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The Liver and the Pancreas. Alex Turner Hannah Soulsby. The Liver: General Structure….
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The Liver and the Pancreas Alex Turner Hannah Soulsby
The Liver: General Structure… We don’t need to learn all the parts of the liver exactly, but be able to recognise that the portal vein comes in part from the small intestine and the blood delivered is rich in glucose and amino acids as well as from the pancreas, bringing high concentrations of insulin and glucagon. The blood is brought to the liver in the hepatic artery and is returned to the heart in the hepatic vein.
…and Location The liver is fairly high up on the digestive system. It sits to the left of, and slightly above, the stomach and is attached to the gallbladder.
Function Overview The liver carries out a huge number of functions for the body. It metabolizes carbohydrates and is also responsible for the following processes: Lipid metabolism Protein metabolism Synthesis of plasmaproteins Producing bile Detoxification
Histology of the liver • The liver is made up of over 100,000 lobules and in the centre of each is a branch of the hepatic artery and the hepatic portal vein. • At the other end of the cell collection are branches of the hepatic vein. • Blood runs from the hepatic portal vein or the hepatic artery, through a channel between rows of the cells to the other end where the blood flows into the hepatic vein. • The cells of the lobules are called hepatocytes and the channels running between them sinusoids. • The rows of hepatocytes never exceed two cells’ thickness so that each cell is in close proximity with the blood and the sinusoids. Kpuffer cells line the sinusoids and are essentially phagocytic macrophages, there to destroy bacteria in the hepatic blood
Carbohydrate • The liver interconverts glucose and glycogen which helps to maintain a constant level of glucose in the blood. • Hormones adrenaline and glucagon trigger an increase in blood sugar levels. (glycogenlysis) • Insulin increases the rate in which glucose is converted back in to glycogen. • Other substances can be used to form glucose when levels are low e.g. when fasting. Amino acids can be converted to glucose, known as gluconeogenesis. Dr. Robert Atkins says ‘carbs are the enemy’.
Lipid Metabolism • The liver can use fatty acids as an energy source. • It converts fatty acids to acetoacetate which moves in the blood and can be uses as a respiratorysubstrate by other tissues. E.g. cardiac muscle. • The liver is also responsible for the conversion of excess proteins and carbohydrates into fats. These are triglycerides and move through the body as lowdensitylipoproteins. • The liver also synthesizes cholesterol which is needed for cell membranes and some hormones e.g. testosterone. It also helps the skin to remain waterproof and allows the absorption of vitamin D.
Protein Metabolism • The liver also converts certain type of amino acids to others, and helps the process of deamination. • The liver can convert a type of amino acid which is in ample supply to one which is in short supply. This process is called transamination, but it can’t make essential amino acids. • The body can’t store excess amino acids, so it takes the amino part of the acid and turns it into something useful e.g. fats or carbohydrates. • However this leaves a highly toxic product of ammonia, the liver combines this with carbon dioxide to produce urea. This process occurs in a series of steps known as the ‘ornithinecycle’
Plasma Proteins • Plasma proteins which are found in blood and synthesized by the liver. • Fibrinogen and prothrobin are involved in blood clotting. • Albumin is the most common plasma protein and it prevents too much water leaving the blood capillaries. • Their size stops them from leaving through the small holes in the capillary membrane, so not too much tissue fluid is formed. Fibrinogen fibres form Red Blood Cell
Producing Bile • Bile is made using cholesterol from the liver. It’s stored in the gall bladder before being released. • Bile contains salts which emulsify fats in the small intestine. • It also breaks down haemoglobin which has a life cycle of about 120 days, into the excretory product of bilirubin. In the lobules, some hepatocytes produce bile and the bile runs in channels called bile canalucili. Bile flows against the blood flow and enters the bile duct. From here it travels to the gall bladder.
Detoxification • Ethanol is an extremely dangerous product as it moves into cells very easily. • Liver breaks it down, forming large amounts of reduced NAD. • However too much reduced NAD inhibits the oxidation of fatty acids. These fats accumulate in the liver and regular drinkers can develop cirrhosis so avoid getting like this too often…
Function Summary The liver carries out a lot of metabolic processes and metabolises carbohydrates, lipids, proteins and ethanol. It recieves blood from the heart via the hepatic artery and from the small intestine via the hepatic portal vein, in which the blood is rich in absorbed nutrients. The liver is made up of groups of cells called lobules, of which the individual cells are called hepatocytes. Blood runs from the hepatic artery/portal vein along sinusoids (channels) past rows of hepatocytes to the hepatic vein which returns the blood to the heart. During this time nutrients are absorbed and metabolised in the hepatocytes. Bile is also produced in the liver by some hepatocytes and runs into the bile duct via the bile canaliculi.
The Pancreas: General Structure… These cells secretepancreatic juice into the lumen which is carried to the duct These cells secrete hormones (discussed later) into the blood
…and Location The pancreas is ‘hidden’ just behind the stomach and just below the liver but above the small intestine. It connects to the duodenum.
The Pancreas As with all organs (and cells that are specialised for a particular job) the structure of the pancreas can be related to its function. The pancreas is interesting as it has two contrary functions; one exocrine and one endocrine. So… the pancreas has two sets of physiological features, each relating to the function that part is responsible for. With a fairly simple function, accompanied by knowledge from AS and module 4, the structure of the pancreas isn’t too hard to remember…
Function Overview The pancreas has an endocrine function as well as an exocrine function so is fairly unique. Endocrine a.k.a endosecretory function is the secretion of substances directly into the blood or lymph. In the case of the pancreas it is to help control blood sugar by secreting the hormones insulin and glucagon. An exocrine function is different; exocrine cells secrete molecules into a duct, not directly into the blood or lymph. For the pancreas, the job of the exocrine cells is to secrete digestiveenzymes that make up pancreaticjuice. Pancreatic Juice
FUNCTION The exocrine cells of the pancreas have 3 jobs: To produce the hormone glucagon, carried out by alpha 2 cells; To manufacture the hormone insulin, taken care of by beta cells; And to produce the regulatory agent somatostatin which is done by the alpha 1 cells. These hormones are secreted directly into the bloodstream, and together they regulate the level of glucose in the blood. STRUCTURE The three cell types lie within the isletoflangerhans. They have a good blood supply so that the molecules that stimulate them to secrete their respective molecules can reach them easily. The islets make up nearly half of the tissue of the pancreas that isn’t the pancreatic duct. Pancreatic Endocrine Function
FUNCTION To secrete the components of pancreaticjuice such as the enzymes it consists of. These are released into the pancreatic duct, which runs into the duodenum where the juice joins bile from the liver. STRUCTURE Roughly half of the pancreatic tissue is clusters of cells which secrete the enzymes trypsinogen, chymotripsinogen, pancreaticlipase, and amylase into a central lumen that forms the pancreaticduct. Pancreatic Exocrine Function This x-ray shows the outflow of bile from the pancreas in the smaller highlighted tube. The ‘s’ shaped curve in it is obstructing the outflow of pancreatic juice
This is an important component of the digestive process, it is secreted by the exocrine part of the pancreas and contains a lot of interesting stuff! It includes the enzymes trypsinogen, chymotripsinogen, pancreaticlipase, and amylase. With its high concentration of bicarbonate ions, the juice is alkaline in nature. This helps to neutralize the acidic gastric juice from the stomach. Nerve impulses from the vagusnerve, as well as detection of the hormone secretin, stimulate the secretion of pancreatic juice into the pancreaticduct. Pancreatic Juice
Function Summary The pancreas is split 50/50 between exocrine cells and isletsoflangerhans containing within them alpha an betacells. The islets of langerhans are responsible for producing hormones involved in regulating blood glucose levels. This is the endocrine function and the hormones are released directly into the blood where they travel to their target organs. The other half of the pancreatic cells are exocrine and carry out the job of secreting digestivehormones including trypsinogen, chymotripsinogen, pancreatic lipase, and amylase. These are released into the pancreaticduct which empties into the duodenum where the juice mixes with bile from the liver. The pancreatic juice is alkaline so it neutralises gastric acid from the stomach.
