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The Model |
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| The animated model shows a
hamster papilla in cross section, and then in a longitudinal section. To
concentrate the urine in the collecting ducts, water must be removed from
the collecting ducts in excess of solutes. Part of this water removal is
caused by the accumulation of solutes in the papillary interstitium.
Mathematical models, however, cannot explain the actual concentrations
reached by some animals. The model presented here tries to explain how the
hydrostatic pressure generated by the pelvic wall peristalsis could
contribute to the removal of water from the collecting duct lumen; it does
not deal with the solutes.
Water movement through a membrane results from a difference in water potential in the two compartments separated by the membrane. Water potential is decreased by solutes in solution, and increased by hydrostatic pressure. Water moves through water channels, aquaporins, as shown by Mark Knepper and his colleagues. Water leaves the collecting duct lumen through the water channels in the plasma membranes of the collecting duct cells. Aquaporin-2, the anti-diuretic hormone sensitive water channels are present in the apical membranes of the collecting duct cells. Water molecules move through the aquaporins by single file diffusion without entrainment of solutes. Urea moves by diffusion through urea transporters. Water can leave the collecting duct cells through the water channels, aquaporin3 and aquaporin-4, which also permit solutes to pass through. In the animated model, the sequence of events occurring in the papilla are shown at low urine flow. |
Legends to Simulations
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