The liver performs metabolic functions requiring a constant, large supply of blood to deliver raw materials and remove waste products. At any given moment, the liver holds approximately 13% of the body’s entire blood volume. The continuous flow of blood is the mechanism by which the liver filters, processes, and manages nearly everything absorbed from the digestive tract. This high-volume circulation system enables the liver to act as the body’s processing plant.
The Liver’s Unique Dual Blood Supply
The liver receives its blood supply from two distinct sources, a feature known as dual circulation. Roughly 70 to 75% of the blood flows into the liver via the hepatic portal vein, which collects nutrient-rich blood from the gastrointestinal tract, the spleen, and the pancreas. This blood is rich in newly absorbed nutrients like sugars, amino acids, and fats, but it is relatively low in oxygen.
The remaining 25 to 30% of the blood supply comes through the hepatic artery, which branches directly from the systemic circulation. This arterial blood is highly oxygenated, and its purpose is to deliver oxygen to the liver cells, or hepatocytes, for their high metabolic needs. Even though the hepatic portal vein carries the majority of the blood volume, the oxygen supply is roughly split, with the portal vein providing about half of the liver’s oxygen requirements.
Both the portal vein blood and the arterial blood merge within the liver’s specialized capillaries, called sinusoids. These sinusoids are lined with highly permeable cells, allowing the blood to interact closely with the hepatocytes. After this exchange and processing, the blood collects in the central veins of the liver lobules and ultimately drains out through the hepatic veins, which empty into the inferior vena cava.
Essential Functions of Hepatic Blood Flow
The constant flow of blood through the liver enables metabolic processing. As the nutrient-rich blood from the intestines passes through the sinusoids, the liver cells begin managing the influx of absorbed substances. For instance, excess glucose is quickly removed from the bloodstream and converted into glycogen for storage, helping to stabilize blood sugar levels.
The blood flow also facilitates the processing of fats and proteins, helping to synthesize various proteins needed for plasma and regulating the levels of amino acids in the circulation. The liver cells convert ammonia, a byproduct of protein metabolism, into a less toxic substance called urea. This urea is then released back into the blood to be transported to the kidneys for excretion in urine.
The blood flow supports detoxification, where the liver filters and chemically alters numerous compounds. This includes drugs, alcohol, and other metabolic waste products, making them less harmful or water-soluble so they can be eliminated from the body. Furthermore, the hepatocytes utilize components in the blood to continuously produce bile, an alkaline fluid that aids in the digestion and absorption of dietary fats.
How the Body Regulates Liver Circulation
The body possesses mechanisms to ensure the liver receives a stable amount of blood, even when systemic circulation changes. A notable mechanism is the Hepatic Arterial Buffer Response (HABR), an intrinsic ability of the liver to stabilize total blood flow. If the flow from the portal vein suddenly decreases, the hepatic artery automatically widens, or dilates, to increase its own flow and compensate for the loss.
Conversely, if the flow from the portal vein increases, the hepatic artery constricts to prevent a surge in total blood volume within the liver. This reciprocal relationship between the two inflow vessels maintains a steady level of perfusion, which is necessary for consistent metabolic function. Following a meal, blood flow to the liver’s portal system naturally increases to handle the influx of absorbed nutrients, directing more resources to the liver for processing.
Consequences of Impaired Hepatic Blood Flow
Disruptions to the liver’s circulatory system can lead to health complications. The most common cause of impaired flow is cirrhosis, a condition where healthy liver tissue is replaced by stiff, non-functional scar tissue. This scar tissue acts as a physical barrier, obstructing the flow of blood through the liver’s sinusoids.
The blockage causes a backup of pressure in the hepatic portal vein system, termed portal hypertension. When pressure builds high enough, the blood is forced to reroute, bypassing the liver through alternate, smaller veins that are not designed to handle high pressure or volume. These collateral veins, particularly those in the esophagus and stomach, can become enlarged, leading to swollen vessels called varices.
Portal hypertension also drives the formation of ascites, which is the accumulation of fluid in the abdominal cavity. When blood flow is severely restricted, especially if the hepatic artery’s oxygen supply is compromised, the liver tissue can suffer from ischemia, or a lack of oxygen. Prolonged ischemia can result in the death of liver cells, compounding the damage and metabolic dysfunction already present.