Quantitative branching pattern of blood vessels in the outer medulla

Kidneys were perfusion-filled with India ink and immuno-stained by an anti-smooth muscle alpha-actin antibody to confirm and extend existing knowledge of the rat medullary vasculature. Of 23 kidneys prepared, 5 provided new information on the pattern of blood vessels in the outer medulla (OM). In a kidney with efferent arterioles (EA) but not peritubular capillaries (PTCs) filled, 60% of 88 EAs traversed the outer stripe (OS) as a single main tributary, while 40% divided to form 2-4 large tributaries. Single EAs had 3 (1-6) [mean (range)] side branches to PTCs which served a tissue volume of 0.01 - 0.05 mm³. In the region of the inner stripe (IS), 4 (2-8) EA tributaries converged, each forking 7 (5-9) times (n) to form n+1 descending vasa recta (DVR) in the IS. (Total DVR per vasa recta bundle (VRB) = n+1 × #EA). Most DVR left the VRB laterally, with each DVR forming PTCs which perfused a tissue volume of about 0.008 mm³. From one VRB, the sum of these laterally exiting DVR perfused a cylindrical tissue volume of about 0.196 mm³. Intra-bundle collecting venules (OM ascending vasa recta), some beginning near the inner medulla, emptied into arcuate or cortical radial veins after receiving perfusate from PTCs at many levels of the IS. Results support the view of 3 distinct major pathways for blood flow through the kidney medulla.