The liver is the largest gland in the human body, sitting in the upper right abdomen and weighing about 1.5 kg in an adult. At GCSE, you need to know five key functions: deamination of excess amino acids, glycogen storage and release, production of bile, detoxification of alcohol and drugs, and heat production to maintain body temperature.
What is the liver's position and blood supply?
The liver has a uniquely dual blood supply that reflects its central role in metabolism:
- Hepatic portal vein: carries nutrient-rich but oxygen-poor blood directly from the small intestine to the liver. After a meal, this blood is loaded with absorbed glucose, amino acids, and fatty acids.
- Hepatic artery: delivers oxygenated blood from the aorta, fuelling the liver's own high metabolic rate.
- Hepatic vein: returns processed blood from the liver to the inferior vena cava and back to the heart.
The liver contains hundreds of billions of hepatocytes (liver cells), each performing many reactions simultaneously. Because the liver processes virtually everything absorbed from the gut, it is both the first line of defence against dietary toxins and the body's main metabolic regulator.
What is deamination and why is it necessary?
The body cannot store excess amino acids. When more protein is eaten than the body needs for growth and repair, the surplus amino acids must be broken down. This happens in the liver in a two-step process:
- Deamination: the amino (–NH₂) group is removed from the amino acid. This produces a molecule of ammonia (NH₃) — a toxic waste product — and an organic acid (which can enter respiration pathways or be converted to glucose or fat for energy).
- Urea synthesis (ornithine cycle): ammonia is immediately combined with carbon dioxide to form urea, which is far less toxic and is soluble in water. Urea passes in the blood to the kidneys, which filter it out and excrete it in urine.
| Stage | Reactants | Products | Location |
|---|---|---|---|
| Deamination | Amino acid | Ammonia + organic acid | Liver (hepatocytes) |
| Urea synthesis | Ammonia + CO₂ | Urea + water | Liver (ornithine cycle) |
| Excretion | Urea in blood | Urea in urine | Kidneys |
How does the liver regulate blood glucose?
The liver acts as a glucose buffer for the whole body, working closely with the hormones insulin and glucagon secreted by the pancreas:
- After a meal (blood glucose rises): insulin signals the liver to convert excess glucose to glycogen (a branched storage polysaccharide) by a process called glycogenesis. This prevents dangerous hyperglycaemia.
- Between meals (blood glucose falls): glucagon signals the liver to break glycogen back down to glucose by glycogenolysis, releasing it into the blood and preventing hypoglycaemia.
- During prolonged starvation: the liver can also synthesise new glucose from amino acids and glycerol, a process called gluconeogenesis.
The liver can store enough glycogen to supply the body with glucose for approximately 12–18 hours.
What is bile and what does it do?
Bile is a greenish-yellow alkaline fluid produced by the liver and stored in the gallbladder (a small sac attached to the liver). After a meal, bile is released down the bile duct into the duodenum (first section of the small intestine).
Bile performs two main functions:
- Emulsification of fats: bile salts break large fat globules into millions of tiny droplets (a process called emulsification). This dramatically increases the surface area of fat available for lipase enzymes to act on, speeding up fat digestion.
- Neutralisation of stomach acid: bile is alkaline (pH ~7.6–8.6). This neutralises the acidic chyme (partially digested food) arriving from the stomach, providing the correct pH for pancreatic enzymes in the small intestine to work.
Note: bile does not chemically digest fats — it only emulsifies them. The actual chemical digestion is done by lipase.
How does the liver detoxify harmful substances?
The liver contains enzymes that chemically alter toxic substances, converting them into harmless (or less harmful) compounds that can be excreted:
- Alcohol (ethanol): metabolised to ethanal (acetaldehyde) by alcohol dehydrogenase, then to ethanoate (acetate), and finally to CO₂ and water. Ethanal is itself toxic and causes hangover symptoms. Regular heavy drinking overwhelms the liver, leading to fatty liver, alcoholic hepatitis, and eventually cirrhosis (scarring that destroys liver tissue).
- Drugs and medicines: the liver breaks down many prescription and over-the-counter drugs; this is why drug doses must be calibrated carefully — too much overwhelms the liver's capacity.
- Hormones: the liver deactivates excess hormones (such as insulin and oestrogen) by chemically modifying them so they cannot bind to their receptors.
How does the liver contribute to temperature regulation?
The liver is metabolically one of the most active organs in the body, carrying out hundreds of exothermic (energy-releasing) chemical reactions simultaneously. This continuous metabolic activity generates a large amount of heat — the liver is one of the primary sources of body heat. Blood flowing through the liver picks up this heat and distributes it around the body, contributing to the maintenance of core body temperature at approximately 37 °C.
Frequently asked questions
Why does the liver produce urea rather than simply excreting ammonia?
Ammonia is highly toxic — even small concentrations in the blood cause brain damage. Urea is far less toxic and is easily dissolved in water, making it safe to transport through the bloodstream to the kidneys at the concentrations needed for excretion. Converting ammonia to urea (via the ornithine cycle) requires energy, but the benefit of producing a safer molecule outweighs the cost.
What happens to the liver in type 2 diabetes?
In type 2 diabetes, cells (including liver cells) become resistant to insulin. The liver fails to respond normally to insulin's signal to store glucose as glycogen. As a result, the liver continues to release glucose into the blood even when blood glucose is already high (continued gluconeogenesis and glycogenolysis). This worsens hyperglycaemia. Many people with type 2 diabetes also develop non-alcoholic fatty liver disease (NAFLD), in which excess fat accumulates in liver cells, reducing their function.
Why is bile stored in the gallbladder rather than the liver?
Bile is produced continuously by the liver but is only needed after meals. The gallbladder acts as a reservoir, concentrating and storing bile between meals, then releasing it in a large pulse when food (particularly fat) enters the duodenum. If the gallbladder is surgically removed (cholecystectomy), bile still flows directly from the liver into the duodenum but cannot be stored, so fat digestion may be slower immediately after a meal, though the body usually adapts.
How does cirrhosis affect the liver's functions?
Cirrhosis is irreversible scarring of the liver caused by prolonged damage (most commonly from chronic alcohol misuse or hepatitis infection). Scar tissue replaces functional hepatocytes, reducing the liver's ability to deaminate amino acids, produce bile, store glycogen, and detoxify substances. This leads to a cascade of serious consequences: accumulation of toxins in the blood (including ammonia, causing hepatic encephalopathy), impaired blood glucose regulation, reduced bile for digestion, and blood clotting problems (the liver also produces clotting factors).
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