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Home >>Zoology Dictionary >> Liver - Lymphocyte
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Liver - An exocrine gland associated with digestion but also performing many other functions in vertebrate animals. Digestive glands of invertebrate animals are also often called livers.
The vertebrate liver is the largest gland in the body. It arises as a pouchlike outgrowth from the gut, and in the ancestors of the vertebrates it is likely that the liver was solely a producer of digestive enzymes.
Though it still retains something of this activity in producing bile, its main functions concern the management of food stuffs fat, carbohydrate and protein storing them and/or converting them into the molecules required by the tissues. Protein in excess of the body's requirements is broken down (deaminated), the nitrogen is converted into urea which is carried by the blood to the kidneys, and the remaining molecules are burnt to provide energy
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Its other roles include the breakdown of harmful substances, such as alochol, the metabolism and storage( rage of iron and copper, and the making of vitamin A (an activity that making of vitamin A (an activity that accounts for the rich supply of this vitamin in oils obtained from fish livers). Special cells remove dead bacteria and the like from the blood. A recently discovered role is the storage of vitamin B 12 (cobalamine) A substance necessary for the proper formation of red blood cells. Lack of this vitamin causes pernicious anaemia. It is thought that the liver releases vitamin B12 when the level in the diet is low. It circulates in the blood, reaching the sites where red blood cells are manufactured.
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The liver consists essentially of a series of five or six sided columns (lobules), each made up of chains of cells (trabeculae) radiating from the centre. Between the trabeculae are blood filled spaces (sinusoids) and other 'spaces' (canaliculi) into which bile is released. A blood vessel passes through the centre of each lobule. It collects blood from the sinusoids which are supplied by branches of the hepatic portal vein around the edge of the lobule. The central veins join to form the hepatic vein that carries blood back to the heart.
The liver is strategically placed in relation to the gut and the blood supply it receives from there. The gut itself has a rich blood supply to absorb the digested food supply to absorb the digested food material. Blood rich in food molecules is conveyed by way of the hepatic portal vein to the liver before it joins the main circulation.
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The liver, by a multitude of chemical processes, is then able to act on the food, before releasing to the tissues the substances that they require. This is also part of the activity by which the liver controls the composition of the blood.
The food materials arriving in the liver from the intestine obviously vary from one part of a day to another, and from day to day, depending on the quality of the food eaten.
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The activities of the liver will also vary, therefore, but not merely because of the food intake, for the demands of the tissues also vary from time to time. The muscles require little fuel while the body is resting, but. at a time when they are working rapidly. enormous supplies of fuel will be required. One of the principal activities of the liver in this respect is the storage of glycogen-a starch like compound whose molecules are made up from numerous glucose molecules. When the blood contains more glucose than the tissues require, the liver cells join the glucose molecules together to form the larger glycogen molecules, and in this form they are stored. The liver is prompted to do this under the inf1uence of insulin produced by the pancreas. When the tissues require further supplies of glucose, the glycogen molecules are broken down and glucose is released into the bloodstream.
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The liver also stores fat. The latter can he broken down to release the energy needed to power chemical processes or to produce heat. The smaller molecules produced heat. The smaller molecules produce heat. The smaller molecules produced can be reassembled to form glycogen. Thus fats can be converted into carbohydrates. In a similar way the liver cells can break down amino acids and convert them into carbohydrates. (They cannot build up amino acids from simpler units. Most of the amino acids that the body requires have to be taken in with the food.) The amino (-NH2) groups remaining are incorporated into urea molecules and this waste product is passed in the bloodstream to the kidneys forremoval to the bladder. The liver breaks down only those amino acids that are surplus to the body's requirements forbuilding up proteins and other molecules containing them. It normally passes on the amino acid molecules that it receives from the gut to the tissues that need them.
The liver cells release bile into the spaces (canaliculi) between the chains of liver cells. The tiny bile canaliculi join to form large bile channels that eventually join and enter the gall bladder. This is a 'pocket' that stores bile. Leading from the gall bladder is the bile duct which carries the bile to the intestine.Bile is an alkaline secretion containing certain organic salts- the bile salts the bile pigments, and such substances as cholesterol and lecithin. The bile salts reduce the surface tension of fats in the food, breaking them down into tiny droplets. This increases their surface area to such an extent that the fat spliting enzymes in the pancreatic juice can act upon them. The bile pigments are breakdown products of haemoglobin fromdead blood cells. They are removed from the body cells with the faeces.
Apart from the production of bile and the management of food substances, a most important function of the liver is detoxication the conversion of harmful substances into harmless ones. This may be carried out in a number of ways. Thus ammonia, a highly toxic substance, formed in the process of deamination, is converted into urea for excretion by the kidneys. A substance such as benzoic acid is combined with an amino acid-glycine and converted to the harmless hippuric acid. Other substances are acetylated that is,- COCH3 grouping are added to the molecules.
This is how the body gets rid ofsulphonamides (a group of drugs). Associated with these processes is the removal of dead bacteria and
other foreign matter by special cells Kupffer cells. All may be considered protective functions of the liver.
The liver stores both iron and copper. It obtains iron from the food and also retains that released during the breakdown of haemoglobin. Copper is not itself part of the haemoglobin molecule, but in some way it enables the iron atoms to be incorporated into the haemoglobin molecules.
Lizard - (See Squamate).
Lumbar Vertebra - Vertebra of the waist region between the ribs and the sacrum.
Lung - Air breathing organ of vertebrates but also applied to the modified mantle cavity of slugs and snails. The vertebrate lung arises as a pouch in the ventral gut wall and it was present in some of the earliest fishes. It remains in many modern fishes as the swim bladder, but it lies above the gut and has normally lost its connection with the gut. In the air-breathing vertebrates the lungs open into the pharynx by way of two bronchi and the trachea. Amphibians obtain much of their oxygen through their skin and the surface area of their lungs is small. The reptile lung is considerably more complicated while the surface area of man's lungs may equal about half a tennis court. In the mammalian lung the bronchi break up into smaller tubes called bronchioles which end eventually in tiny blind pockets called alveoli. Each alveolus is surrounded by blood vessels and oxygen from the air taken into the lungs passes through the alveolus wall and into the blood stream. The lungs of a bird do not have nearly such a complicated lining but there are a number of accessory air sacs which act as pumps and ensure that a fresh supply of air reaches the absorbing surfaces of the lungs at each breath. This does not happen in mammals: the alveoli retain a certain amount of 'stale' air all the time and oxygen has to diffuse through this stale air before reaching the blood. The enormous number of alveoli makes up for this drawback in mammals.
Lung Book - Respiratory organ of spiders and scorpions, consisting of numerous sheets of tissue, wellsupplied with blood, suspended in a
pocket of the body wall, opening to the outside by a small pore.
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Lung Fish - (See Dipnoi)
Lymph - Fluid that bathes ,the cells and tissues of the body, providing them with food and oxygen. Lymph is actually blood plasma minus its proteins and cells that cannot diffuse out from the blood capillaries. The lymph is returned to the heart via the lymphatic system.
Lymphatic System - A special set of vessels returning lymph from the tissues to the heart. Its vessels reach nearly all parts of the body. The lymph capillaries are blind tubes a little larger in diameter than the blood capillaries. They are in close contact with the tissue cells or the spaces round them. Although the tubes are blind, molecules of all shapes and sizes (even bacteria) can squeeze through the walls between cells. The smaller vessels join up to form larger ones in much the same way as veins. At intervals along, the lymph channels are swellings the lymph nodes. These are essentially networks of connective tissue that contain phagocytes and also other white blood cells called lymphocytes.
The lymph nodes are particularly important at times of infection when the phagocytes actively consume bacteria. They are the 'swollen glands' that we feel in the armpits from an infected finger or in the neck from a bad tooth. Thus a major function of the lymphatic system is as a filter for bacteria and other foreign particles.The lymphatic system is also concerned with maintaining the fluid balance of the tissue cells and the spaces round them. When channels become blocked by disease the affected part may swell considerably, a condition called oedema. The lymph vessels (lacteals) of the intestine Lymh is moved very slowly through the lymph vessels as a result of body movements. Valves similar to those of veins prevent its backward flow.
Lymph Node - (See Lymphatic System)
Lymphocyte - Large type of white blood cell possibly concerned with the formation of antibodies.
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