The Digestive System. May 16, The Digestive System.notebook

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1 3 The Digestive System.ntebk The Digestive System THE BASIC CONCEPTS Animals btain energy by breaking fd mlecules int smaller pieces. The basic fuel mlecules are amin acids, lipids and carbhydrates. The prcessing f fd has fur basic steps: (1) Ingestin (2) Digestin (3) Absrptin (4) Eliminatin Digestin is the breakdwn f large fd mlecules int smaller mlecules that can be used by cells. It ccurs in tw different ways Mechanical digestin - breaking fd int smaller pieces. Chemical digestin - breaking fd int smaller mlecules. Basic Cncepts Schematic Diagram 1

2 3 The Digestive System.ntebk The Digestive System FOOD ENTERS THE DIGESTIVE SYSTEM THROUGH THE MOUTH Chewing breaks fd int smaller particles s that chemical digestin can ccur faster. Teeth are imprtant t animal digestin fr capturing, tearing, and chewing fd. Carnivres pssess pinted teeth fr capture, cutting and shearing. Herbivres have large, flat teeth suited fr grinding plant materials. Omnivres have bth types, frnt like carnivres, back like herbivres. Fd is mistened and lubricated in the muth. Tngue mixes fd with saliva - it is stimulated by the presence f fd. Saliva is secreted by salivary glands. Cntains the enzyme salivary amylase t begin the breakdwn f starch. There is cntinuus secretin f saliva t keep the muth mist and t lubricate fd. Fd Enters 2

3 3 The Digestive System.ntebk FOOD PASSES THROUGH THE ESOPHAGUS TO THE STOMACH Mucus lubricates and helps hld the chewed fd tgether in a clump called a blus. The tngue is muscular and is used t mve fd. It pushes fd t the back f the muth where it is swallwed. As the tngue mves the blus t the back f the muth, the swallwing reflex takes ver t mve the blus dwn the esphagus. The respiratry and digestive passages meet in the pharynx. They separate psterir t the pharynx t frm the esphagus (leading t the stmach) and the trachea (leading t the lungs). When swallwing, the epiglttis autmatically mves up t blck the passage t the trachea s that fd des nt end up in the lungs. The blus is mved t the stmach by peristalsis (rhythmic muscle cntractins). Peristalsis Peristalsis animatin 3

4 3 The Digestive System.ntebk A muscle called the cardiac sphincter cntrls the mvement f fd frm the esphagus t the stmach. This muscle prevents fd in the stmach frm reentering the esphagus. Smetimes stmach acid will splash up int the esphagus causing what we call heartburn. Cardiac Sphincter THE STOMACH Preliminary digestin ccurs in the stmach. The lining f the stmach is highly flded and expands as it fills with fd. The stmach has an extra layer f muscle t churn fd. The mixture f partly digested fd and gastric juice is called chyme. Cells f the stmach lining secrete hydrchlric acid (HCl) which keeps the stmach cntents at a ph f abut 2, helping t digest prteins. The lw ph als kills mst bacteria. The cells f the lining are prtected by a cating f mucus that is cnstantly replaced as the acid destrys it. These cells als secrete pepsingen, a prtein digesting enzyme. Inactive pepsingen is cleaved t frm pepsin - the active frm f the enzyme. It is imprtant t prtect the cells that prduce pepsin frm being digested themselves. Pepsin is mst active in a ph f 2. Pepsin cuts prteins int shrt pieces and they are nt further digested until they reach the small intestine. Prduce HCl Stmach 4

5 3 The Digestive System.ntebk Carbhydrates and fats are nt digested in the stmach. The stmach prduces abut 2 L f acid and gastric secretins per day. Seeing, smelling, tasting, r thinking abut fd can result in the secretin f gastric juice. If t much stmach acid is prduced, it can disslve a hle thrugh the prtective mucus cating and a gastric ulcer results. Ulcers can als be caused by certain bacteria which can survive in the acidic envirnment f the stmach. The grwth f the bacteria n sectins f the stmach lining prevents it frm secreting mucus, making it susceptible t the digestive actin f pepsin. Dudenal (intestinal) ulcers are mre cmmn, resulting when excessive acidic chyme is passed t the dudenum. Very little absrptin ccurs in the stmach, with sme exceptins such as water, aspirin and alchl. All ther absrptin ccurs in the intestine. Ulcers THE SMALL INTESTINE The passage f fd frm the stmach t the small intestine is regulated by a muscle called the pylric sphincter, which allws chyme t enter the dudenum in small spurts. The capacity f the small intestine is limited and digestin takes time s nly small amunts f chyme are permitted t enter at a time. The small intestine is where mst digestin ccurs. Absrptin f mlecules als takes place in the small intestine. The length f the small intestine is apprximately three meters. The first part, the dudenum, is abut the first 25 centimeters. The ther tw sectins are the jejunum and ileum. Small Intestine 5

6 3 The Digestive System.ntebk At this pint, prteins and carbhydrates are nly partially digested and lipid digestin has nt begun. In the dudenum, chyme, pancreatic enzymes, and bile frm the liver and gallbladder are mixed. The presence f fd in the dudenum triggers the release f varius hrmnes The intestine must be prtected frm the acid cntained in gastric juice. When chyme enters the small intestine, the cells f the dudenum release the hrmne secretin. This hrmne stimulates the pancreas t prduce sdium bicarbnate, which neutralizes the acidic chyme and shuts ff pepsin. It als stimulates the liver t secrete bile. Anther hrmne (CCK, chlecystkinin), stimulates the gallbladder t release bile and the pancreas t prduce pancreatic enzymes. Anther hrmne (GIP, gastrin inhibitry prtein) inhibits gastric glands in the stmach and inhibits the mixing and churning mvement f stmach muscles. This slws the rate f stmach emptying when the dudenum cntains fd. Digestin cntinues and absrptin ccurs in all three sectins f the small intestine. Digestive enzymes secreted by the cells f the small intestine digest lactse, sucrse and ther sugars. Sme adults lse the ability t prduce lactase, resulting in the cnditin called lactse intlerance. Hrmnes Absrptin f fd in the intestine: The walls f the small intestine are cvered with small prjectins called villi. These increase the surface area f the small intestine t increase absrptin. The villi themselves are cvered with many tiny prjectins called micrvilli, which increase the surface area even further. The mlecules resulting frm the digestin f prteins and carbhydrates are absrbed by cells f the intestinal lining. The villi cntain capillaries which allw efficient transfer f these mlecules t the bld. Intestinal absrptin 6

7 3 The Digestive System.ntebk The prducts f fat digestin are absrbed thrugh the villi int the lymphatic system. They enter the bld stream near the neck where the lymphatic system jins the circulatry system. Villi Intestine micrgraph 7

8 3 The Digestive System.ntebk THE PANCREAS The pancreas is lcated belw the stmach and secretes enzymes, sdium bicarbnate and hrmnes. Secretins f the pancreas reach the dudenum via the pancreatic duct. The fluid cntains The prtein digesting enzyme trypsin. The starch digesting enzyme pancreatic amylase. The fat digesting enzyme lipase. Bicarbnate t neutralize HCl frm the stmach. The pancreas als prduces hrmnes that regulate levels f sugar in the bld. These hrmnes, insulin and glucagn, are prduced by clusters f cells called islets f Langerhans. Pancreas Digestive juices 8

9 3 The Digestive System.ntebk THE LIVER The liver prduces bile and regulates bld cmpsitin. Rle in Digestin: Old red bld cells are destryed by the liver and the hemglbin frm these cells is used t make bile. The remains f the red bld cells are eliminated with feces and give it its characteristic brwn clr. Bile is stred and cncentrated in gall bladder. The presence f fatty fd in the dudenum triggers the gallbladder t release bile. Bile travels thrugh the cmmn bile duct t the dudenum. Bile salts are sluble in bth lipids and water. This enables them t break apart fat drplets in chyme t create smaller drplets. This increases the surface area fr lipase t wrk n and increases the speed f their digestin. Chlesterl can bind bile tgether causing crystals t frm. If these crystals becme large enugh they can blck the cmmn bile duct. If the cmmn bile duct becmes blcked, bile salts can accumulate in the skin and prduce a yellw clr in a cnditin called jaundice. Rle in Digestin Regulatin f bld cmpsitin. Anther rle f the liver is t remve txins frm the bld. It chemically mdifies them t prevent them frm remaining in circulatin. Ammnia prduced by the digestin f prteins is cnverted t a less txic cmpund called urea by the liver. Urea is remved frm the bld by the kidneys and eliminated in urine. Alchl and drugs are metablized by liver cells int less harmful cmpunds. Other txins, pesticides, carcingens are als detxified. These less harmful cmpunds are returned t the bld and are remved by the kidneys. If the liver is chrnically expsed t txins, the cells becme damaged and die. The result is cirrhsis f the liver. Reg. f Bld Cmp. 9

10 3 The Digestive System.ntebk Bld sugar Regulatin f bld glucse levels. It is imprtant t maintain a cnstant cncentratin f bld glucse s that cells have a steady supply. This is especially imprtant fr brain cells which stre little glucse, and cannt use fat r amin acids as an energy surce. Vertebrates eat spradically when fd is available and, ften, a perid f fasting ccurs between meals. Als, mst fd is digested rapidly, and the resulting mlecules (including glucse) enter the bld stream. Withut sme cntrl, the level f glucse (and ther cmpunds) in the bld wuld be quite variable. The liver remves glucse frm the bld, cnverting it int glycgen. Glycgen is stred in bth the liver and in skeletal muscle. The cnversin f glucse t glycgen is stimulated by the pancreatic hrmne, insulin. If bld glucse is high (such as after eating) the release f insulin frm the pancreas causes the liver t stre glucse. If bld glucse level is lw (such as between meals), the ppsing pancreatic hrmne glucagn causes the liver t secrete glucse int the bld. The liver stres enugh glycgen fr abut 10 hurs f fasting. If fd is still unavailable after that, amin acids (frm muscles) and fats (frm fat stred in fat cells) are used as a surce f energy. Reg. f Bld Glucse. 10

11 3 The Digestive System.ntebk 1. Skin ulcer 3. Prgressive infectin and amputatin 2. Infectin 4. Majr infectin and tissue lss Diabetes THE LARGE INTESTINE The large intestine, r cln, cncentrates slids by reabsrbing water. It cmprises the last meter f the digestive tract. It has n digestive functin, but functins t absrb water. If water is nt absrbed, as can happen during certain bacterial infectins, diarrhea can result, causing dehydratin and salt lss. The daily ttal vlume f fd and water we take in frm eating is abut 2 L (including abut 800 g f slids). The bdy adds abut 7 L f its wn fluids making a ttal f 9 L. 1.5 L saliva and salivary enzymes 2 L f gastric secretins 1.5 L f pancreatic secretins 0.5 L f bile frm the liver 1.5 L f intestinal secretins Nearly all fluids and slids are absrbed s that nly 50 g f slids and 100 ml f liquid leave as feces. The large intestine (alng with sme absrbed by the small intestine) recvers abut 90% f the water that enters the digestive system. Undigested material is cmpacted and stred until the cln is full. When the cln is full, a signal t empty it is sent by sensrs in the walls f the cln. Bacteria (mstly E. cli) live and reprduce in the cln. Anaerbic digestin (fermentatin) f material by these bacteria prduces gas in the cln. They als prduce vitamins fr the hst, including vitamin K and sme B vitamins. Bacteria are lst when feces is eliminated, making expsure t feces dangerus. Fiber (cellulse) tends t fill up the cln and cause it t be emptied. Lw fiber diets result in slwer passage f fd thrugh the cln and have been linked t cln cancer. The rectum is the last sectin f the large intestine. Feces pass int the rectum by peristaltic cntractins and material exits thrugh the anus, a sphincter muscle. Feces is cmpsed f apprximately 75% water and 25% slids. One third f the slids is intestinal bacteria, 2/3 is undigested materials. Large Intestine 11