Liver Anatomy and Function

The liver is an important as well as structurally and functionally complex organ. One of the important functions of the liver is to receive and take up nutrients and other molecules absorbed from the intestines before the blood is returned to the heart via the inferior vena cava. The liver has dual blood supplies, one venous and one arterial. The portal vein, which carries blood from the spleen, pancreas, and intestines, contributes about 75% of the blood volume to the liver. This blood is rich in nutrients and other absorbed molecules, but relatively poor in oxygen content.

The hepatic artery, a branch of the celiac trunk from the abdominal aorta, contributes about 25% of blood volume entering the liver. This blood is well oxygenated, but nutrient poor. Blood from the portal vein and hepatic artery mixes as it enters and passes through a liver labial, the classical structural unit of the liver. Blood then percolates through the labial and is collected by the hepatic or central vein. Central veins unite to form larger sub-lobular veins, which ultimately join hepatic veins. Blood flows to the heart via the inferior vena cava.

Bial comprises the exocrine secretory product of the liver. It is stored in the gallbladder. During digestion, bile is released from the gallbladder through the cystic duct and to the duodenum to aid in the digestion of fats. The filtration of blood and the production of bile takes place within the structural and functional unit of the liver. The lobule, where the terminal branches of the vessel's carrying incoming and outgoing blood meet. The liver lobule is roughly hexagonal in cross-section. In the center of the lobule lies the central vein, which collects blood from the lobule and carries it to the hepatic veins.

Located at the lobule periphery are branches of the vessel supplying incoming blood, the portal vein and the hepatic artery. Also located here is the bile duct. Collectively, the portal vein, hepatic artery and the bile duct together comprise the portal triad. Taking a horizontal cross-section of the lobule gives us an arrangement typically seen in histological preparations. In this view, we can identify the central veins and associated portal triads. There are two prevailing views of liver lobule organization, the classic liver lobule or anatomical model and the liver assonus.

The classic liver lobule is based on the structure of the lobule we just discussed. It is hexagonal in shape and is defined by portal triads surrounding a single central vein. To identify a lobule in a histological slide, first find a central vein. Next, locate the portal triad surrounding that vein, usually about six at the corners of the hexagonal prism. An imaginary line connecting the triads represents the outside border of the lobule. In the classic liver lobule, blood flows from the portal triads toward the central vein. Bile flows in the opposite direction, from the central position toward the portal triads of the lobule periphery.

Remember, the classic liver lobule model emphasizes the anatomical structure of the lobule. The liver assonus is another way to view liver structure. This model is important for understanding liver physiology and pathology. The structure of the assonus overlaps two classic liver lobules. The assonus is diamond or oval shaped. It involves approximately two portal triads and two central veins. To identify the assonus, find two adjacent portal triads. This makes the short axis. Next, find the closest central vein on either side of the triads. This is the long axis.

The resulting shape is a diamond or oval. Blood still flows from the portal triad to the central vein, but within the shape of the assonus, the flow is from the center to the periphery. Bile flows in the opposite direction, from the periphery towards the center. The assonus model is important because it emphasizes the metabolic differences and hepatocytes between the portal triads and the central vein. The cells in the assonus are divided into three zones and are numbered in the direction of blood flow, from one to three.

Cells in zone one lie along the short axis, between the portal triads. Thus, these cells are first to receive everything in blood, including oxygen, absorbed nutrients, and absorbed xenobiotics. Cells in zone two have characteristics intermediate between the cells of zones one and three. Cells in zone three are closest to the central vein, and therefore are the last to receive incoming blood. As blood moves towards the central vein, it becomes depleted of oxygen, nutrients, and xenobiotics. The liver lobule is the anatomical model. It is hexagonal in shape. It involves one anatomical liver lobule, and blood flows from the periphery to the center.

The liver assonus focuses on function. It is diamond or oval shaped. It involves two adjacent anatomical liver lobules. The blood flows from the middle, which we discuss as a short axis, to the periphery.