2003A10 Describe the factors influencing hepatic blood flow.

 

List:

·         Hepatic circulation summary

·         Hepatic arterial flow

·         Portal venous flow

·         Hepatic arterial buffer response

 

Hepatic circulation:

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 in the liver

·   Entrance to lobule via hepatic arteriole or portal vein

·   Centripetal transit through sinusoid

·   Exit from lobule via central vein

 

Determinants of hepatic arterial flow:

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, ↑pO2/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

Arterial pressure

·   mHAP = mAP = CO x TPR

·   ↓CO -> ↓mAP -> ↓Q (e.g. hypovolaemia, bradycardia, cardiogenic shock)

·   ↓TPR -> ↓mAP -> ↓Q (e.g. sepsis, anaphylaxis)

Venous pressure

·   HVP = CVP

·   PPV: ↑HVP -> ↓Q

·   Laparoscopy: ↑HVP -> ↓Q

·   Heart failure: ↑HVP -> ↓Q

 

Determinants of portal venous flow:

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 -> α1 constriction -> ↑R -> ↓Q

·   Adrenaline -> α1 constriction -> ↑R -> ↓Q

·   Cirrhosis: scarring -> ↓R (results in portal hypertension)

Portal venous pressure

·   ?

Central venous pressure

·   As above

 

Hepatic arterial buffer response:

Definition

·   Semi-reciprocal supply relationship

·   Hepatic arterioles autoregulate, portal venules do not

·   Blood supply can reach 50%-50%

Mechanism

·   ↓Liver blood flow -> ?↑adenosine release -> hepatic arteriolar dilatation

 

 

 

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