The liver’s network of blood vessels, known as the liver vasculature, is fundamental to its operation. This system is responsible for supplying the liver with blood and draining it after it has been processed. The liver performs approximately 500 metabolic functions, including detoxifying the body and synthesizing proteins. Unlike any other organ, the liver receives blood from two distinct sources, a feature that allows it to manage substances absorbed from our diet while receiving the oxygen it needs to survive.
The Liver’s Dual Blood Supply
The liver’s blood supply is divided between two major vessels: the hepatic artery and the portal vein. The hepatic artery branches from the aorta and is responsible for delivering oxygen-rich blood to the liver, accounting for about 25-30% of the liver’s total blood flow. This arterial supply provides the necessary oxygen for the liver’s cells to function and remain healthy.
The majority of the liver’s blood, about 70-75%, comes from the portal vein. This large vein collects nutrient-dense blood from the entire gastrointestinal tract, including the stomach, intestines, spleen, and pancreas. This blood is filled with substances absorbed during digestion. The portal vein operates under low pressure, between 3 to 5 mm Hg, which facilitates the liver’s processing functions. This dual system ensures the liver receives both oxygen for its own metabolic needs and the raw materials from digestion that it is tasked with processing for the rest of the body.
The Internal Vascular Network
Once inside the liver, blood from the hepatic artery and portal vein enters a complex internal network. The vessels from these two sources subdivide repeatedly, eventually mixing together within a specialized type of capillary called a liver sinusoid. These sinusoids are wider than typical capillaries and are designed to facilitate a slow, low-pressure blood flow. This slow pace maximizes the contact time between the blood and the liver’s main functional cells, the hepatocytes.
Hepatocytes are organized into functional units called hepatic lobules, which are roughly hexagonal structures. Blood flows from the portal triads, located at the corners of the lobule, through the sinusoids toward a single central vein at the lobule’s core. As blood percolates through this system, hepatocytes absorb nutrients for storage or conversion, detoxify harmful substances like alcohol and drugs, and produce proteins and bile. The slow flow and direct contact in the sinusoids give hepatocytes the opportunity to perform their vast array of metabolic tasks.
How Blood Exits the Liver
After blood has been processed by the hepatocytes, it needs to be returned to the general circulation. The journey out of the liver begins as the blood collects in the central veins located at the heart of each lobule. These individual central veins then merge, much like small streams flowing into larger rivers, into progressively larger vessels known as hepatic veins.
This convergence continues until three main hepatic veins are formed: the right, middle, and left hepatic veins. The final step in this vascular circuit is the connection of the hepatic veins to the body’s largest vein, the inferior vena cava. The hepatic veins empty the newly filtered and nutrient-balanced blood directly into the inferior vena cava, which then transports it back to the heart to be re-oxygenated and pumped out to the rest of the body.
When Liver Blood Flow Is Disrupted
Disruptions to the liver’s vascular system can have significant consequences. The most common issue arises when blood flow through the liver is obstructed, leading to a condition known as portal hypertension. This is a serious complication of underlying liver problems, most frequently cirrhosis. Cirrhosis involves the replacement of healthy liver tissue with hard scar tissue, which impedes the flow of blood through the sinusoids.
This scarring creates resistance within the liver, making it difficult for blood from the portal vein to pass through. As a result, pressure builds up in the portal vein system, rising well above the normal low-pressure state. This elevated pressure forces blood to find alternative routes back to the systemic circulation, often through smaller, ill-equipped veins.
The high pressure can cause fluid to leak from the surface of the liver and intestines, accumulating in the abdomen, a condition called ascites. It can also cause veins in the esophagus and stomach to swell, creating fragile varices that are prone to bursting and causing life-threatening bleeding.