Abstract
BackgroundThe likelihood of a Randall’s plug composed of calcium oxalate monohydrate (COM) forming by the free particle mechanism in a model of kidney with a structure recently described by Robertson was examined at the most favourable conditions for the considered mechanism.MethodsThe Robertson model of the kidney is used in the following development. The classical theory of crystallization was used for calculations.ResultsInitial COM nuclei were assumed to form at the beginning of the ascending loop of Henle where the supersaturation with respect to COM has been shown to reach the threshold level for spontaneous nucleation. Nucleation proceeds by a heterogeneous mechanism. The formed particles are transported in the nephron by a laminar flow of liquid with a parabolic velocity profile. Particles travel with a velocity dependent on their position in the cross-section of the nephron assumed to be straight tubule with smooth walls and without any sharp bends and kinks. These particles move faster with time as they grow as a result of being surrounded by the supersaturated liquid. Individual COM particles (crystals) can reach maximum diameter of 5.2 × 10−6 m, i.e. 5.2 μm, at the opening of the CD and would thus always be washed out of the CD into the calyx regardless of the orientation of the CD. Agglomeration of COM crystals forms a fractal object with an apparent density lower than the density of solid COM. The agglomerate that can block the beginning of the CD is composed of more crystals than are available even during crystaluria. Moreover the settling velocity of agglomerate blocking the opening of the CD is lower than the liquid flow and thus such agglomerate would be washed out even from upward-draining CD.ConclusionsThe free particle mechanism may be responsible for the formation of a Randall’s plug composed by COM only in specific infrequent cases such as an abnormal structure of kidney. Majority of incidences of Randall’s plug development by COM are caused by mechanism different from the free particle mechanism.
Highlights
The likelihood of a Randall’s plug composed of calcium oxalate monohydrate (COM) forming by the free particle mechanism in a model of kidney with a structure recently described by Robertson was examined at the most favourable conditions for the considered mechanism
This model describes the ascending Henle loop as a circular truncated cone with average length LHL of 1.2 × 10−2 m and internal radius rLH increasing from 1.12 × 10−5 to 1.20 × 10−5 m, the following distal tubule as a cone with a length LD of 6 × 10 −3 m and internal radius rDL decreasing from 1.2 × 10−5 to 1 × 10−5 m, and the collecting duct (CD) as a cone with a length LCD of 2.7 × 10−2 m and internal radius rCD increasing from 1 × 10−5 to 4 × 10−5 m at its opening at the papilla
For simplicity let Liquid surrounding calcium oxalate (CaOx) particles is supersaturated with respect to COM and particles increase in size by regular crystal growth
Summary
The likelihood of a Randall’s plug composed of calcium oxalate monohydrate (COM) forming by the free particle mechanism in a model of kidney with a structure recently described by Robertson was examined at the most favourable conditions for the considered mechanism. One type results from a subepithelial calcification (hydroxyapatite) of the renal papillary tissue as a consequence of a previous injury. Two mechanisms for blocking collecting ducts by solid concretions, i.e. Randall’s plug, have been suggested, the free and fixed particle mechanisms [2]. The free particle mechanism assumes that during the residence time of Grases and Söhnel BMC Urology (2017) 17:80 urine in the nephron and collecting duct a spherical object that can obstruct the opening into the calyx develops as a result of growth of a single particle or by agglomeration of smaller particles. The fixed particle mechanism assumes that a large concretion develops on a solid nidus firmly attached to the collecting duct wall. A crystal of calcium oxalate (CaOx) formed in urine during its residence time in the nephron attached to a Bellini’s duct or to subepithelial calcification (hydroxyapatite) of the renal papillary tissue followed by an injury can serve as such a solid nidus
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