The Mammalian Liver: Structure
External structure
- The liver carries out several important roles in the body, including the breakdown of unwanted or toxic substances and the production of excretory waste
- In order to carry out these roles successfully, the liver requires a constant blood supply
- The liver is actually supplied with blood from two different sources
- It receives oxygenated blood from the heart via the hepatic artery
- It also receives deoxygenated blood from the digestive system via the hepatic portal vein. This allows the liver to absorb and metabolise many of the nutrients that are absorbed into the blood in the small intestine
- Deoxygenated blood then leaves the liver in the hepatic vein and flows back to the heart
- The liver is also connected directly to the gall bladder
- Bile salts (that help to digest fats) and bile pigments (a waste product from the breakdown of haemoglobin) are stored in the gall bladder as part of the bile
- The bile is then released into the duodenum via the bile duct
The external structure of the liver, its blood supply and its connection to the gall bladder and duodenum
Internal structure
- The internal structure of the liver is fairly simple
- Unlike some organs, the liver is not made up of lots of different cell types that are all specialised for different functions
- Instead, the liver is mainly made up of cells known as hepatocytes that carry out almost all the functions required
- The liver is divided into many lobules, which are separated from each other by connective tissue (a tissue that consists of cells that secrete an extracellular matrix)
- Branches of the hepatic artery and the hepatic portal vein supply each lobule with blood
- This blood is carried in wide capillaries known as sinusoids
- The sinusoids are lined with an incomplete layer of endothelial cells
- This allows blood to reach the hepatocytes, ensuring substances can be exchanged between the blood and these cells
- Each hepatocyte has a large surface area in contact with the blood to maximise the exchange of substances
- Each lobule is also connected to a branch of the hepatic vein that drains blood away from the lobule
- The lobule is known as the functional unit of the liver, as all the functions of the liver occur within each individual lobule
Labelled photomicrograph of a liver lobule (x30)
How the blood vessels, sinusoids and hepatocytes are arranged inside a liver lobule
