The liver is a large metabolic organ that helps maintain the chemical balance of the body. It processes nutrients absorbed from the intestine, stores energy, makes important blood proteins, and removes harmful substances from circulation. Because blood from the digestive tract passes through the liver before reaching the rest of the body, the liver is positioned to regulate what enters the systemic circulation.
Its functions are essential for energy balance, digestion, drug handling, and waste removal.
At the microscopic level, hepatocytes are arranged in lobules where blood from the portal vein and hepatic artery flows past liver cells toward the central vein. As blood moves through sinusoids, hepatocytes take up glucose, amino acids, lipids, bilirubin, and drugs, then modify, store, detoxify, or secrete them. The liver also produces bile, which carries bilirubin, cholesterol, and bile salts into the intestine to support fat digestion and waste excretion.
Damage to these pathways can lead to jaundice, coagulopathy, hypoglycemia, and toxin buildup.
Understanding Liver Functions
The liver changes its work according to the body’s fed or fasting state. After a carbohydrate rich meal, insulin signals liver cells to remove glucose from blood and join many glucose units into glycogen. Extra energy can be turned into fat for longer storage.
Between meals, glucagon signals the liver to break down glycogen and release glucose. During a longer fast, the liver can make new glucose from lactate, glycerol, and some amino acids.
This control helps the brain and red blood cells receive a steady fuel supply. Poor liver function can therefore cause dangerously low blood glucose, especially in children or people who have not eaten.
Protein processing creates a serious waste problem. When amino acids are used for energy or converted into other molecules, nitrogen is removed and ammonia forms. Ammonia is toxic to brain cells.
Liver cells use the urea cycle to package this nitrogen into urea, which travels in blood to the kidneys and leaves in urine. The liver makes albumin as well. Albumin helps keep fluid inside blood vessels and carries substances such as hormones and drugs.
It produces several clotting factors, so severe liver disease may lead to easy bruising or prolonged bleeding. Vitamin K is needed for some of these factors, linking normal fat absorption to normal blood clotting.
Detoxification does not simply mean that the liver destroys every chemical. Many substances must first be changed into forms that can leave the body in urine or bile. Enzymes, including cytochrome P450 enzymes, can alter drugs, alcohol, and environmental chemicals.
A later step often attaches another small molecule, making the product easier to remove. These steps can sometimes create a more reactive intermediate before it becomes safer. This is one reason why an overdose of acetaminophen can injure the liver.
Alcohol, certain medicines, and herbal products can affect the same enzyme systems. A medicine dose that is safe for one person may be harmful when combined with another substance or when liver disease is present.
Bile gives the liver a route for handling fats and pigments. Bile salts surround large fat droplets in the small intestine, breaking them into smaller droplets that digestive enzymes can work on. After helping with absorption, most bile salts are absorbed again near the end of the small intestine and returned to the liver.
This recycling conserves materials. Bilirubin comes from the breakdown of old red blood cells. The liver modifies it so it can enter bile.
If bile flow is blocked by a gallstone or if bilirubin handling fails, bilirubin can build up in blood and cause yellow skin or eyes. Pale stools and dark urine can be clues that bile is not reaching the intestine.
When learning liver tests, it is important to separate signs of cell injury from signs of reduced liver performance. High ALT or AST levels suggest damaged cells are leaking enzymes, but they do not alone measure how well the liver is functioning. Albumin level, clotting time, bilirubin, and blood glucose give different information.
The liver has a large reserve and can keep working despite early damage. Long term injury from viral hepatitis, alcohol, fatty liver disease, or toxins can replace healthy tissue with scar tissue. This scarring can restrict blood flow, raise pressure in the portal circulation, and reduce the organ’s ability to perform its many connected jobs.
Key Facts
- The liver receives dual blood supply: about 75% from the portal vein and about 25% from the hepatic artery.
- Glycogenesis stores glucose as glycogen, and glycogenolysis releases glucose from glycogen to help maintain blood glucose.
- Ammonia is converted to urea in the liver: 2 NH3 + CO2 -> urea + H2O.
- Cytochrome P450 enzymes in hepatocytes carry out phase I detoxification, often followed by phase II conjugation to increase water solubility.
- Bile salts emulsify fats and help absorption of lipids and fat soluble vitamins A, D, E, and K.
- Normal total bilirubin is about 0.2 to 1.2 mg/dL, and elevated levels can indicate impaired bilirubin processing or bile flow.
Vocabulary
- Hepatocyte
- A hepatocyte is the main functional liver cell that carries out metabolism, detoxification, protein synthesis, and bile production.
- Hepatic lobule
- A hepatic lobule is the microscopic structural unit of the liver, organized around a central vein with portal triads at the edges.
- Sinusoid
- A sinusoid is a specialized capillary in the liver where blood flows slowly past hepatocytes for exchange of substances.
- Bilirubin
- Bilirubin is a yellow pigment formed from heme breakdown that is processed by the liver and excreted in bile.
- Portal triad
- The portal triad is a group of three structures at the edge of a lobule consisting of a branch of the portal vein, hepatic artery, and bile duct.
Common Mistakes to Avoid
- Thinking the liver only detoxifies drugs, which is wrong because it also regulates carbohydrate, lipid, and protein metabolism, produces bile, and synthesizes plasma proteins.
- Confusing blood flow with bile flow, which is wrong because blood moves from the portal triad through sinusoids to the central vein, while bile flows in the opposite direction toward bile ducts.
- Assuming all bilirubin in blood is already ready for excretion, which is wrong because unconjugated bilirubin must first be taken up and conjugated by hepatocytes.
- Believing detoxification always makes substances inactive, which is wrong because phase I reactions can sometimes create reactive intermediates that require further conjugation.
Practice Questions
- 1 A patient has a total bilirubin level of 2.4 mg/dL. If the upper normal value is 1.2 mg/dL, by what factor is the patient's bilirubin elevated?
- 2 The liver receives 1500 mL of blood per minute, with 75% from the portal vein and 25% from the hepatic artery. Calculate the flow rate from each source.
- 3 Explain why obstruction of the bile duct can reduce absorption of dietary fats even if hepatocytes are still metabolically active.