The liver is the largest solid organ inside the body — about 1.4-1.7 kg in adults — wedged under the right diaphragm and tucked partly behind the ribs. It handles something like 500 different biochemical jobs: metabolizing almost everything you eat, synthesizing clotting factors and albumin, storing glycogen and vitamins, producing bile, and breaking down drugs and toxins. It also has the unusual ability to regenerate to full mass from as little as 25% of itself.
At a glance
What it does
Runs whole-body metabolism. It sits directly downstream of the gut via the portal vein, which means roughly 75% of its blood supply is nutrient- and toxin-rich blood arriving from the intestines before it reaches systemic circulation. Everything you absorb gets a first pass through here.
Core jobs fall into a few categories. Glucose handling: stores excess as glycogen when insulin is high, releases it during fasting, and performs gluconeogenesis from amino acids and lactate. Lipid handling: synthesizes cholesterol, packages fats into VLDL, processes dietary fats arriving via chylomicron remnants, and makes ketones in prolonged fasting. Protein synthesis: albumin, clotting factors, many transport proteins, acute phase reactants. Detoxification: phase I (mostly cytochrome P450 oxidation) and phase II (conjugation — glucuronidation, sulfation) biotransform drugs, hormones, and toxins. Bile production: emulsifies fat for absorption and excretes bilirubin, cholesterol, and toxins into stool. Vitamin and mineral storage: retinol, vitamin B12, vitamin D, iron, copper.
How it works
Hepatocytes are the workhorse cells, arranged in plates around sinusoids — specialized capillaries lined with fenestrated endothelium and Kupffer cells (liver-resident macrophages). Blood filters slowly through the sinusoids, giving hepatocytes time to extract, modify, and dump substances back into the bloodstream. Bile produced on the apical side drains into canaliculi, then bile ducts, then the common bile duct, which empties into the duodenum via the ampulla of Vater.
The liver is organized into functional units called lobules, each with a central vein draining outward and portal triads (portal vein branch, hepatic artery branch, bile duct) at the periphery. Zonation matters: hepatocytes near the portal triads (zone 1) see blood first and are well-oxygenated; hepatocytes near the central vein (zone 3) see blood last, are more hypoxic, and are the first to die in ischemic insult and the primary site of acetaminophen toxicity.
Regeneration is one of the liver's wilder tricks. Living donor transplants remove about 60% of the donor's liver, and both the donor remnant and the recipient graft regenerate to near-normal mass in about 3 months. The mechanism involves cytokine signaling (IL-6, TNF) and growth factors (HGF, EGF) that drive hepatocyte proliferation. This is why partial hepatectomy for tumor can work even though it sounds insane.
When it goes wrong
Non-alcoholic fatty liver disease (NAFLD, recently renamed MASLD — metabolic dysfunction-associated steatotic liver disease) is the defining liver epidemic of the modern era. Prevalence is roughly 25-30% in adults globally, and higher in people with obesity or Type 2 diabetes. It progresses in a subset to NASH (with inflammation and fibrosis), then cirrhosis, then hepatocellular carcinoma. Treatment is weight loss, exercise, and increasingly GLP-1 agonists and resmetirom.
Alcoholic liver disease follows a similar progression — steatosis, hepatitis, cirrhosis — but driven by ethanol. The dose-response is real but nonlinear; current evidence supports that "any alcohol is cleaner than some alcohol was once thought" is closer to the truth than "one drink is fine." Risk rises with consumption above about 20 g/day for men and less for women, with substantial individual variability.
Hepatitis viruses (A, B, C, D, E) cause acute or chronic liver inflammation. Hepatitis B and C are the dominant global drivers of cirrhosis and liver cancer; Hep B is preventable by vaccine and Hep C is curable with direct-acting antivirals (sofosbuvir plus others) in about 8-12 weeks.
Drug-induced liver injury is common and often under-recognized. Acetaminophen overdose is the leading cause of acute liver failure in the US. Many prescription and herbal products can also cause DILI — supplement manufacturers like to pretend otherwise.
The "liver detox" industry is selling nonsense. The liver detoxifies things; you do not need to detoxify the liver. Milk thistle, lemon water cleanses, juice fasts, and similar products have no meaningful evidence of benefit and occasionally cause harm. What actually reduces liver disease burden is weight loss, reduced alcohol intake, and management of hepatitis infection.
Interactions
The liver modulates hormones. It clears cortisol (via 11-beta-HSD), metabolizes sex hormones (via conjugation), synthesizes IGF-1 in response to growth hormone, and makes sex hormone-binding globulin. Hepatic dysfunction produces endocrine downstream effects — cirrhotic men often have elevated estradiol and gynecomastia from impaired clearance.
Insulin resistance is both cause and consequence of fatty liver. Hepatic insulin resistance drives inappropriate gluconeogenesis, which contributes to fasting hyperglycemia in Type 2 diabetes. Weight loss, low-carb diets, and GLP-1 agonists all reduce hepatic fat, often before they produce visible weight change.
Drug interactions are dominated by the cytochrome P450 system. Grapefruit inhibits CYP3A4 (raising levels of many drugs). Inducers like rifampin and phenytoin speed metabolism and can reduce efficacy of hormonal contraception, warfarin, and antiretrovirals.
Honest take
Fatty liver is the quiet public health story of the century and most people carrying it have no idea. If you are overweight, sedentary, and eating the standard industrial diet, your liver probably has measurable fat and you will never see a symptom until something else goes wrong. The cheap fix is the same boring intervention that fixes most of the others: lose 7-10% of body weight, move most days, cap alcohol hard, and if you are diabetic get treated aggressively. The expensive fix — GLP-1 agonists, resmetirom for confirmed NASH — is now real and working. "Liver cleanse" products are a scam; your liver does not need your help clearing itself, and milk thistle is not going to rescue you from a bad diet.
Sources
- Rinella et al., Hepatology (2023) — new MASLD nomenclature and practice guideline.
- Harrison et al., NEJM (2024) — resmetirom for NASH with fibrosis.
- Ghany et al., Hepatology — AASLD hepatitis C guidance.
- Lazarus et al., Nature Reviews Gastroenterology and Hepatology — global epidemiology of NAFLD/MASLD.