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Liver Functions
Metabolism, Detoxification, and Bile
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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.
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.