Anatomy and physiology

It would be difficult to overestimate the importance of the liver to the healthy functioning of the human body. It is a truly remarkable organ. The liver acts as a processing plant, a battery, a filter, a warehouse and distribution centre all in one. Without it we would not survive.

The number and range of tasks and functions it carries out is staggering:

• It transforms the food we eat into energy to build cells and tissues in our body.
• It detoxifies all the poisonous chemicals that enter our bodies such as alcohol, drugs and pollutants.
• It stores vitamins, fat, sugars, and minerals and sends them out around the body as and when they are needed.
• It makes a digestive juice called bile so that fats can properly be broken down and absorbed by the body.
• It produces hormones that regulate sexual desire and function.
• It is engaged in so many activities that the energy that it creates carrying them out warms the blood that passes through it and helps maintain the bodys temperature.

The immune system, digestive tract, kidney, brain and cardio-vascular system all depend on a healthy and well-functioning liver. This is why liver diseases such as hepatitis C can have such varied symptoms, as all the bodys major systems and organs can potentially be affected by a diseased liver.

It is because of this that it is so important to understand both how the liver works and how to look after it.

The liver is located under the ribs on the right hand side of the body. It lies just below the lungs under the top of the diaphragm to which it is attached. The diaphragm is the muscle beneath the lungs that regulates our breathing. The liver is partly protected by the rib cage and is packed in so tightly that it can leave a slight impression on the top of the liver.

The liver is the largest organ in the body. It is shaped like a wedge, weighing between 1.2 and 1.5kg and is about 5 inches across and about 7 inches along its diagonal. It is divided into two main lobes, the right and the left. Fibrous ligaments separate the two lobes on the underside. The right lobe is the larger of the two and has two smaller lobes itself. All the lobes of the liver perform the same functions. The whole liver is encased in a protective fibro-elastic sheath called Glissons capsule.

The liver is attached to the gallbladder (via the cystic duct). Together they make up the hepatobiliary system. Bile is a greenish digestive juice which is manufactured inside liver cells. It then passes to the gallbladder for storage until it is needed. When food enters the intestine, bile is secreted into the intestine, via the common bile duct. The pancreas also secretes fluid containing digestive enzymes into the gut as well as producing the hormones insulin and glucagon which are released into the blood stream to control blood sugar levels. Insulin prevents blood-sugar levels from rising too high, while glucagon quickly raises levels when they are too low.

The liver is attached to the gall bladder. The liver secretes bile (a digestive fluid) into the hepatic ducts. Bile flows into the gall bladder to be stored and then drains into the duodenum during digestion. The liver, gallbladder and hepatic ducts are collectively known as the Hepatobiliary System. The pancreas also secretes digestive juices into the duodenum and is sometimes considered a subsidiary part of the hepatobiliary system.

The liver has two blood supplies.

• 25% of the blood it receives comes as oxygen-rich blood direct from the lungs through the Hepatic Artery.
• 75% comes through the Portal Venous System. Blood laden with the products of digestion travels from the intestines and joins up with blood from the spleen and the pancreas in the Portal Vein before entering the liver. After the blood has travelled through the liver it returns to the heart via the vein called the Vena Cava.

The liver is reddish brown in colour because it is saturated in blood. A gallon of blood passes through the livers complicated network of arteries, veins and capillaries every two and half minutes.

The liver, unlike any other organ in the body, has two blood supplies. The largest amount of blood (about 75%) comes through the portal vein system. This is a network of blood vessels that transports blood through the intestine, stomach, the spleen and the pancreas draining into the portal vein and then into the liver. All the products of digestion from nutrients to toxins pass into the liver through this route. The Spleen releases iron broken down from destruction of red blood cells for use by the liver.

The second blood supply to the liver is via the hepatic artery which delivers highly oxygenated blood from the lungs. Once blood has been de-oxygenated and processed by the liver it is transported through the liver to the central hepatic vein and then out of the liver to the heart.

The portal vein and hepatic artery enter the liver through a fissure called the Porta Hepatis and then both divide into branches to the right and left lobes of the liver.

Hepatocytes

The primary and most common cells of the liver (making up 90 per cent of the livers cells) are called hepatocytes. These are highly sophisticated cells that carry out most of the many functions of the liver. Hepatocytes are all identical. The multiple tasks that the liver performs are not split up or shared out between them. They are all capable of carrying out the same functions.

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 liver cells or 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 supplying each lobule.

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 and attack and neutralise bacteria and viruses. Pit cells are a type of immune cell called natural killer cells . Under certain cicumstances Hepatic Stellate cells can produce collagen fibres leading to scarring or fibrosis

Biliary system

Running parallel to the sinusoids through the lobules are small channels called bile canaliculi. The bile produced by hepatocytes flows into these canaliculi and then moves to the outside of the lobule to branches of the bile duct which drain into progressively larger ducts until it reaches the right and left hepatic ducts. These are the final channels out of the liver from each lobe of the liver. 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.

Lymph

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 through the body 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 out into the bodys main lymphatic system.

How the liver is held together

The sinusoids, lymph vessels and the bile ducts are all contained in and supported by connective tissue that branches 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.