Question 3

Compare and contrast renal and hepatic blood flow, and their regulation.

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College Answer

It was expected candidates would describe the salient features of the anatomy, distribution and 
content of blood flow and influences on each circulation. Answers with a clear organisation and 
context for the normal influences of blood flow on the functioning of each organ system scored 
highly. Anatomy was often sufficiently covered, but candidates often did not take advantage of 
that by linking the anatomical features to the functional concepts. Figures should be clearly and accurately labelled to score well. Many answers failed to demonstrate a depth of understanding 
of key concepts. For example tubuloglomerular feedback, relationship between hepatic arterial 
and portal venous flows and autoregulation within both those systems was often poorly 
described.

Discussion

This question is identical to Question 11 from the first paper of 2012, except with better grammar. 

A Comparison of Hepatic and Renal Regional Circulations
Domain Hepatic circulation Renal circulation
Anatomy of blood supply Portal vein (70%)
Hepatic artery (30%)
Renal arteries (100%)
Blood flow (ml/min) total = 1500 ml/min total = 1100 ml/min
Blood flow
(% of CO)
25% of cardiac output 20% of cardiac output
Venous drainage Hepatic veins  Renal veins
Oxygen consumption 6ml O2/100g/min 2ml O2/100g/min
Blood flow distribution Arterial and portal venous blood mix in hepatic sinusoids 95% of flow goes to the renal cortex, 5% to medulla
Capillary beds Hepatic sinusoids (large diameter low-pressure capillaries with discontinuous endothelium) Glomerular capillary bed;
peritubular capillary bed
Function Metabolic activity
Blood storage reservoir
Filtration
Clearance of waste substances
Regulatory mechanisms Portal venous flow regulation:
  • Flow rate is mainly determined by splanchnic arterial flow rate
  • Resistance changes in response to:
    • Humoural signals (eg. catecholamines), in shock
    • Local endocrine signals (eg. VIP), causing vasodilation following a meal

Hepatic arterial flow regulation:

  • Standard arterial regulatory mechanisms: myogenic, flow(shear)-mediated, conducted vasomotor responses, immunologically mediated by inflammatory molecules.
  • Hepatic arterial buffer response: hepatic arterial flow increases if portal venous flow decreases, and vice versa.

Local arterial autoregulation:

  • Myogenic stretch response of the afferent arteriole
  • Metabolic autoregulation (products of cellular metabolism)

Systemic autoregulation:

  • RAAS: angiotensin II vasoconstricts afferent and efferent arterioles
  • Other humoural influences: endothelin, adenosine, nitric oxide, dopamine, bradykinin and acetylcholine

Extrarenal factors affecting flow:

  • Dietary protein intake increases renal blood flow
  • Hyperglycaemia increases renal blood flow
  • Sympathetic stimulation activates RAAS

References

Eipel, Christian, Kerstin Abshagen, and Brigitte Vollmar. "Regulation of hepatic blood flow: the hepatic arterial buffer response revisited." World journal of gastroenterology: WJG 16.48 (2010): 6046.

Abshagen, K., et al. "Anatomy and physiology of the hepatic circulation." PanVascular Medicine (2015): 3607-3629.

Greenway, C. V., and R. D. Stark. "Hepatic vascular bed." Physiological reviews 51.1 (1971): 23-65.

Dancygier, Henryk. "Hepatic Circulation." Clinical Hepatology. Springer, Berlin, Heidelberg, 2010. 55-

Wilkinson, Grant R., and David G. Shand. "A physiological approach to hepatic drug clearance." Clinical Pharmacology & Therapeutics 18.4 (1975): 377-390.