Outline the functional anatomy, and the physiological factors, that determine oxygen delivery to the renal medulla.
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.
The examiners chose to ask this question in a weird roundabout way here, by phrasing it in terms of oxygen delivery to the medulla. This wording will probably have led 96% of the candidates to assume that the physiological and anatomical factors determining renal medullary blood flow were being asked about. To discuss the oxygen content of whole blood and cardiac output would not have occurred to most people, as of course the kidney has little to do with them, and those people would have lost marks.
Factors that influence oxygen delivery and blood flow to the medulla:
Pallone, Thomas L., Zhong Zhang, and Kristie Rhinehart. "Physiology of the renal medullary microcirculation." American Journal of Physiology-Renal Physiology 284.2 (2003): F253-F266.
Zimmerhackl, Bernd, Channing R. Robertson, and Rex L. Jamison. "The microcirculation of the renal medulla." Circulation research 57.5 (1985): 657-667.