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Medical and Healthy Living Glossary - L

This page of our online glossary of medical and healthy living is for medical and healthy living terms beginning with the letter 'L'.

Lactase - The enzyme, secreted in the small intestine, that breaks down lactose from milk into its constituent sugars. Some people have a hereditary lactase deficiency, which makes them lactose intolerant so they are unable to digest milk sugar.
Lacteal - Small lymph vessel found in intestinal villi. Lacteal helps the digestive process in the small intestine by absorbing fat.
Lactic Acid - A sugar which is formed in cells and tissue, and also in sour milk, cheese and yogurt. Lactic acid is produced when the body uses up sugar during exercise. Excessive amounts of lactic acid in the body can produce muscle cramp.
Lactose - A sugar found only in milk. Lactose consists of two simple sugars, glucose and galactose, and is broken down in the small intestine by the enzyme lactase.
Lecithin - A constituent of our cell membranes and lipoproteins that helps in the transport and absorption of fats in the blood. It has been suggested that lecithin may help to combat against disorders ranging from arterial disease to viral infections and gallstones. It is found naturally in egg yolks, liver, whole wheat and nuts. Lecithin is used in the food industry as an emulsifier in such products as mayonnaise.
Legumes - Any member of the pea family, such as chickpeas, runner beans, soya beans and lentils. The seeds of these plants are known as pulses.
Leucocyte or Leukocyte - A white blood cell, so called because it contains a nucleus but has no hemoglobin. Leucocytes are formed in the bone marrow, spleen, thymus and lymph nodes. There are three types: granulocytes, comprising 70 per cent of all white blood cells, lymphocytes (25 percent) and monocytes (5 percent). Granulocytes are produced by the bone marrow, and help to combat bacterial and viral infection and may be involved in allergies. Lymphocytes destroy foreign bodies, either directly or through production of antibodies. Monocytes ingest bacteria and foreign bodies by the process called phagocytosis (engulfing microorganisms and cell debris to remove them from the body). In disease, immature forms of leucocytes may appear in the blood, ultimately forming both red and white blood cells.
Ligaments - Strong, fibrous tissues that connects the bones at a joint and forms the joint capsule.
Linoleic Acid - One of the omega-6 family of essential fatty acids. It is found naturally in vegetable oils such as those made from corn (maize) and soya as well as in some animal fats. It is always added (along with linolenic acid) to milk formula preparations for babies.
Linolenic Acid - Another essential fatty acid - part of the omega-3 family - which is found in green leafy vegetables and rapeseed oil.
Lipids - General term used to describe fats, oils and waxes, together with more complex molecules. They are insoluble in water. They are esters of fatty acids and form three groups: simple lipids, including fats, oils and waxes; compound lipids, which includes phospholipids; and derived lipids which includes steroids. Lipids float in the blood and can attach themselves to the walls of arteries causing atherosclerosis.
Lipoproteins - Particles made of proteins and lipids which enable insoluble fats to be transported in the bloodstream. Lipoproteins are classified according to the percentage of protein which they carry. Low-density lipoproteins (LDLs), transport cholesterol to the body cells, where it forms a component of the cell walls and plays a part in other essential functions. A high level of LDLs in the blood can reflect a high cholesterol level, which raises the risk of heart disease. The reasons for a high level of LDLs in the blood may be hereditary or diet-related, or a combination of the two. High-density lipoproteins (HDLs) remove surplus cholesterol from the tissues and carry it to the liver for excretion. A high level of HDLs in the blood indicates a lower than average risk of heart disease.
Litre or Liter - A metric unit of liquid volume, equal to 1000 millilitres. One litre is the same as 1.75 pints and there are 4.55 litres in 1 gallon.
Liver - A very important organ of the body, with many functions critical in regulating metabolic processes. The liver is the largest gland in the body, it occupies the top righthand part of the abdominal cavity next to the stomach and is made up of four lobes. It is fastened to the abdominal wall by ligaments and sits beneath the diaphragm and on the right kidney, large intestine, duodenum and stomach. There are two blood vessels supplying the liver: the hepatic artery delivers oxygenated blood, while the hepatic portal vein transports digested food from the stomach. Among its functions, the liver converts excess glucose to glycogen for storage as a food reserve; excess amounts of amino acids are converted to urea for excretion by the kidneys; bile is produced for storage in the gall bladder and lipolysis occurs; and some poisons are broken down (detoxified). Thus the hepatic portal vein has a beneficial effect in carrying blood to the liver rather than it going around the body first. The liver also synthesizes blood-clotting substances such as fibrinogen and prothrombin and the anticoagulant heparin; it breaks down red blood cells at the end of their life and processes the haemoglobin for iron, which is stored; vitamin A is synthesized and stored, and it also stores vitamins B12, D, E and K. Diseases which affect the liver include hepatitis and cirrhosis; the symptom of liver disease is often jaundice.
Lymph Fluid - A colourless, watery fluid that surrounds the body tissues and circulates in the lymphatic system. Lymph fluid carries waste matter away from tissues to the veins. It is derived from blood and is similar to plasma, comprising 95 per cent water with protein, sugar, salts and lymphocytes. The lymph is circulated by muscular action, passing through the lymph nodes, which act as filters, and eventually returning to the blood via the thoracic duct (one of the two main vessels of the lymphatic system). Lymph fluid is an essential part of the body’s defence against infection.
Lymphatic System or Lymphatics - The network of vessels, valves, nodes, etc, that carry lymph fluid from the tissues to the bloodstream and help maintain the internal fluid environment of the body. Lymph fluid drains into capillaries and larger vessels, passing through nodes and eventually into two large vessels (the thoracic duct and right lymphatic duct), which return it to the bloodstream by means of the innominate veins.
Lymph Nodes (Lymph Glands) - Small oval structures that occur at various points in the lymphatics. They are found grouped in several parts of the body, including the neck, groin and armpit. Their main functions are to remove foreign particles and produce lymphocytes, which are important in the immune response.
Lymphocytes - A type of white blood cell produced in the bone marrow and also present in the spleen, thymus gland and lymph nodes, which forms a vital component of the bodies' immune system. There are two types: B cells and T cells. B cells produce antibodies and search out and bind with particular antigens. T cells circulate through the thymus gland where they differentiate. When they come in contact an antigen, large numbers of T cells are generated which secrete chemical compounds to assist the B cells in destroying foreign bodies such as bacteria.