The primary and most common cells of the liver (making up to 90% of the liver’s cells) are called hepatocytes. These highly sophisticated cells carry out most of the many functions which the liver performs. Hepatocytes are all identical. The multiple tasks that the liver performs are not split up or shared out between them. The hepatocytes are capable of carrying out the same function.
Hepatocytes are split up into groups or batches to form lobules. These are the functional units of the liver. There are up to a million lobules in the liver.
Each lobule is organised around a central vein. Plates or strips of hepatocytes radiate out from this central vein like spokes from a wheel. On the outside of each lobule are branches of both the portal vein and the hepatic artery.
Small blood channels called sinusoids run from these branches of the portal vein and hepatic artery through the lobules. Every cell in the lobule is bathed in this blood flow. There are over one billion sinusoids in the liver. The consequence of this stacked system is that blood runs very slowly through the sinusoids. This allows time for the liver cells to both take in what they need from the bloodstream and to export other products.
The sinusoids are lined by tissue made up of endothelial cells. Other cells in the sinusoids include Kupffer cells, Pitt Cells and Hepatic Stellate or fat cells.
Kupffer cells remove aged and damaged red blood cells, as well as attacking and neutralising bacteria and viruses. Pit cells are a type of immune cell called ‘natural killer cells’. Under certain circumstances Hepatic Stellate cells can produce collagen fibres leading to scarring or fibrosis.
Running through the lobules, parallel to the sinusoids, are small channels called bile canaliculi. The bile produced by hepatocytes flows into these canaliculi and then moves to the outside of the lobule. There it flows into branches of the bile duct which drain it into progressively larger ducts until it reaches the right and left hepatic ducts. These are the final channels out of the liver from each of its lobes. They join up outside the liver in the common hepatic duct. This drains into the gallbladder where bile is stored until it is needed for fat digestion.
The liver also manufactures lymph which regulates the fluid balance within the body. Lymph is a colourless fluid that flows through a network of channels called the lymphatic system. Approximately half of the lymph made in the body is formed in the liver. Fluid from the tissues of the liver flows into an area between the sinusoids and the hepatocytes called the Space of Disse where lymph is formed. It then flows through small lymphatic channels and out into the body’s main lymphatic system.
How the liver is held together
The sinusoids, lymph vessels and the bile ducts are contained in and supported by connective tissue. This tissue branches out and extends throughout the liver. This tissue provides the scaffolding around and through which all these various channels and ducts are threaded. It also separates each lobule from its neighbours.