The liver has four important roles which include:
- producing proteins to help digestion,
- storing essential proteins, carbohydrates, vitamins and minerals,
- cleaning up toxic substances and dead cells (detoxification) and
- producing essential proteins and substances required for body functions.
Digestion: Food is made up of carbohydrates (commonly called sugars and starches), proteins, fats, vitamins and minerals. Food enters the stomach for the first phase of digestion, then passes on to the small intestine for further digestion. The liver assists in the process by making bile salts and digestive enzymes – special proteins that help to break down food to give the body energy. Bile salts are made by the liver but stored in the gall bladder. Bile is a complex fluid containing bile salts and other substances that are secreted into the small intestine. Bile acts like a detergent, making the fat more easily broken down and absorbed. Many waste products are secreted into bile before leaving the body in faeces. Cholesterol and bilirubin (a waste product from old red blood cells) are also excreted with bile. When the liver does not function properly, fat digestion is particularly affected. Fat-soluble vitamins such as vitamin A, D, E and K may not be well absorbed under these circumstances.
Storage: The liver stores essential substances such as iron and some vitamins, including vitamin B12. It is also critical in the maintenance of brain function, as it takes up glucose absorbed after a meal and turns it into glycogen (a form of glucose or sugar suitable for storage). Glucose is the only form of energy that the brain can use, so this store is essential to ensure that there is always enough glucose for the brain. When liver glycogen reserves become depleted (after not eating for hours), the liver begins to make glucose from other forms of energy such as proteins in the body. Excess carbohydrates and proteins are also converted into fat for storage, and fats are also broken down for energy when needed.
Detoxification: When digestion has occurred, the broken down parts of carbohydrates, proteins and fats, vitamins and minerals are absorbed directly into the liver. The liver also acts as a filter against toxic substances (chemical and infectious) entering the main blood system. Toxic substances can also be changed into substances that can be removed by the kidneys. For example, ammonia is a toxic substance that comes from the breakdown of proteins. The liver changes ammonia into urea which is easily handled by the kidneys.
Essential proteins and substances for body function: Proteins and other substances play major roles in the growth, reproduction and control of the body cells. The liver is crucial in the production of hormones (like oestrogen, progestogens, testosterone, cortisol and insulin). It also produces proteins needed for circulation (like plasma proteins to help keep the water balance in the body just right, and the blood pressure okay), proteins for blood clotting (like prothrombin and fibrinogen) and antibodies needed by the immune system. The liver makes lymph as well as kupffer cells, both of which help remove toxic substances from circulation.
The liver has a large blood supply that not only provides oxygen and nutrients to working liver cells, but also carries blood that is rich in food from the gut for processing. A large volume of blood flows through the liver every minute.
