Arterial–Venous Balance in the Human Renal Vasculature: Renal–Aortic and Renal–Caval Ratios and Factors on Contrast-Enhanced Computed Tomography

Background/Objectives: A single absolute vessel diameter is hard to compare between individuals of differing body size. We defined two venous proportional parameters, the renal–caval ratio (R-IVCr) and factor (R-IVCf), as counterparts of the established renal–aortic ratio (R-Ar) and factor (R-Af), and asked whether the arterial and the venous bed respond to multiplicity of vessels in the same direction. Methods: Biphasic contrast-enhanced computed tomography of the abdomen in 1196 patients (2392 kidneys) was analysed retrospectively. Ratios related the dominant vessel diameter to that of the parent trunk; factors related the summed cross-sectional area of a kidney’s vessels to that of the corresponding trunk segment. Two angles, measured towards the left (α) and the right (β) from an anteroposterior reference axis, were recorded at both trunks. Results: Mean R-Ar was 0.2381 (±0.0573) and mean R-IVCr was 0.4651 (±0.0943). Dominant-vessel ratios were lower in the multiple variant and area-summing factors were higher, in parallel on both sides (p < 0.001). All four were higher in men, with moderate effect sizes arterially and negligible ones venously. α and β inverted between the two trunks, which was the largest effect recorded, and multiple veins showed a 2.3-fold right-sided predominance. Conclusions: The four parameters describe arterial inflow and venous outflow within a single proportional system in which both beds respond to multiplicity in parallel; their value in donor-kidney assessment and anastomosis planning awaits prospective validation.