Compare and contrast renal and hepatic blood flow and its regulation

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College Answer

Another fundamental physiology topic that required candidates to understand, and 
synthesize knowledge from multiple areas. Generally well done with some very good 
answers. A tabular format worked well. Candidates were expected to mention values for 
renal and hepatic flow (total flow, % of cardiac output and oxygen consumption), basic 
anatomical comparisons, distribution (e.g. renal cortex 95% , renal medulla 5%,), two
capillary beds (glomerular and peritubular) for renal, and hepatic triad and sinusoids for the 
liver, function (filtration for renal blood flow and metabolic activity for hepatic) and 
regulatory mechanisms for each (e.g. myogenic, autonomic, metabolic and humoral for 
both and tubuloglomerular feedback for renal).

Discussion

These college comments are commendably detailed, and they were used to compose the answer included here, as they are helpfully prescriptive about the format and content. With the local renal section in advanced stages of decomposition, the author had no original thoughts about the kidney, and referred extensively to Judith Askew's excellent answer.

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.