The liver is the largest visceral organ in the human body, receiving 25% of the cardiac output. It has a unique circulatory system that connects capillaries from various gastrointestinal organs to the liver’s capillaries. This article will discuss hepatic circulation and its functions.
Function
The liver’s circulatory system supplies the liver with nutrients and allows the liver to process toxins before the blood reaches the systemic circulation. To understand the circulatory system of the liver, we should briefly review the anatomy of the liver.
Structure
The liver is made up of lobules that consist of a central vein and a portal triad. The portal triad consists of:
- Arteriole (branch of the hepatic artery)
- Venule (branch of the portal vein)
- Bile ductule (branch of the bile duct)
- Lymphatic vessels
- Branch of the vagus nerve
Circulation
The liver receives a dual blood supply from the hepatic artery and portal vein. The hepatic artery is a branch of the coeliac trunk and provides 30% of blood to the liver. The portal vein, formed by the mesenteric and splenic veins, supplies 70% of the blood to the liver. Blood from the portal vein blood contains absorbed nutrients (such as glucose, fatty acids and amino acids) from the GI tract.
Firstly, oxygen is delivered to the hepatic circulation from both of these arteries. Approximately 50% of oxygen demand is met by the portal vein and 50% by the hepatic artery. The hepatic artery carries oxygenated, nutrient-poor blood to the liver whereas the portal vein carries nutrient-rich, oxygen-poor blood to the liver. The blood then flows through the liver and provides it with oxygen and nutrients.
Fig 1 – Structure of a single hepatic lobule.
The venous supply is drained from the gastrointestinal organs, including the colon, small intestine, large intestine, pancreas, stomach and spleen. This deoxygenated, nutrition-rich blood enters the liver via the portal vein into the sinusoids (low-pressure capillary-like vascular channels), which allow efficient transfer of nutrients in the liver.
The liver then metabolises the carbohydrates, lipids and proteins, while also removing toxins and bacteria from the blood. After this, approximately 50% of water-soluble nutrients are taken up from the venous blood and stored. Subsequently, blood collects in the central veins of each lobule and drains into the hepatic veins, into the inferior vena cava and back to the heart.
The portal vein is known to deliver nutrient-rich blood to the liver. After a meal, when the demand for nutrient absorption is higher than other structures, there is a shift of blood supply to the liver. 90% of the blood being delivered to the liver now comes from the portal vein. This is known as the post-prandial effect.
Clinical Relevance – Portal Hypertension
Portal hypertension is high blood pressure in the hepatic portal system and is a common sign of cirrhosis (a chronic liver disease resulting in scarring of the liver). The hepatic circulation is a high-volume, low-pressure system – despite the liver receiving a large portion of cardiac output, the pressure of blood entering and leaving the liver is approximately 9mmHg and 1mmHg, respectively.
Fig 2 – A cirrhotic liver
If the liver becomes damaged from alcoholism, hepatitis or a poor diet, the specialised capillaries in the liver (called sinusoids) become more fibrotic. This causes vasoconstriction in the liver, increasing pressure and resistance and thus leading to portal hypertension. This portal hypertension causes blood to back up into the portal vein and vessels of the stomach, spleen and colon, causing them to become dilated and tortuous.
These dilated vessels are known as varices. Varices can sometimes be seen on the abdomen, called caput medusae. Varices are weak and are not designed to carry large volumes of blood, meaning they can leak or rupture, causing life-threatening bleeding.