these findings are consistent with anticipated trends for a healthy tissue, wherein a vascular network is assumed to be a fractal structure, with branching pattern driven by optimised delivery of blood to the whole organ. interestingly, within truncated strahler orders 8 – 6, the mean branching angle was approximately 150°, decreasing to 130° for truncated strahler orders 3 – 1 ( fig. 4c ). importantly, the latter value of 130° is the predicted optimal theoretical branching angle for volume - constrained vascular growth. simulation of kidney haemodynamics has previously been performed using μct data from the rat kidney. to facilitate comparison between existing rat data and our human hip - ct results, we aligned our network based on the strahler order allocated to the segmental arteries, thus aligning strahler order 9 in the previously published rat dataset to truncated strahler order 8 of our human data. we then related normalised vessel metrics from each species, matching anatomically defined vessel types. the expected increase in vessel radius with order followed a similar trend between human and rat kidney ( fig. 5a ). however human kidney vessel radii increased to a greater extent across strahler orders than in the rat kidney, evaluated based on a fit of log ( radius ) to strahler order ( fig. 5b ), ( p < 0. 0001 sum - of - f test f ( dfn, dfd ) = 700. 6 ( 2, 12 ) ). to provide additional insights into this difference observed between human and rat kidneys, we extracted radial scaling exponents of the human vascular network. the radial scaling exponent provides insight into how the network has structurally developed with respect to its functions such as the efficiency of blood flow and nutrient delivery to meet metabolic demands and minimise flow resistance. exponents of 0. 33 and 0. 5 each have theoretical bases in different models, ( i ) murray ’ s law ( expected exponent of 0. 33 for all the whole network, derived from considering the energy balance between energy of flow and viscous drag ), ( ii ) the west - brown - enquist ( wbe ) model which predicts 0. 5 in larger vessels and 0. 33 in smaller vessels, resulting from balancing the energy for metabolic distribution of blood across a fractal - like network. however, deviations from these exponent values and other variants of vascular scaling models have been widely reported. previously in the rat kidney, nordsletten et al. demonstrated a deviation from murray ’ s law by ~ 1 % for the rat kidney. figure 5ci shows the cubed parent radius plotted against the cube sum of the child radii for our dataset, the theoretical murray ’ s law is overlaid ( orange hatched ) to allow qualitative comparison to previous literature. to quantitatively evaluate whether our data are better represented by murray ’ s law ( expected radial scaling exponent 0. 33 ) the wbe model ( radial scaling exponent 0. 33 in small and 0. 5 in large vessels ) or another model, we extracted the number of downstream terminal ends of the network for each vessel segment and the radius of that segment. through a log - log plot of the data ( fig. 5cii ), the theoretical value of the exponent a was found to be 0. 55. this value is higher than murray ’ s law and closer to the web model and the values found in ref. for the human pulmonary artery system. we then sought to compare heterogeneity in morphology of the human kidney vasculature according to anatomical regions within the human kidney, which may reflect specialised vascular functions. for example, the medulla of the kidney is predominantly vascularised by vasa recta; specialised capillaries which possess low oxygen tension. this configuration leads to physiological hypoxia that is inherent to the medulla ’ s urinary concentration mechanisms. further reflecting the importance of vascular morphology is the longstanding hypothesis, supported by blood oxygenation level - dependent mri studies, that vascular rarefaction in ckd results in hypoxia within the kidney cortex. in turn, this stimulates neighbouring cells into a pro - fibrotic phenotype, manifesting in replacement of normal kidney tissue by fibrosis and heralding loss of organ function. thus, regional heterogeneity of vascular morphology is fundamental for sustaining local microenvironmental features, such as hypoxia, that influence specialised organ functions. however, regional heterogeneity in vascular structure has not been quantitatively explored in the human kidney. leveraging the contrast - free approach of hip - ct, we were able to segment the kidney into known anatomical compartments, including hilum, medulla, intramedullary kidney columns and cortex ( fig. 6ai ). we compartmentalised the vascular network according to these anatomical compartments ( fig. 6aii ). the total tissue volume of each compartment, in addition to the number of vessels, length, radius and volume of segmented vessels within each compartment, were quantified ( table 2 ). most of the tissue volume of the human kidney was occupied by the cortex ( 63. 7 % ) as compared with the medulla ( 23. 5 % ), hilum ( 8. 7 % ) or intermedullary pillars ( 4. 1 % ). the number of segments of the vascular network within each compartment followed this trend. we then quantified ( fig. 6aiii ) and mapped ( fig. 6bi – biii ) the inter - vessel distance, compartmentalised by hilum, medulla, cortex, and intermedullary pillars. mean inter - vessel distances were calculated for each compartment, assessing the distribution of inter - vessel distance from the renal artery down to interlobular arteries ( table 2 ). the medulla had the highest inter - vessel distance. whilst the cortex had a comparatively smaller inter - vessel distance than medulla and hilum, a large standard deviation for this value was noted within the cortex. this is illustrated by the heatmap in fig. 6bi, bii, which identified small areas with inter - vessel distance > 4. 5 mm localised towards the kidney capsule. table 2 human kidney vascular branching metrics compartmentalised by spatial zone with the organ cortex medulla hilum intermedullary pillars organ volume of tissue, ×10 µm ( % of total ) 8. 70 ( 63. 7 % ) 3. 21 ( 23. 5 % ) 1. 18 ( 8. 66 % ) 0. 57 ( 4. 14 % ) 13. 7 ( 100 % ) number of segments * ( % of total ) 6141 ( 60. 27 % ) 554 ( 5. 4 % ) 151 ( 1. 5 % ) 727 ( 7. 1 % ) 10, 190 ( 100 % ) mean segment length, µm ± std 1999 ± 1374 1493 ± 1113 3993 ± 3568 1720 ± 1386 2260 ± 1720 mean segment radius, µm ± std 48 ± 12. 6 95 ± 49 496 ± 335 136 ± 80 71 ± 87 mean inter - vessel distances, ×10 µm ± std 1. 10 ± 0. 677 1. 55 ± 0. 881 1. 55 ± 1. 312 0. 664 ± 0. 543 1. 2 ± 0. 833 mean segment volume, ×10 µm ± std 0. 148 ± 0. 116 0. 623 ± 1. 12 69. 0 ± 147 1. 73 ± 4. 48 1. 65 ± 20. 6 mean segment tortuosity ± std 1. 14 ± 0. 17 1. 08 ± 0. 13 1. 08 ± 0. 1 1. 1 ± 0. 12 1. 15 ± 0. 18 * segments that crossed over two regions were excluded. human kidney vascular branching metrics compartmentalised by spatial zone with the organ source paper: pmc12408821 * segments that crossed over two regions were excluded. owing to the limited volume of tissue that can be imaged at high resolution using ex vivo 3d imaging modalities, such as μct and lightsheet microscopy, and insufficient resolution of technologies routinely used in clinical practice, such as ct and mri; it had previously been impractical to capture the vascular network of the intact adult human kidney beyond the very largest arteries. here, using a synchrotron - phase contrast tomography technique, termed hip - ct, we were able to image, segment and quantify the human kidney arterial network within an intact human kidney from renal artery to down to the level of interlobular arteries, without the need for exogenous contrast agents. with hip - ct, we show that vessels which have not been imaged in the intact human kidney previously, namely the interlobar to interlobular arteries, occupy approximately 20 % of the arterial vascular volume of the organ.