Final answer:
The liver's cardiac output is mainly supplied by the portal vein, which brings in nutrient-rich blood from the gastrointestinal tract. The hepatic artery also supplies the liver with oxygenated blood. After processing, blood exits the liver via hepatic veins to the inferior vena cava.
Step-by-step explanation:
The cardiac output to the liver is mainly supplied by the portal vein, and not directly from the left ventricle. Therefore, the correct answer is Option 4: 'Mainly supplied by the portal vein.'
The liver receives blood from two primary sources: the hepatic artery, which delivers oxygenated blood from the heart, and the portal vein, which brings in nutrient-rich and partially deoxygenated blood from the gastrointestinal tract and spleen. After processing the blood, the liver sends it back into circulation via the hepatic veins to the inferior vena cava, completing the hepatic portal circulation.