By Steve Morgan & Sophie Connolly Welcome back to the Basic Science Clinic podcast on ICN. Post-hiatus we are ready to reinvigorate the examination of oxygen cascade physiology, from the prevailing atmosphere down to the only organelle that boasts its own bespoke genome, the mitochondrion. In the last podcast we decomposed the minutiae of passive respiratory gas diffusion across the alveolar capillary membrane. Prior to expounding the pre-eminence of V/Q ratios in determining gas exchange sufficiency, we need to publically vivisect the pulmonary circulation to bring you the belated Raw Science 11, pulmonary perfusion. This detailed inspection of pulmonary perfusion is the longest podcast yet, no doubt we got slightly carried away and thus we have broken it up into three more comfortably digested sections. Section 1 (start to 18:24) will include the historical bit and both adult and fetal anatomy. Section 2 (18:30 to 40:26) examines pulmonary haemodynamics and the integrated control of pulmonary vascular tone. Section 3 (40:35 to end) details the protean functions of the pulmonary endothelium and endothelial glycocalyx, the determinants of transvascular fluid flux in the lung, all with reference to the pathophysiology of acute lung injury. The pulmonary circulation participates in gas exchange, blood filtration, metabolic regulation of endogenous vasoactive mediators, drug uptake, metabolism and excretion and the regulation of lung interstitial fluid homeostasis, a dexterous function devastated by the clinical syndrome of ARDS. Understanding the idiosyncracies of the pulmonary circuit is imperative for interpreting heart-lung interactions that influence V/Q distribution & gas exchange efficiency as well as overall cardiovascular performance, particularly during positive pressure ventilation in the context of concomitant shock states.