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Ngược lại, máu chảy về phía ngã ba corticomedullary trong AVR được pha loãng bằng cách khuếch tán ra ngoài này để chất tan là bị mắc kẹt và tái chế. Nghiên cứu gần đây đã chỉ ra rằng mô tả của một trao đổi hoàn toàn khuếch tán "utube" là đơn giản. Loại bỏ thẩm thấu của nước từ DVR | 448 Part II Organ Microvascular Adaptations toward the papillary tip. Conversely blood flowing toward the corticomedullary junction in AVR is diluted by diffusive efflux so that solute is trapped and recycled. Recent studies have shown that this depiction of a purely diffusive U-tube exchanger is oversimplified. Osmotic removal of water from DVR across water channels occurs sieving NaCl and urea to concentrate these solutes in DVR plasma. Thus both molecular sieving and diffusion contribute to equilibration of DVR plasma with the interstitium. As discussed later shunting of water from DVR to AVR in vascular bundles might play a role in optimization of urinary concentrating ability by reducing blood flow rate to the deep medulla. Efflux of Water from DVR DVR plasma protein concentration rises along the direction of blood flow indicating water loss from plasma to renal medullary interstitium. Thus DVR and nephrons deposit water to the renal medulla and AVR take up that water accounting for overall mass balance. Water efflux from DVR raises two paradoxes. First the purpose of depositing water from DVR plasma to the hypertonic medullary interstitium seems enigmatic. Second Starling forces hydraulic and oncotic pressure do not account for the direction of DVR water transport because intraluminal oncotic pressure that favors water uptake exceeds the hydraulic pressure that favors efflux. NaCl and urea gradients generated by the lag in equilibration of DVR blood with interstitium favor water efflux and could account for osmotic water abstraction from DVR but this requires the presence of a small pore pathway across which such small solutes exert effective osmotic driving force. The discovery of the aquaporins led to the molecular identification of that route Figure 2A . Blockade of aquaporin-1 AQP1 with mercurial agents or AQP1 knockout eliminates water efflux driven by abluminal hypertonic NaCl 3 5 . Thus transport of water across the DVR wall must be described by at .
