Liver Function
Your body's 500-job chemical plant. How the liver filters blood, stores energy, makes bile and processes almost everything you eat.
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What Is the Liver?
The liver is the largest solid organ in the body, weighing roughly 1.5 kilograms in an adult. It sits in the upper right side of the abdomen, tucked beneath the ribcage and above the stomach. Its reddish-brown colour comes from the enormous amount of blood constantly flowing through it.
If the heart is the body's pump and the lungs are its air exchanger, the liver is its chemical factory — a processing hub that performs an astonishing variety of jobs simultaneously. Scientists have identified over 500 distinct functions, though most people are only familiar with a handful of them.
Understanding liver function helps explain why this organ is so central to metabolism, detoxification, digestion and even blood clotting — and why looking after it matters throughout life.
Structure of the Liver
The liver is divided into two main sections called lobes — the larger right lobe and the smaller left lobe. These are further divided into thousands of tiny functional units called lobules, each roughly hexagonal in shape and about one millimetre across.
At the centre of each lobule runs a small vessel called the central vein. Around the edges are branches of the portal vein and hepatic artery, which feed blood inward. Blood flows through microscopic channels called sinusoids, past the liver's main working cells — the hepatocytes — before draining into the central vein.
This design ensures that almost every hepatocyte is in close contact with the blood it processes. The liver also contains specialised immune cells called Kupffer cells, which patrol the sinusoids and destroy bacteria, dead cells and other particles arriving from the gut.
Metabolism and Energy Management
The liver sits at the crossroads of the body's energy supply. After a meal, nutrient-rich blood from the intestines arrives via the portal vein. The liver takes what it needs, stores what the body might need later, and sends the rest onward into general circulation.
Carbohydrate Handling
When blood glucose rises after a meal, the liver converts excess glucose into a storage molecule called glycogen and packs it away. This process, called glycogenesis, protects the body from excessively high blood sugar.
When blood glucose later falls — between meals or during exercise — the liver breaks glycogen back down into glucose and releases it into the blood (glycogenolysis). If glycogen stores run low, the liver can also manufacture glucose from non-carbohydrate sources such as amino acids and fats, through a process called gluconeogenesis.
This constant glucose regulation keeps the brain and other glucose-dependent organs fuelled around the clock, even during fasting.
Fat Processing
The liver is central to fat metabolism. It breaks down fatty acids for energy through a process called beta-oxidation, packages fats into particles called lipoproteins (including VLDL and HDL) for transport in the blood, and synthesises cholesterol — a molecule essential for making cell membranes, bile and hormones such as testosterone and oestrogen.
The liver also assembles triglycerides from surplus calories and ships them out to fat tissue for storage. When energy is scarce, it retrieves these fats and converts them back into usable fuel.
Protein Metabolism
Amino acids arriving from the gut undergo deamination in the liver — the removal of their nitrogen-containing group. This nitrogen is converted into urea, which travels in the blood to the kidneys and is excreted in urine. The carbon skeletons left behind are either used for energy or converted into glucose or fat.
The liver also synthesises many important proteins, including albumin (which maintains blood pressure and carries molecules), fibrinogen and other clotting factors (essential for blood to clot after injury), and complement proteins (part of the immune system).
| Category | Function |
|---|---|
| Energy regulation | Stores glycogen; releases glucose when needed; synthesises glucose from non-carbs |
| Fat metabolism | Packages lipoproteins; synthesises cholesterol; converts fat to energy |
| Protein metabolism | Deaminates amino acids; converts nitrogen to urea; makes albumin and clotting factors |
| Detoxification | Processes alcohol, drugs, toxins and metabolic waste products |
| Digestion | Produces bile to emulsify fats in the small intestine |
| Blood management | Stores iron and vitamins; filters old red blood cells; makes immune proteins |
Detoxification: Cleaning the Blood
One of the liver's best-known roles is detoxification. All blood coming from the digestive organs passes through the liver before reaching the general circulation — a design called the hepatic portal system. This gives the liver first access to anything absorbed from the gut, including nutrients, medications, alcohol and potentially harmful substances.
The liver neutralises toxins through two main pathways, often called Phase I and Phase II reactions. In Phase I, a family of enzymes (the cytochrome P450 enzymes) chemically modify toxins — sometimes making them temporarily more reactive. In Phase II, the liver attaches water-soluble molecules to these modified toxins, making them safe enough to dissolve in blood and be excreted by the kidneys or gut.
Alcohol is one substance the liver processes in this way. It converts alcohol into acetaldehyde (a toxic intermediate) and then into harmless acetate. The liver can process only about one standard drink per hour, which is why drinking faster leads to alcohol building up in the blood.
Medications are also processed by the liver, which is why liver health matters for drug dosing and why some drugs can be hard on the liver with prolonged use. Always follow medical guidance on medication use — this is for educational context only.
Bile Production and Fat Digestion
The liver produces roughly 500 to 1,000 millilitres of bile every day. Bile is a greenish-yellow digestive fluid containing bile salts, cholesterol, bilirubin (a waste product from old red blood cells), water and electrolytes.
Bile salts act as emulsifiers — they break large fat globules into tiny droplets, enormously increasing the surface area available for digestive enzymes to work on. Without bile, the body would struggle to absorb fats and the fat-soluble vitamins A, D, E and K.
Bile is stored and concentrated in the gallbladder between meals. When fatty food arrives in the small intestine, a hormone called cholecystokinin (CCK) signals the gallbladder to contract and release bile through the bile duct into the duodenum. See our digestive system guide for more about how bile fits into the overall digestion process.
Blood Management and Immune Roles
The liver is a major blood-processing centre. It stores significant amounts of iron and releases it as needed for making new red blood cells. It also stores fat-soluble vitamins — particularly vitamins A, D, E, K and B12 — acting as a reserve supply during periods of low dietary intake.
Old or damaged red blood cells are broken down partly in the liver. The haemoglobin they contain is split into its components: the iron is recycled and stored, while the remaining haem group is converted into bilirubin. Bilirubin is incorporated into bile and eventually excreted. This is why bile is yellow-green and why faeces are brown — both colours come from bilirubin pigments.
The Kupffer cells lining the liver's sinusoids form an important immune barrier, filtering bacteria and foreign particles from blood arriving from the gut. The liver also produces several acute-phase proteins that increase rapidly during infection or injury, helping coordinate the inflammatory response. To explore immunity more broadly, see our immune system guide.
Remarkable Regeneration
The liver has a truly extraordinary capacity for self-repair. If up to 70% of the liver is removed or destroyed, the remaining tissue can regenerate to restore the organ to roughly its original size. This is possible because hepatocytes — normally long-lived, slowly dividing cells — can switch rapidly into a high-growth mode when liver tissue is lost.
The signals driving this regeneration include growth factors such as hepatocyte growth factor (HGF) and various cytokines. Remarkably, the liver stops growing once it reaches the appropriate size, controlled by feedback signals that remain an active area of research.
This regenerative ability makes living-donor liver transplantation possible: a donor can give a large portion of their liver and both the donated section and the remaining donor liver will regrow over several months.
The Liver and Hormonal Signalling
The liver plays an important but often overlooked role in hormonal balance. Steroid hormones — including oestrogen, testosterone and cortisol — are synthesised partly from cholesterol, and once they have done their job, the liver is responsible for breaking them down and preparing them for excretion.
The liver also produces insulin-like growth factor 1 (IGF-1) in response to growth hormone signals from the pituitary gland. IGF-1 is the primary mediator of growth hormone's effects on tissues, stimulating cell growth and division throughout the body. This makes the liver a crucial link in the body's growth and development pathways.
Another important molecule the liver synthesises is thrombopoietin, which stimulates the bone marrow to produce platelets — the tiny cell fragments essential for blood clotting. Combined with the clotting factor proteins the liver makes directly, this means that liver health is intimately tied to the body's ability to stop bleeding after injury.
When the liver is severely damaged, the failure to produce adequate clotting factors and thrombopoietin leads to impaired clotting — one of the signs clinicians watch for when assessing liver function. Blood tests that measure clotting time (such as prothrombin time) are therefore used as indirect measures of how well the liver is functioning.
How the Liver and Kidneys Work Together
The liver and kidneys are closely linked in the business of waste removal. The liver converts toxic ammonia (from amino acid breakdown) into the less harmful molecule urea and releases it into the blood. The kidneys then filter urea out of the blood and excrete it in urine.
Similarly, the liver processes bilirubin and releases it into bile, while the kidneys excrete a smaller fraction in urine — giving urine its characteristic pale yellow colour. When either organ is compromised, the other faces a heavier burden.
For a detailed look at how the kidneys handle filtration, fluid balance and waste, visit our kidney function guide. And for a picture of how metabolism shapes what the liver receives to process, our metabolism explained guide is a useful companion.
When the Liver Is Under Stress
Because of its central role in processing everything the body encounters, the liver is vulnerable to damage from a variety of sources. Understanding the main threats is valuable educational context — though any concerns about liver health should be discussed with a healthcare professional.
Hepatitis means inflammation of the liver, and it can be caused by viruses (hepatitis A, B, C and others), alcohol, toxins, medications or autoimmune conditions. Viral hepatitis ranges from acute illnesses that resolve on their own to chronic infections that can damage the liver over many years.
Cirrhosis is the result of long-term liver damage that has progressed to widespread scarring. Scar tissue cannot perform the liver's normal functions and cannot be reversed, though progression can sometimes be slowed or halted by treating the underlying cause.
Non-alcoholic fatty liver disease (NAFLD) — more recently renamed metabolic dysfunction-associated steatotic liver disease (MASLD) — occurs when fat accumulates in liver cells in the absence of heavy alcohol use. It is closely linked to obesity, type 2 diabetes and metabolic syndrome, and is now one of the most common liver conditions worldwide. In most cases it causes no symptoms in its early stages, which is why it is often detected incidentally during tests for other purposes.
The liver's remarkable regenerative ability means it can recover from many forms of acute damage — but chronic repeated stress erodes this capacity over time. Looking after metabolic health through a balanced diet and regular physical activity supports the liver's ability to keep pace with its extensive workload.
Supporting Liver Health
Because the liver filters everything we consume, diet and lifestyle choices have a direct effect on how hard it has to work. A balanced diet rich in vegetables, fruit and wholegrains provides fibre and antioxidants that reduce the liver's processing load.
Excessive alcohol consumption is one of the most significant burdens on the liver, as it must constantly prioritise processing alcohol over other tasks, and prolonged high intake can lead to progressive liver damage. This is an educational note — for personal guidance, always consult a healthcare provider.
Maintaining a healthy weight is also important, as excess body fat (particularly around the abdomen) is associated with fat accumulation in liver cells. Regular physical activity and adequate hydration support overall metabolic health and, by extension, liver health.
You can explore your daily energy balance with the daily calorie estimator and look up nutrition information with the anatomy glossary. You may also find the body systems explorer useful for seeing how the liver connects to other organ systems.