Outline the functional anatomy, and the physiological factors, that determine oxygen delivery to the renal medulla.

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1 Outline the functional anatomy, and the physiological factors, that determine oxygen delivery to the renal medulla. Oxygen delivery = Blood flow CaO 2 Where Blood flow determined by (arterial pressure venous pressure) / vascular resistance Hagen-Poisuille Equation ( pressure) πr 4 /8 viscosity length CaO 2 = ([Hb] SaO ) + (0.03 p a O 2 ) Kidneys - Lie in the posterior wall of the abdomen outside peritoneum (retroperitoneal) - Each kidney approx 150g - Medial side of each kidney = hilum Renal artery and vein Lymphatic supply Nerves Ureter - Kidney surrounded by tough fibrous capsule - Kidney organized into outer cortex and inner medulla - Nephrons = functional unit of the kidney (1 million / kidney) - Medulla Organised into mutliple pyramids with the base at the corticomedullary junction and apices the papilla at the hilum draining into the ureters - Anatomy of renal blood flow Renal blood flow ~1.25L via renal artery As renal artery enters through hilum devides interlobar arteries arcuate arteries interlobular arteries afferent arterioles

2 glomerular capillaries efferent arterioles peritubular capillaries Juxtaglomerular capillaries have specialized vasa recta which supply the LoH and is important in renal concentration of urine venou system interlobular veins arcuate veins interlobar veins Peritubular capillaries supply blood to the renal tubules Physiology of renal blood flow - Renal blood flow Renal vascular resistance maintained by interlobular arteries, afferent arterioles and efferent arterioles Autoregulated between 80~170mmHg Neural Sympathetic All renal vescles richly innervated by sympathetic nerves Minimal influence on GFR except in severe acute disturbances (severe haemorrhage) Decrease blood flow due to vasoconstriction Hormonal Adrenaline and noradrenaline Vasoconstrict decrease blood flow Endothelin Vasoconstrict decrease blood flow Angiotensin II Constricts efferent arterioles decrease blood flow Nitric oxide Vasodilate increase blood flow Prostaglandins Vasodilate increase blood flow - Starling resistors

3 Increased intra-abdominal pressure will decrease blood flow in starling resistor model Increased intra-capsular pressure will decrease renal blood flow Medullary blood flow - 1~2% of total renal blood flow 0.6ml/g/min in inner medulla 2.5ml/g/min in outer medullar 5ml/g/min in cortex) - 20~30% nephrons are juxtaglomerular with specialized vasa recta - Blood from post-glomerulus therefore decreased plasma (filtered) proportion and increased viscosity Decreased flow Factors the increase filtration fraction decrease medullary flow (e.g. angiotensin II) - Oxygen supply determined by state of oxyhb dissociation curve Right shift favours oxygen supply Renal medullary po 2 = 15mmHg (compared with 50mmHg in cortex) Increased O 2 offloading - Renal oxygen consumption Increases with sodium resorption

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6 Examiners Comments A good answer required mentioning factors that affect systemic oxygen capacity and delivery (eg Hb, cardiac output, PaO2, etc), Hb-HBO2 dissociation and a description of the anatomy and regulation of renal medullary blood flow % of nephrons have glomeruli deep in the cortex and long Loop of Henle that go deep into medulla. Here blood supply differs long efferent arterioles from glomeruli into outer medulla and inner cortex that then divide into vasa recta deep in medulla. These vessels carry post glomerular blood so have less serum, mostly plasma, ie more viscous and concentrated. Factors that influence medullary blood flow include: Sympathetic stimulation decrease (via efferent arteriolar constriction), Angiotensin II (via tubuloglomerular feedback) decrease (via efferent arteriolar constriction), Endothelin decrease (via efferent arteriolar constriction), Prostaglandins increase, Bradykinin increase, high protein meal increase, high glucose levels increase. Candidates performed poorly due to a lack of knowledge of the topic and/or failure to logically present their answer. Common mistakes were to give no value for renal blood flow, to discuss the function of the medulla which was not required and to not describe the factors that influence medullary blood flow.