2003A10 Describe the factors influencing hepatic blood flow.

· Hepatic circulation summary

· Hepatic arterial flow

· Portal venous flow

· Hepatic arterial buffer response

Arterial supply

· Total 1400mL/min = 100mL/min/100g = 30% of cardiac output

· Blood: 75% portal vein, 25% hepatic artery

· Oxygen: 50% portal vein, 50% hepatic artery

Venous drainage

· Hepatic vein 1400mL/min

Flow pattern

· Entrance to lobule via hepatic arteriole or portal vein

· Centripetal transit through sinusoid

· Exit from lobule via central vein

Equations	· Hepatic arterial flow = (mHAP – HVP) / hepatic arteriolar resistance · HAP = mAP = 70mmHg · HVP = CVP = 0-5mmHg · R = (8 x length x viscosity) / (π x radius⁴)
Vascular resistance	· Myogenic autoregulation: ↓mAP -> ↓stretch -> reflex vasodilatation -> ↓R -> ↑Q · Metabolic autoregulation: ↓pO₂, ↑pCO₂/H⁺/K⁺/adenosine -> vasodilatation -> ↓R -> ↑Q · Neural: SNS -> α₁ vasoconstriction -> ↑R -> ↓Q · Hormonal: adrenaline -> α₁ constriction then β₂ dilatation; overall minimal change · Anaesthesia: halothane inhibits hepatic arteriolar myogenic response to hypotension
Hepatic arterial pressure	· mHAP = mAP = CO x TPR · ↓CO -> ↓mAP -> ↓Q (e.g. hypovolaemia, bradycardia, cardiogenic shock) · ↓TPR -> ↓mAP -> ↓Q (e.g. sepsis, anaphylaxis)
Hepatic venous pressure	· HVP = CVP · PPV: ↑HVP -> ↓Q · Laparoscopy: ↑HVP -> ↓Q · Heart failure: ↑HVP -> ↓Q

Equations	· Portal venous flow = (PVP – HVP) / portal venous resistance · PVP = 5-10mmHg · HVP as above (normal gradient <5mmHg) · R = (8 x length x viscosity) / (π x radius⁴)
Vascular resistance	· Metabolic autoreg: ↑GIT activity -> ↓R -> ↑Q · SNS/adrenaline -> α₁ constriction -> ↑R -> ↓Q · Cirrhosis: scarring -> ↑R -> ↓Q (and ↑pressure = portal hypertension) · Post-prandial splanchnic vasodilation -> ↓R -> ↑Q
Portal venous pressure	-
Hepatic venous pressure	· Hepatic vein thrombosis: ↑HVP -> ↓Q

Definition

· Semi-reciprocal supply relationship

· Hepatic arterioles autoregulate, portal venules do not

· Blood supply can reach 50%-50%

Mechanism

· ↓Liver blood flow -> ↓adenosine removal -> hepatic arteriolar dilatation