Hepatic portal circulation
The liver is unusual in that it receives the majority of its metabolic requirements from a venous source. The normal liver gets about 70% of its O2 requirement via the portal vein. The portal vein also delivers the dietary carbohydrates used to fuel liver activity.
The portal system begins in the capillaries and venules of the digestive system. It collects venous blood from the lower esophagus, stomach, duodenum, jejunum, ileum, colon, spleen and delivers it to the liver via portal vein. Consequently, portal blood contains the substances absorbed by the digestive tract.
Normal portal flow and pressure vary depending upon: cardiac output, intra-abdominal pressure, disease process, positioning, feeding schedule, time of day, etc. Circadian variations begin to increase portal pressure around 19:00 hours, reaching peak pressures around 09:00. Portal pressure decreases from 09:00 to about 19:00 hours. Interestingly, peak reports of bleeding varicies correspond with 09:00 and 23:00 hours.*
Normal hepatic circulation is a high flow - low resistance system. Branches of the portal vein deliver 1000-1500 ml/min of blood into the sinusoids of the hepatic lobules. Normal portal venous pressure is 5-10 mm Hg.* The blood passes through the sinusoids and drains into the inferior vena cava. Inferior vena cava pressure ranges from -5 to +5 mmHg.* Portal pressure >10mm Hg may indicate portal hypertension.*
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