Liver Structure

The liver is the largest internal organ in a mammal and is metabolically the most versatile. It carries out more than 500 known functions, from storing glycogen to producing clotting factors, from detoxifying alcohol to synthesising plasma proteins. Understanding how the liver performs these functions requires understanding its structure — the arrangement of its blood supply, its microscopic lobules, and the specialised cells (hepatocytes, Kupffer cells, endothelial cells) that carry out its work. This lesson covers mammalian liver anatomy at the gross and microscopic levels, matching OCR A-Level Biology A specification module 5.1.2(c).

Key Definitions:

• Hepatocyte — the principal cell of the liver, responsible for most of its metabolic functions.
• Lobule — the structural and functional unit of the liver, roughly hexagonal in cross section.
• Sinusoid — a specialised capillary inside a liver lobule, lined with hepatocytes and Kupffer cells.
• Kupffer cell — a macrophage-type cell found in the liver sinusoids that phagocytoses bacteria and old red blood cells.

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Gross Anatomy of the Liver

In a human, the liver weighs about 1.5 kg and occupies much of the upper right abdominal cavity, just below the diaphragm. It has four main lobes and is connected to the digestive system via the gallbladder and bile duct. Three blood vessels and one duct enter or leave the liver:

• Hepatic artery — carries oxygenated blood from the aorta to the liver. It supplies oxygen for the high metabolic activity of hepatocytes.
• Hepatic portal vein — carries deoxygenated but nutrient-rich blood from the gut (stomach, intestines, pancreas, spleen). This is the liver's largest blood supply by volume. The portal vein system is unusual: it begins and ends in capillary beds (one in the gut, one in the liver), without an intervening heart.
• Hepatic vein — carries deoxygenated blood from the liver to the inferior vena cava, returning it to the right side of the heart.
• Bile duct — carries bile from the liver to the gallbladder (for storage) and to the duodenum (for fat emulsification and excretion of bile pigments).

The liver receives around 25 % of resting cardiac output — reflecting its central role in processing absorbed nutrients and removing toxins.

flowchart LR
    A[Hepatic artery<br/>oxygenated blood from aorta] --> D[Liver]
    B[Hepatic portal vein<br/>nutrient-rich blood from gut] --> D
    D --> C[Hepatic vein<br/>to vena cava]
    D --> E[Bile duct<br/>to gallbladder / duodenum]

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Microscopic Structure: the Lobule

Internally, the liver is organised into millions of lobules. Each lobule is roughly hexagonal in cross section and about 1 mm across. OCR expects you to be able to label a diagram of a lobule and explain how its structure supports its function.

At each corner of the hexagon is a portal triad, containing:

• A branch of the hepatic artery (bringing oxygenated blood).
• A branch of the hepatic portal vein (bringing nutrient-rich blood from the gut).
• A bile duct (carrying bile away from the hepatocytes).

Blood from the hepatic artery and hepatic portal vein mixes as it flows through the lobule. At the centre of each hexagonal lobule is the central vein, also called the centrilobular vein. Blood drains from the sinusoids into the central vein, which then joins others to form the hepatic vein.

Sinusoids

Between the hepatocytes, blood flows through wide, leaky capillaries called sinusoids. These differ from ordinary capillaries:

• They are wider — up to ~40 µm in diameter, allowing slow, intimate contact between blood and hepatocytes.
• Their endothelium is fenestrated (perforated with gaps), and there is little or no basement membrane, so even large plasma proteins can pass from blood to the hepatocyte surface.
• They are lined partly by Kupffer cells — resident macrophages that filter the blood.

The low flow rate and large exchange surface of the sinusoids allow hepatocytes to extract absorbed nutrients, remove toxins, and secrete newly made plasma proteins efficiently.

Canaliculi and Bile Flow

Bile is produced by hepatocytes and secreted into narrow channels called bile canaliculi, which run between rows of hepatocytes. These canaliculi flow in the opposite direction to the blood — from the central vein towards the portal triad at the lobule corner. At the edge of the lobule, they drain into bile ducts, which converge into the hepatic ducts and finally the common bile duct.

flowchart LR
    A[Portal triad] --> B[Sinusoid]
    B --> C[Central vein]
    C --> D[Hepatic vein]
    A -.bile flows opposite.-> E[Bile canaliculi]
    E --> F[Bile duct]

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Cells of the Liver

Hepatocytes

Hepatocytes are the dominant cells of the liver, making up roughly 80 % of its mass. They are highly metabolically active and adapted accordingly: