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

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College Answer

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. 20 – 30% 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.