8/7/2019 ANATOMY - digestive system and liver

http://slidepdf.com/reader/full/anatomy-digestive-system-and-liver 1/6

THE LIVER

 The liver, is the largest organ of the human body. This organ plays amajor role in metabolism and has a number of functions in the body, includingglycogen storage, decomposition of red blood cells, plasma protein synthesis,hormone production, and detoxification. It lies below the diaphragm in thethoracic region of the abdomen. It produces bile, an alkaline compound whichaids in digestion, via the emulsification of lipids. It also performs and regulatesa wide variety of high-volume biochemical reactions requiring highly specializedtissues, including the synthesis and breakdown of small and complex molecules,many of which are necessary for normal vital functions.

ANATOMY 

An adult human liver normally weighs between 1.4–1.6 kg (3.1–3.5 lb),[4] and is a soft, pinkish-brown, triangular organ. It is both the largest internal organ(the skin being the largest organ overall) and the largest gland in the humanbody.

It is located in the right upper quadrant of the abdominal cavity, resting just below the diaphragm. The liver lies to the right of the stomach and overliesthe gallbladder.

Blood Flow

 The liver receives a dual blood supply from the hepatic portal vein andhepatic arteries. Supplying approximately 75% of the liver's blood supply, the

hepatic portal vein carries venous blood drained from the spleen,gastrointestinal tract, and its associated organs. The hepatic arteries supplyarterial blood to the liver, accounting for the remainder of its blood flow. Oxygenis provided from both sources; approximately half of the liver's oxygen demand is met by the hepatic portal vein, and half is met by the hepatic arteries.[5]Blood flows through the sinusoids and empties into the central vein of eachlobule. The central veins coalesce into hepatic veins, which leave the liver andempty into the inferior vena cava.

Biliary Flow

 The term biliary tree is derived from the arboreal branches of the bileducts. The bile produced in the liver is collected in bile canaliculi, which merge toform bile ducts. Within the liver, these ducts are called intrahepatic (within theliver) bile ducts, and once they exit the liver they are considered extrahepatic(outside the liver). The intrahepatic ducts eventually drain into the right and lefthepatic ducts, which merge to form the common hepatic duct. The cystic duct from the gallbladder joins with the common hepatic duct to form the commonbile duct.

8/7/2019 ANATOMY - digestive system and liver

http://slidepdf.com/reader/full/anatomy-digestive-system-and-liver 2/6

Bile can either drain directly into the duodenum via the common bile ductor be temporarily stored in the gallbladder via the cystic duct. The common bileduct and the pancreatic duct enter the second part of the duodenum together atthe ampulla of Vater.

PHYSIOLOGY 

 The various functions of the liver are carried out by the liver cells orhepatocytes. Currently, there is no artificial organ or device capable of emulating all the functions of the liver. Some functions can be emulated by liverdialysis, an experimental treatment for liver failure.

Synthesis

Further information: Proteins produced and secreted by the liver

• A large part of amino acid synthesis•  The liver performs several roles in carbohydrate metabolism:o Gluconeogenesis (the synthesis of glucose from certain aminoacids, lactate or glycerol)o Glycogenolysis (the breakdown of glycogen into glucose)o Glycogenesis (the formation of glycogen from glucose)(muscletissues can also do this)•  The liver is responsible for the mainstay of protein metabolism, synthesisas well as degradation•  The liver also performs several roles in lipid metabolism:o Cholesterol  synthesiso Lipogenesis, the production of triglycerides (fats).•  The liver produces coagulation factors I (fibrinogen), II (prothrombin), V,VII, IX, X and XI, as well as protein C, protein S and antithrombin.• In the first trimester fetus, the liver is the main site of red blood cell production. By the 32nd week of gestation, the bone marrow has almostcompletely taken over that task.•  The liver produces and excretes bile (a greenish liquid) required foremulsifying fats. Some of the bile drains directly into the duodenum, and some isstored in the gallbladder.•

 The liver also produces insulin-like growth factor 1 (IGF-1), a polypeptide protein hormone that plays an important role in childhood growth and continuesto have anabolic effects in adults.•  The liver is a major site of thrombopoietin production. Thrombopoietin isa glycoprotein hormone that regulates the production of platelets by the bonemarrow.

Breakdown

•  The breakdown of insulin and other hormones

8/7/2019 ANATOMY - digestive system and liver

http://slidepdf.com/reader/full/anatomy-digestive-system-and-liver 3/6

•  The liver breaks down hemoglobin, creating metabolites that are addedto bile as pigment (bilirubin and biliverdin).•  The liver breaks down or modifies toxic substances (e.g., methylation)and most medicinal products in a process called drug metabolism. Thissometimes results in toxication, when the metabolite is more toxic than itsprecursor. Preferably, the toxins are conjugated to avail excretion in bile or

urine.•  The liver converts ammonia to urea.

Other functions

•  The liver stores a multitude of substances, including glucose (in the formof glycogen), vitamin A (1–2 years' supply), vitamin D (1–4 months' supply),vitamin B12, iron, and copper.•  The liver is responsible for immunological effects- the reticuloendothelialsystem of the liver contains many immunologically active cells, acting as a'sieve' for antigens carried to it via the portal system.•  The liver produces albumin, the major osmolar component of bloodserum.•  The liver synthesizes angiotensinogen, a hormone that is responsible forraising the blood pressure when activated by renin, an enzyme that is releasedwhen the kidney senses low blood pressure.

8/7/2019 ANATOMY - digestive system and liver

http://slidepdf.com/reader/full/anatomy-digestive-system-and-liver 4/6

THE DIGESTIVE SYSTEM

 The whole digestive system is around 9 meters long. In a healthy humanadult this process can take between 24 and 72 hours.

Oral Cavity

In humans, digestion begins in the oral cavity where food is chewed. Saliva issecreted in large amounts (1-1.5 litres/day) by three pairs of exocrine salivaryglands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed withthe chewed food by the tongue.. The saliva serves to clean the oral cavity andmoisten the food, and contains digestive enzymes such as salivary amylase, whichaids in the chemical breakdown of polysaccharides such as starch into disaccharides 

such as maltose. It also contains mucin, a glycoprotein which helps soften the foodinto a bolus. Swallowing transports the chewed food into the esophagus, passingthrough the oropharynx and hypopharynx.

Pharynx

 The pharynx is the part of the neck and throat situated immediately posteriorto (behind) the mouth and nasal cavity, and cranial, or superior, to the esophagus. Itis part of the digestive system and respiratory system. Because both food and airpass through the pharynx, a flap of connective tissue, the epiglottis closes over thetrachea when food is swallowed to prevent choking or asphyxiation. The pharynx isalso divided into 3, the oropharynx, nasopharynx, and the

hypopharynx/laryngopharynx.

Esophagus

 The esophagus is a narrow muscular tube about 25 centimeters long whichstarts at pharynx at the back of the mouth, passes through the thoracic diaphragm,and ends at the cardiac orifice of the stomach. The wall of the esophagus is madeup of two layers of smooth muscles. The inner layer of muscles is arrangedcircularly in a series of descending rings, while the outer layer is arrangedlongitudinally. At the top of the esophagus, is a flap of tissue called the epiglottis 

8/7/2019 ANATOMY - digestive system and liver

http://slidepdf.com/reader/full/anatomy-digestive-system-and-liver 5/6

that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles. It takes only about seven seconds for food to passthrough the esophagus and no digestion takes place.

Stomach

 The stomach is a small,'J'-shaped pouch with walls made of thick, elasticmuscles, which stores and helps break down food. Stomach churning has the effectof assisting the physical disassembly begun in the mouth. Food enters the stomachthrough the cardiac orifice where it is further broken apart and thoroughly mixedwith gastric acid, pepsin and other digestive enzymes to break down proteins. Theenzymes in the stomach also have an optimum, meaning that they work at aspecific pH and temperature better than any others. The acid itself does not breakdown food molecules, rather it provides an optimum pH for the reaction of theenzyme pepsin and kills many microorganisms that are ingested with the food. Itcan also denature proteins. This is the process of reducing polypeptide bonds anddisrupting salt bridges which in turn causes a loss of secondary, tertiary or

quaternary protein structure. The parietal cells of the stomach also secrete aglycoprotein called intrinsic factor which enables the absorption of vitamin B-12.Other small molecules such as alcohol are absorbed in the stomach, passingthrough the membrane of the stomach and entering the circulatory system directly.Food in the stomach is in semi-liquid form, which upon completion is known aschyme.

 The transverse section of the alimentary canal reveals four distinct and welldeveloped layers within the stomach:

• Serous membrane, a thin layer of mesothelial cells that is the outermost wallof the stomach.

• Muscular coat, a well-developed layer of muscles used to mix ingested food,composed of three sets running in three different alignments.

• Submucosa, composed of connective tissue that links the inner muscularlayer to the mucosa and contains the nerves, blood and lymph vessels.

• Mucosa is the extensively folded innermost layer. This is the secreting-layerof the stomach.

Small intestine

After being processed in the stomach, food is passed to the small intestine viathe pyloric sphincter. The majority of digestion and absorption occurs here after the

milky chyme enters the duodenum. Here it is further mixed with three differentliquids:

• Bile, which emulsifies fats to allow absorption, neutralizes the chyme and isused to excrete waste products such as bilin and bile acids.

• Pancreatic juice made by the pancreas.• Intestinal enzymes of the alkaline mucosal membranes. The enzymes include

maltase, lactase and sucrase (all three of which process only sugars), trypsin and chymotrypsin.

8/7/2019 ANATOMY - digestive system and liver

http://slidepdf.com/reader/full/anatomy-digestive-system-and-liver 6/6

As the pH level changes in the small intestines and gradually becomes basic,more enzymes are activated further that chemically break down various nutrientsinto smaller molecules to allow absorption into the circulatory or lymphatic systems.Small, finger-like structures called villi, each of which is covered with even smallerhair-like structures called microvilli improve the absorption of nutrients byincreasing the surface area of the intestine and enhancing speed at which nutrientsare absorbed. Blood containing the absorbed nutrients is carried away from thesmall intestine via the hepatic portal vein and goes to the liver for filtering, removalof toxins, and nutrient processing.

 The small intestine and remainder of the digestive tract undergoes peristalsis totransport food from the stomach to the rectum and allow food to be mixed with thedigestive juices and absorbed. The circular muscles and longitudinal muscles areantagonistic muscles, with one contracting as the other relaxes. When the circularmuscles contract, the lumen becomes narrower and longer and the food issqueezed and pushed forward. When the longitudinal muscles contract, the circularmuscles relax and the gut dilates to become wider and shorter to allow food toenter.

In the small intestines, the duodenum provides critical pH balancing to activatedigestive enzymes. The liver secretes bile into the duodenum to neutralise theacidic conditions from the stomach. Also the pancreatic duct empties into theduodenum, adding bicarbonate to neutralize the acidic chyme, thus creating aneutral environment. The mucosal tissue of the small intestines is alkaline, creatinga pH of about 8.5, thus enabling absorption in a mild alkaline in the environment.[dubious – discuss]

Large intestine

After the food has been passed through the small intestine, the food entersthe large intestine. Within it, digestion is retained long enough to allow fermentationdue to the action of gut bacteria, which breaks down some of the substances whichremain after processing in the small intestine; some of the breakdown products areabsorbed. In humans, these include most complex saccharides (at most threedisaccharides are digestible in humans). In addition, in many vertebrates, the largeintestine reabsorbs fluid; in a few, with desert lifestyles, this resorption makescontinued existence possible.

In humans, the large intestine is roughly 1.5 meters long, with three parts:the cecum at the junction with the small intestine, the colon, and the rectum. Thecolon itself has four parts: the ascending colon, the transverse colon, thedescending colon, and the sigmoid colon. The large intestine absorbs water fromthe bolus and stores feces until it can be egested. Food products that cannot gothrough the villi, such as cellulose (dietary fiber), are mixed with other wasteproducts from the body and become hard and concentrated feces. The feces isstored in the rectum for a certain period and then the stored feces is eliminatedfrom the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.