The Pathway Of Blood Flow Through A Liver Lobule

The liver is the largest gland and internal organ in the body. It can be found in the right upper quadrant of the abdomen and carries out many vital processes such as detoxification, protein synthesis, and production of various biochemicals needed for digestion. It is generally divided into two lobes: the right and left lobes. The right lobe is further divided into the quadrate and caudate lobes. The liver is unique in that it receives blood from the hepatic portal vein and the hepatic arteries. Majority of the liver’s blood supply (70%) is venous blood from the portal vein. All venous blood returning from the small intestine, stomach, pancreas and spleen are drained into the portal vein. Therefore, the liver firstly receives everything absorbed in the small intestine which makes the blood nutrient rich. The remaining 30% of the liver’s blood supply is oxygenated arterial blood from the hepatic arteries.

The lobes of the liver are made up of microscopic units called hepatic lobules. Hepatic lobules are the anatomical units of the liver and are roughly hexagonal in shape. They consist of rows of hepatocytes radiating outwards from a central vein. The hepatocytes are close linked with sinusoids and lie adjacent to canaliculi where bile is secreted and flows away from the central vein, in the opposite direction to blood. Around the perimeter of the lobules, there are branches of the bile duct, the hepatic artery, and the hepatic portal vein which cluster at the corner of each lobule to form the portal triad. The hepatic portal vein and hepatic artery of the portal triad empty together and mix as they enter the sinusoids in the lobules, delivering oxygen and nutrients to the sinusoids. Blood flows through the sinusoids, allowing many metabolic processes to take place, and empties into the central vein of each lobule, removing metabolic end products and detoxified substances. The central veins of each lobule merge into hepatic veins which leave the liver and empty into the inferior vena cava.

The lobules can be characterized by metabolic zones which define the hepatic acinus - the functional component of the liver. There are three zones, and each zone centers on the line connecting two portal triads and extends outwards towards the central veins. The first zone is the Periportal Zone I. It is closest to the portal triad thus it receives the most oxygenated blood. This makes it the least sensitive to ischemic injury. Because it is the most oxygenated, it is specialized for oxidative functions of the liver including gluconeogenesis, cholesterol synthesis, bile salt extraction and β-oxidation of fatty acids. The second zone is the Midzonal Zone II and it lies between the first and third zones. The third zone is the Centrilobular Zone III and it is the least oxygenated. It lies closer to the central vein and is the most sensitive to ischemic injury. It is specialized for lipogenesis, glycolysis, and drug detoxification.