Women in Mathematical Biology by Anita T. Layton & Laura A. Miller

Author:Anita T. Layton & Laura A. Miller
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The fate of a urea bolus injected into a superficial proximal straight tubule differs significantly from that of NaCl. Because the short loop of Henle has low to moderate urea permeability and no active urea transport, a much bigger fraction (∼ 52%) of the urea bolus reaches the outer-medullary collecting duct, and ∼ 50% reaches the inner-medullary collecting duct. See Fig. 4 for urea bolus concentration profiles at selected times. Urea permeability remains low along the initial segment of the inner-medullary collecting duct (0. 25 × 10−5 cm/s), but increases exponentially in the terminal inner medulla to 110 × 10−5 cm/s. Thus, much of the urea bolus is reabsorbed from the terminal inner-medullary collecting duct; it then enters the descending and ascending limb segments of the long loops, which are highly urea permeable. As we will see below, this “urea cycling” greatly increases the residence time of urea within the medulla, especially in the inner medulla.

Fig. 4Concentration profiles of a urea bolus injected into the superficial proximal straight tubule of the anti-diuretic model. Notations are analogous to Fig. 3

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