Renal microcirculation in acute kidney injury: bridging haemodynamics, inflammation and repair.

Background: The kidneys receive a disproportionately high share of cardiac output relative to their mass, reflecting their central role in solute clearance and fluid regulation. The renal microcirculation is anatomically complex and regionally heterogeneous, with steep gradients in perfusion and oxygen tension from cortex to medulla. This architecture supports efficient filtration and reabsorption but renders specific nephron segments supply-dependent and susceptible to injury during haemodynamic instability, hypoxia, inflammation, or exposure to toxins. These vulnerabilities can lead to acute kidney injury (AKI), where microvascular dysfunction often precedes changes in global renal perfusion. Main body: The renal microvasculature comprises glomerular capillaries followed by efferent arterioles supplying either cortical peritubular capillaries or the medullary vasa recta. Regional endothelial heterogeneity, limited medullary oxygen availability, countercurrent oxygen shunting, and the high metabolic demand of tubular sodium transport create intrinsic susceptibility to injury. In AKI, sepsis, ischaemia-reperfusion, surgery, nephrotoxins, and venous congestion may trigger early endothelial activation and glycocalyx injury, promoting increases permeability, leukocyte and platelet adhesion, altered vascular tone, capillary obstruction, and interstitial oedema. These changes impair perfused capillary density and oxygen diffusion. Microvascular dysfunction may persist despite normalisation of systemic haemodynamics, reflecting a loss of haemodynamic coherence and contributing to regional hypoxia and reduced glomerular filtration.Tubular injury further amplifies these abnormalities. Reduced NaCl reabsorption increases delivery to the macula densa, activates tubuloglomerular feedback, and promotes afferent vasoconstriction, while tubular obstruction, filtrate backleak, and interstitial pressure oppose filtration and compromise peritubular perfusion. The relative contribution of these mechanisms differs across sepsis-associated, ischaemia-reperfusion, perioperative, nephrotoxic, congestive, and predominantly haemodynamic AKI phenotypes.When repair is incomplete, persistent endothelial dysfunction, pericyte detachment, capillary rarefaction, chronic hypoxia, and fibrosis promote nephron dropout, loss of renal functional reserve, and progression from AKI to chronic kidney disease. Clinical assessment currently relies on complementary surrogates, including perfusion imaging, venous congestion assessment, oxygenation monitoring, and endothelial or tubular biomarkers. Conclusion: Renal microvascular dysfunction contributes to the initiation, functional expression, persistence, and long-term consequences of AKI. A phenotype-oriented approach integrating perfusion, congestion, oxygenation, endothelial injury, and tubulo-vascular interactions may improve mechanistic understanding and support the development of targeted preventive and therapeutic strategies.