The liver is the largest gland in the body, weighing about three pounds in an adult. It is also one of the most important organs. In addition to being an accessory digestive organ, it plays a number of roles in metabolism and regulation. The liver lies inferior to the diaphragm in the right upper quadrant of the abdominal cavity and receives protection from the surrounding ribs.

The liver is divided into two primary lobes: a large right lobe and a much smaller left lobe. In the right lobe, some anatomists also identify an inferior quadrate lobe and a posterior caudate lobe, which are defined by internal features. The liver is connected to the abdominal wall and diaphragm by five peritoneal folds referred to as ligaments. These are the falciform ligament, the coronary ligament, two lateral ligaments, and the ligamentum teres hepatis. The falciform ligament and ligamentum teres hepatis are actually remnants of the umbilical vein, and separate the right and left lobes anteriorly. The lesser omentum tethers the liver to the lesser curvature of the stomach.

The porta hepatis (“gate to the liver”) is where the hepatic artery and hepatic portal vein enter the liver. These two vessels, along with the common hepatic duct, run behind the lateral border of the lesser omentum on the way to their destinations. As shown in image, the hepatic artery delivers oxygenated blood from the heart to the liver. The hepatic portal vein delivers partially deoxygenated blood containing nutrients absorbed from the small intestine and actually supplies more oxygen to the liver than do the much smaller hepatic arteries. In addition to nutrients, drugs and toxins are also absorbed. After processing the bloodborne nutrients and toxins, the liver releases nutrients needed by other cells back into the blood, which drains into the central vein and then through the hepatic vein to the inferior vena cava. With this hepatic portal circulation, all blood from the alimentary canal passes through the liver. This largely explains why the liver is the most common site for the metastasis of cancers that originate in the alimentary canal.

The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs.

Salivary Glands

Three pairs of major salivary glands (parotid, submandibular, and sublingual glands) and numerous smaller ones secrete saliva into the oral cavity, where it is mixed with food during mastication. Saliva contains water, mucus, and enzyme amylase. Functions of saliva include the following:

• It has a cleansing action on the teeth.
• It moistens and lubricates food during mastication and swallowing.
• It dissolves certain molecules so that food can be tasted.
• It begins the chemical digestion of starches through the action of amylase, which breaks down polysaccharides into disaccharides.

Liver

The liver is located primarily in the right hypochondriac and epigastric regions of the abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface, the liver is divided into two major lobes and two smaller lobes. The functional units of the liver are lobules with sinusoids that carry blood from the periphery to the central vein of the lobule.

The liver receives blood from two sources. Freshly oxygenated blood is brought to the liver by the common hepatic artery, a branch of the celiac trunk from the abdominal aorta. Blood that is rich in nutrients from the digestive tract is carried to the liver by the hepatic portal vein.

The liver has a wide variety of functions and many of these are vital to life. Hepatocytes perform most of the functions attributed to the liver, but the phagocytic Kupffer cells that line the sinusoids are responsible for cleansing the blood.

Liver functions include the following:

• secretion
• synthesis of bile salts
• synthesis of plasma protein
• storage
• detoxification
• excretion
• carbohyrate metabolism
• lipid metabolism
• protein metabolism
• filtering

Gallbladder

The gallbladder is a pear-shaped sac that is attached to the visceral surface of the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main components of bile are water, bile salts, bile pigments, and cholesterol.

Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of hemoglobin are excreted from the body in the bile.

Pancreas

The pancreas has both endocrine and exocrine functions. The endocrine portion consists of the scattered islets of Langerhans, which secrete the hormones insulin and glucagon into the blood. The exocrine portion is the major part of the gland. It consists of pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between the cells. Pancreatic enzymes include anylase, trypsin, peptidase, and lipase. Pancreatic secretions are controlled by the hormones secretin and cholecystokinin.

The liver has three main components: hepatocytes, bile canaliculi, and hepatic sinusoids. A hepatocyte is the liver’s main cell type, accounting for around 80 percent of the liver's volume. These cells play a role in a wide variety of secretory, metabolic, and endocrine functions. Plates of hepatocytes called hepatic laminae radiate outward from the portal vein in each hepatic lobule.

Between adjacent hepatocytes, grooves in the cell membranes provide room for each bile canaliculus (plural = canaliculi). These small ducts accumulate the bile produced by hepatocytes. From here, bile flows first into bile ductules and then into bile ducts. The bile ducts unite to form the larger right and left hepatic ducts, which themselves merge and exit the liver as the common hepatic duct. This duct then joins with the cystic duct from the gallbladder, forming the common bile duct through which bile flows into the small intestine.

A hepatic sinusoid is an open, porous blood space formed by fenestrated capillaries from nutrient-rich hepatic portal veins and oxygen-rich hepatic arteries. Hepatocytes are tightly packed around the fenestrated endothelium of these spaces, giving them easy access to the blood. From their central position, hepatocytes process the nutrients, toxins, and waste materials carried by the blood. Materials such as bilirubin are processed and excreted into the bile canaliculi. Other materials including proteins, lipids, and carbohydrates are processed and secreted into the sinusoids or just stored in the cells until called upon. The hepatic sinusoids combine and send blood to a central vein. Blood then flows through a hepatic vein into the inferior vena cava. This means that blood and bile flow in opposite directions. The hepatic sinusoids also contain star-shaped reticuloendothelial cells (Kupffer cells), phagocytes that remove dead red and white blood cells, bacteria, and other foreign material that enter the sinusoids. The portal triad is a distinctive arrangement around the perimeter of hepatic lobules, consisting of three basic structures: a bile duct, a hepatic artery branch, and a hepatic portal vein branch.

Your liver is one of the largest and most important organs in your body. You have only one liver. It is the size of a football and weighs about 3 pounds in the average-size person. It is reddish-brown. Your liver is located on the right side of your abdomen behind your lower ribs, and your ribs help to protect your liver.

The liver is a filter

Your liver has many important jobs. One job is to act as a "filter" for your body. The liver filters or detoxifies the blood.

Almost all the blood in your body passes through the liver. As blood passes through the liver, it breaks down substances, such as prescription or over-the-counter drugs, street drugs, alcohol, and caffeine.

Our bodies naturally produce some harmful (toxic) chemicals or poisons, and those are also broken down by the liver. In this way the liver acts as a filter to clean your blood.

The liver is a factory

The liver is also a "chemical factory" -- performing over 500 chemical functions in your body! The liver takes certain materials in your body and turns them into something else. For example, your liver turns proteins and sugars into things that your body needs.

The liver produces blood-clotting factors that are needed to help you heal after an injury.

It also stores vitamins, hormones, cholesterol, and minerals. Your liver lets go of these chemicals and nutrients when your body needs them, and they flow into your bloodstream.

The liver also produces a greenish fluid called bile. Tubes, called "bile ducts," connect the liver and another organ, the gallbladder, to the small intestine. The bile that is made by the liver helps to digest fats in the small intestine.

Summary of liver functions

The liver:

• Filters your blood
• Makes proteins, including blood-clotting factors (needed to help you heal)
• Stores vitamins, sugars, fats, and other nutrients
• Helps regulate hormones
• Releases chemicals and nutrients into the body when needed
• Makes bile needed for digesting fats
• And much more

Lipids are hydrophobic, that is, they do not dissolve in water. Thus, before they can be digested in the watery environment of the small intestine, large lipid globules must be broken down into smaller lipid globules, a process called emulsification. Bile is a mixture secreted by the liver to accomplish the emulsification of lipids in the small intestine.

Hepatocytes secrete about one liter of bile each day. A yellow-brown or yellow-green alkaline solution (pH 7.6 to 8.6), bile is a mixture of water, bile salts, bile pigments, phospholipids (such as lecithin), electrolytes, cholesterol, and triglycerides. The components most critical to emulsification are bile salts and phospholipids, which have a nonpolar (hydrophobic) region as well as a polar (hydrophilic) region. The hydrophobic region interacts with the large lipid molecules, whereas the hydrophilic region interacts with the watery chyme in the intestine. This results in the large lipid globules being pulled apart into many tiny lipid fragments of about 1 µm in diameter. This change dramatically increases the surface area available for lipid-digesting enzyme activity. This is the same way dish soap works on fats mixed with water.

Bile salts act as emulsifying agents, so they are also important for the absorption of digested lipids. While most constituents of bile are eliminated in feces, bile salts are reclaimed by the enterohepatic circulation. Once bile salts reach the ileum, they are absorbed and returned to the liver in the hepatic portal blood. The hepatocytes then excrete the bile salts into newly formed bile. Thus, this precious resource is recycled.

Bilirubin, the main bile pigment, is a waste product produced when the spleen removes old or damaged red blood cells from the circulation. These breakdown products, including proteins, iron, and toxic bilirubin, are transported to the liver via the splenic vein of the hepatic portal system. In the liver, proteins and iron are recycled, whereas bilirubin is excreted in the bile. It accounts for the green color of bile. Bilirubin is eventually transformed by intestinal bacteria into stercobilin, a brown pigment that gives your stool its characteristic color! In some disease states, bile does not enter the intestine, resulting in white (‘acholic’) stool with a high fat content, since virtually no fats are broken down or absorbed.

Hepatocytes work non-stop, but bile production increases when fatty chyme enters the duodenum and stimulates the secretion of the gut hormone secretin. Between meals, bile is produced but conserved. The valve-like hepatopancreatic ampulla closes, allowing bile to divert to the gallbladder, where it is concentrated and stored until the next meal.

Liver disease is caused by damage to the liver. Liver damage can be caused by many things, including:

• Viruses (such as the hepatitis viruses)
• Drinking alcohol heavily
• Being very overweight
• Certain medications--for example, acetaminophen (Tylenol), can cause severe liver damage in people who also have heavy alcohol use
• Exposure to industrial chemicals, including cleaning solvents, aerosolized paints, and paint thinners

Liver damage can lead to livers that are swollen, shrunken, hard, or scarred. Such livers do not work well, and you can get very sick, or even die, if your liver stops working altogether.

Your liver works hard to protect your health. It’s a rugged, strong organ. But certain things—like alcohol, drugs, viruses, and excess weight—can damage it. You may not even realize when your liver is struggling, because liver disease usually has no symptoms until the problem becomes severe. Help your liver to guard your health by avoiding the things that might cause it harm.

The liver is the largest organ inside your body. It’s about the size of a football, and rests just under your ribs against the right side of your stomach.

“The liver performs an amazing set of functions that keep you healthy,” says Dr. Jake Liang, a liver specialist and researcher at NIH.

Your liver helps fight infections. It cleans your blood by getting rid of your body’s natural waste products and other harmful substances, including alcohol and drugs. “The liver also transforms the foods you eat into energy and nutrients your body can use, and it regulates how nutrients flow to different parts of the body when needed,” Liang says.

Your liver can keep working even if part of it is damaged or removed. But if it starts to shut down completely—a condition known as liver failure—you can survive for only a day or 2 unless you get emergency treatment.

Many things can affect liver function. Some liver problems are inherited from your parents, some are caused by viruses (certain kinds of hepatitis, and some are related to your behavior. Certain liver diseases go away on their own. Others can last a lifetime and cause serious illness.

Although liver disease often has no symptoms, warning signs can include a swollen abdomen, nausea, itching, or jaundice (having a yellow tint to the skin and the whites of the eyes).

NIH supports large research networks across the country to learn more about liver disease. For instance, teams of scientists nationwide have joined forces to study rare and often-deadly liver disorders that strike newborns and older children.

“Research networks are important because no single medical center has enough patients with rare diseases to do a rigorous study or test new treatments,” says Dr. Edward Doo, a liver disease expert at NIH. “With this large pediatric network, we can combine the efforts and expertise of many clinical centers that specialize in rare childhood liver diseases.”

Other NIH studies are focusing on an increasingly common type of liver disorder—known as fatty liver disease—that affects both children and adults. A healthy liver contains just a little fat or none at all. But too much fat buildup in liver cells can cause swelling and damage. Over time, the excess fat can lead to cirrhosis, liver cancer, and even liver failure.

“Estimates vary, but 2 different studies in the past decade suggest that about 30% to 45% of Americans have excess fat in the liver,” says Dr. Yaron Rotman, an NIH specialist in fatty liver disease. “It’s also becoming a huge problem for children and teens.”

Drinking too much alcohol can cause fatty liver. But a growing number of people who drink little or no alcohol are also being diagnosed with fatty liver. “The rise seems to be tied to the nation’s obesity epidemic,” says Doo.

Studies suggest that fatty liver disease now also affects about 1 in 10 children nationwide. As with adults, most children with fatty liver disease are overweight and resistant to insulin, a critical hormone that regulates energy.

In its early stages, fatty liver disease usually has no symptoms. It’s often first detected by blood tests for liver function. But these tests can’t tell the difference between mild fatty buildup and more serious damage. And some people with fatty liver disease can have normal blood tests. The only sure way to diagnose the severity of fatty liver disease is by getting a liver biopsy. For this test, a doctor inserts a thin needle through the skin and into the liver to remove a small piece of tissue for analysis.

NIH-funded scientists have been searching for simpler ways to measure the severity of fatty liver disease. They’re also conducting clinical studies to assess possible treatments. There are currently no approved medications for fatty liver or its more severe form called NASH, or non-alcoholic steato-hepatitis.

“To treat fatty liver disease, we recommend lifestyle changes: Weight loss for people who are overweight, and exercise and a healthy diet to help reduce fat,” Rotman says. “In many patients, just a 5-8% reduction in body weight will translate into a large improvement to liver damage.” For people with alcohol-related fatty liver, stopping alcohol use can reverse or prevent further liver injury.

Another common type of liver disease—known as viral hepatitis—can be caused by at least 5 different viruses, named hepatitis A, B, C, D, and E. These infections can injure your liver and keep it from working properly.

“Collectively, about 20% of people worldwide may be affected by a hepatitis virus infection,” Liang says. “It’s a major public health problem.” The most common types in the United States are hepatitis A, B, and C.

Each hepatitis virus causes a different form of liver disease. All the viruses can trigger acute, or short-term, hepatitis. Hepatitis B, C, and D can also cause chronic hepatitis, in which the infection lasts a long time, sometimes for your whole life.

People are often exposed to hepatitis A and E viruses through contaminated food or water. “The other hepatitis viruses often pass through some type of break in the skin barrier, sometimes by injections or by close contact with blood or other body fluids,” Liang adds. Hepatitis B, C, and D can spread through sexual contact.

Because many infected people have few symptoms, they may not realize they have viral hepatitis. They can spread the infection to others without even knowing it.

Viral hepatitis is often treated with antiviral medications. Hepatitis A, B, and D infections can be prevented by vaccines. Practicing good hygiene—such as washing your hands and avoiding contact with infected blood—can also help block the spread of viral hepatitis.

Another potentially dangerous type of liver disease can be caused by taking certain drugs or supplements. “It’s important to be aware that a lot of drugs can cause liver injury,” Liang says. “This especially can be a problem for people who are taking several different medications.”

Taking too much acetaminophen (Tylenol) is the most common cause of sudden liver failure. “It’s particularly dangerous if you mix alcohol with acetaminophen or certain other drugs,” Liang adds. Talk with your doctor or pharmacist about all the medications you take and how they might affect your liver.

Maintain a healthy weight, stay physically active, and limit your alcohol use. Keep your liver healthy, and it will protect you for a lifetime.

For a Healthy Liver

• Eat a balanced, healthy diet and exercise regularly.
• Maintain a healthy weight.
• Limit alcohol use. Talk with your doctor about how much alcohol is OK for you.
• Learn how the medicines you take might affect your liver. Take medications as directed.
• Talk to your doctor or pharmacist about all the medicines you’re taking.
• Avoid direct contact with toxins from insecticides, cleaning products, and other chemicals.
• Don’t smoke.