. In the ascending loop of Henle, water permeability is always low, so almost no water is reabsorbed, despite a large osmotic gradient. Water permeability in the last parts of the tubules-the distal tubules, collecting tubules, and collecting ducts-occurs through aquaporins and can be high or low, depending on the presence or absence of ADH. REABSORPTION OF CHLORIDE, UREA, AND OTHER SOLUTES BY PASSIVE DIFFUSION When sodium is reabsorbed through the tubular epithe-lial cell, negative ions such as chloride are transported along with sodium because of electrical potentials. That is, transport of positively charged sodium ions out of the lumen leaves the inside of the lumen negatively charged, compared with the interstitial fluid causing chloride ions Na reabsorption H₂O reabsorption ↑ Lumen negative potential Luminal ↑ Concentration Luminal Aurea concentration Passive Cl reabsorption Passive urea reabsorption Figure 28-5 Mechanisms whereby water, chloride, and urea reab-sorption are coupled with sodium reabsorption. to diffuse passively through the paracellular pathway. Additional reabsorption of chloride ions occurs because of a chloride concentration gradient that develops when water is reabsorbed from the tubule by osmosis, thereby concentrating the chloride ions in the tubular lumen (Figure 28-5). Thus, active reabsorption of sodium is closely coupled to passive reabsorption of chloride by way of an electrical potential and a chloride concentra-tion gradient. Chloride ions can also be reabsorbed by secondary active transport. The most important of the second-ary active transport processes for chloride reabsorption involves the co-transport of chloride with sodium across the luminal membrane. Urea is also passively reabsorbed from the tubule, but to a much lesser extent than chloride ions. As water is reabsorbed from the tubules (by osmosis coupled to sodium reabsorption), urea concentration in the tubu-lar lumen increases (see Figure 28-5). This increase creates a concentration gradient favoring reabsorption of urea. However, urea does not permeate the tubule as readily as water. In some parts of the nephron, espe-cially the inner medullary collecting duct, passive urea reabsorption is facilitated by specific urea transporters. Yet, only about half of the urea that is filtered by the glomerular capillaries is reabsorbed from the tubules. The remaining urea passes into the urine, allowing the kidneys to excrete large amounts of this waste product of metabolism. In mammals, more than 90% of waste nitrogen, mainly generated in the liver as a product of protein metabolism, is normally excreted by the kidneys as urea. Another waste product of metabolism, creatinine, is an even larger molecule than urea and is essentially imper-meant to the tubular membrane. Therefore, almost none of the creatinine that is filtered is reabsorbed, so virtually all the creatinine filtered by the glomerulus is excreted in the urine. 348 was

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