Liver – Tutorial
Please read Unit 10 – Introduction to Digestive System Tissues and study the image slider of the four major layers of the digestive tract wall prior to completing the activities in this chapter.
Introduction to the Liver
The liver is the largest organ in the abdominal cavity weighing an average of 3.3 pounds (1.5 kg). The liver is composed of four lobes, but most of its mass is from its right lobe which is located in the right upper abdominal quadrant just inferior to the diaphragm and the smaller left lobe located just inferior to the diaphragm and superior to the stomach in the epigastric and left hypochondriac regions. Together, the right and left liver lobes account for over 95% of liver volume. Two smaller lobes known as the caudate and quadrate lobes account for the remaining liver volume. The caudate lobe lies posterior to the right lobe near the inferior vena cava and the quadrate lobe is located between the left lobe and the gall bladder.
The liver is responsible for over 200 different functions, most of which are related to regulating amounts of metabolic fuel molecules, enzymes, lipids, and various proteins circulating in the bloodstream. Although the liver is typically included in the digestive system unit of most anatomy and physiology textbooks, the only function the liver performs that is directly related to digestion is the production of bile. Bile produced by the liver is collected by a system of small bile ducts that drain bile to the duodenum via the common bile duct which delivers bile to the lumen of the duodenum at the duodenal papilla. Bile plays the important roles of emulsifying lipids in the small intestine which improves enzymatic digestion of fats and facilitating the absorption of digested lipids from the lumen of the small intestine into intestinal villi. Bile also serves to buffer acidic chyme entering the duodenum from the stomach.
Blood arrives at the liver from two distinct sources. Approximately one-third of the blood arriving at the liver at any point in time is oxygenated blood supplied by the hepatic artery. The majority of blood reaching the liver arrives via the hepatic portal vein. The hepatic portal system drains venous blood from all organs of the digestive tract within the abdominal cavity, the pancreas, and the spleen and transports them into the liver via the hepatic portal vein.
The hepatic artery and hepatic portal vein continue to branch into smaller vessels that distribute blood to thousands of functional units within the liver known as lobules. Individual lobules are usually shaped like a pentagon or hexagon and are separated from other lobules by thin bands of connective tissue known as interlobular septa. The functional tissue within each lobule is comprised primarily of hepatocytes (liver cells) separated by small vascular spaces for blood to flow known as sinusoids (sinusoidal capillaries). Blood enters sinusoids (vascular passageways) of a lobule from tiny branches of the hepatic artery and hepatic portal vein located in the “corners” of interlobular septa. As blood flows through the sinusoids, the arterial and venous blood mix together and eventually flow out a vein at the center of each lobule known as the central vein which carries blood toward larger hepatic veins and eventually the inferior vena cava. The surfaces of hepatocytes exposed to blood flowing through the sinusoids are covered by short microvilli. As blood flows past hepatocytes, they regulate circulating levels of fuel molecules, lipids, plasma proteins, and various other substances by secreting or absorbing substances into or out of the blood as necessary.
A distinct histological characteristic found within liver tissue is the presence of portal triads (hepatic triads, liver triads) at the “corners” of interlobular septa. Each portal triad consists of three structures: 1) a tiny branch of the hepatic artery known as an interlobular artery, 2) a tiny branch of the hepatic portal vein known as an interlobular vein, and 3) a tiny branch of the bile collecting system known as an interlobular bile duct. The interlobular artery and vein distribute blood to the sinusoids of adjacent lobules as described in the previous paragraph. Bile produced by hepatocytes is collected by tiny bile draining channels between hepatocytes known as bile canaliculi. Bile canaliculi drain bile outward (the opposite direction that blood flows) toward the interlobular septa. Bile from lobules is eventually collected by the interlobular bile ducts of portal triads. Interlobular bile ducts drain bile into increasingly larger ducts that eventually transport all bile from the liver into the right and left hepatic ducts which then merge to form the common hepatic duct which exits the liver. Bile in the common hepatic duct flows into the bile duct (common bile duct) if the entrance into the duodenum at the duodenal papilla is open or it enters the cystic duct if the entrance to the duodenum is closed and is instead transported to the gall bladder for temporary storage.
Tutorial: Use the image slider below to learn more about the structure and characteristics of the liver.
Microscopy: Use the image slider below to learn how to use a microscope to study the liver on a microscope slide.
Tutorial: Use the hotspot image below to learn more about the structure of the liver on a low power image.
Tutorial: Use the hotspot image below to learn more about the structure and function of portal triads (hepatic triads).
