The hepatocytes (epithelial cells of the liver) form branching plates of cells, often only one cell thick, between a system of capillary sinusoids that connect the portal tracts to the central vein.
To facilitate the exchange of a wide variety of substances between the blood and hepatocytes,the hepatocytes are directly exposed to the blood passing though the organ, by being in close contact with the liver blood sinusoids. The sinusoids carry blood from the edges of the lobule to the central vein.
The sinusoids are lined by two types of cell 1) phagocytic cells called Kupffer cells (macrophages), which phagocytose dead red blood cells, particulate matter, and micro-organisms. 2) sinusoid lining cells which are similar to the endothelial cells that line other blood vessels.
The blood enters through portal tracts at the outer edge of the liver lobule, and filters through the sinuisoids which are in close connection with the liver hepatocytes, until it reaches the central hepatic vein, where it drains away. Thus the flow of blood is from the outside to the inside of the lobule.
In contrast, bile flows through small canaliculi formed by the hepatocytes themselves, and it flows from the inside of the liver lobule towards the outside.
This diagram shows how the relationships between the hepatocytes and the liver sinusoids.
The endothelial cells of the liver sinusoids (capillaries) have pores, including large pores called fenestra. Furthermore the basement membrane is discontinuous and non-obstructing. This allows blood plasma to enter a space between the endothelial cells and the hepatocytes, called the space of Disse (perisinuisoidal space). Blood cells and platelets are excluded. The hepatocytes have many microvilli which project into this space, to increase absorption from the plasma.
The space of Disse also contains lipocytes, that store fat, and vitamin A. They can also become contractile, and they make the type III collagen fibres (reticular fibres) also found in the space of Disse.
Hepatocytes synthesise and secrete bile into a system of tiny bile canaliculi which are present between adjacent hepatocytes. These canaliculi do not have a duct-like structure but consist merely of localised enlargements of the intercellular space between adjacent cells. These canaliculi empty into short canals of Hering close to the portal area, which then open into duct cells in a bile ductule in the portal area.
This electron micrograph shows two hepatocyte cells which contain mitochondria and microvilli, together with the space of Disse, and the endothelial cell of the sinusoid.
See if you can identify them for yourselves.
As well as mitochondria, the hepatocytes contain extensive rER and Golgi. Proteins are secreted into the sinusoids. The smooth ER is where damaging compounds such as steroids, alcohol, drugs and toxins are converted or detoxified. Stored glycogen is also metabolised the the smooth ER. The hepatocytes also take up haem, the breakdown product of erythrocytes, produced by the Kuppfer cells. The hepatocytes convert this to bilirubin, an iron depleted form of haem. The sER conjugates bilirubin to glucoronic acid, to make it water blood and some tissues, giving the skin a yellow colour known as jaundice.