Digestive System and its Absorption (16-10-24) PDF

**DIGESTIVESYSTEM** -- -- -- -- ### 1. Mucosa (Mucous Membrane) - **Purpose**: Protects and absorbs nutrients. - **Components**: - **Epithelium**: - In the mouth, throat, esophagus, and anal canal: **Non-keratinized, strat...

**DIGESTIVESYSTEM** -- -- -- -- ### 1. Mucosa (Mucous Membrane) - **Purpose**: Protects and absorbs nutrients. - **Components**: - **Epithelium**: - In the mouth, throat, esophagus, and anal canal: **Non-keratinized, stratified squamous epithelium** (tough and protective). - In the stomach and intestines: **Simple columnar epithelium** (better for absorption). - **Lamina Propria**: - Connective tissue that supports the epithelium. - Contains blood and lymphatic vessels to transport nutrients and immune cells (like Peyer's patches in the ileum) to fight off pathogens. - **Muscularis Mucosa**: - A thin layer of smooth muscle that creates folds in the mucosa, increasing the surface area for digestion and absorption. ### 2. Submucosa - **Purpose**: Connects mucosa to the muscularis and provides support. - **Features**: - Made of dense connective tissue. - Contains blood and lymphatic vessels for nutrient transport. - Houses submucosal glands that secrete digestive juices. ### ### 3. Muscularis - **Purpose**: Responsible for movement in the digestive tract. - **Structure**: - Composed of two layers of smooth muscle: - **Inner Circular Layer**: Constricts and mixes food. - **Outer Longitudinal Layer**: Shortens and moves food along the canal. - These muscle contractions help with digestion and move food through the system. ### 4. Serosa - **Purpose**: Protects and lubricates the outside of the canal. - **Structure**: - A layer of visceral peritoneum (thin membrane) and loose connective tissue. - Found only in the abdominal cavity. - In the mouth, throat, and esophagus, there's no serosa; instead, there's a tough layer called **adventitia** that helps hold these structures in place. +-----------------------------------------------------------------------+ | Summary: | +=======================================================================+ | - The **mucosa** is the innermost layer that interacts with food. | +-----------------------------------------------------------------------+ | - The **submucosa** provides support and nutrient transport. | +-----------------------------------------------------------------------+ | - The **muscularis** enables movement and digestion. | +-----------------------------------------------------------------------+ | - The **serosa** covers and protects the outside of the canal in | | the abdominal region. | +-----------------------------------------------------------------------+ | This structure helps your digestive system effectively break down | | food and absorb nutrients while protecting against harmful bacteria! | +-----------------------------------------------------------------------+ - The first of these processes, **ingestion**, refers to the entry of food into the alimentary canalthrough the mouth. There, the food is chewed and mixed with saliva, which contains enzymesthat begin breaking down the carbohydrates in the food plus some lipid digestion via linguallipase. - Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus.Thisactofswallowing,thelastvoluntaryactuntil defecation,isanexampleof**propulsion** - **Peristalsis** moves food through the digestive tract with alternating waves of musclecontractionandrelaxation.Digestionincludesbothmechanicalandchemicalprocesses.**Mechanical digestion** is a purely physical process that does not change the chemical nature ofthe food.Instead, it makes the food smaller to increase both surface area and mobility. Itincludes**mastication**, orchewing, aswellastonguemovements thathelp breakfoodintosmallerbitsandmixfoodwithsaliva. - **Mechanical digestion involves physically breaking down food substances into smaller particlesto more efficiently undergo chemical digestion**. **Segmentation** is **a muscular movement thattakes place in the small intestine, dividing and mixing the chyme by alternating backwards andforwardmovementof theGItractcontents**. - Chemical digestion, through a process called hydrolysis, **uses water and digestive enzymes tobreakdownthecomplexmolecules** - The process of **absorption**, which takes place primarily within the small intestine. There, mostnutrients are absorbed fromthe lumen of the alimentary canal into the bloodstream throughtheepithelialcellsthatmakeup themucosa. - In **defecation**, the final step in digestion, undigested materials are removed from the body asfeces. **Lingual lipase** is an enzyme produced by glands on the tongue that starts the process of fat digestion. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Propulsion** refers to the movement of food through the digestive tract. This process involves muscular contractions that push food along the alimentary canal, primarily through two mechanisms: **peristalsis** (wave-like contractions that move food forward) and **segmentation** (contractions that mix food and enhance digestion). +-----------------------------------------------------------------------+ | - **Chyme** is the semi-liquid food mixture that comes from the | | stomach. | | | | - In the intestines, muscles contract and relax in a way that | | pushes the chyme back and forth. | | | | - This **backward and forward movement** helps break down the food | | further and mix it with digestive juices, ensuring better | | absorption of nutrients. | | | | So, instead of just moving straight through, the chyme gets stirred | | and mixed, which helps your body digest it more effectively. | +-----------------------------------------------------------------------+ 1. **Oralcavity**ProcessesthatoccurintheupperGItractincludeingestion,mastication(chewing),andswallowing,mechanicalandchemicaldigestion.  2. **Salivaryglands**threepairsofsalivaryglandscommunicatewiththeoralcavitySalivation occurs in response to the taste, smell or even appearance of food. This occursdue to nerve signals that tell the salivary glands to secrete saliva to prepare and moistenthemouth.Eachpairofsalivaryglandssecretessalivawithslightlydifferentcompositions 3. **Pharynx**Thepharynxisthecommontubeconnectingtheoralandnasalcavitiestothetrachea andesophagus. **Figure3Pharynx** 4. **Esophagus*:*** The esophagus is a muscular tube of approximately 25cm in length and 2cmin diameter. It extends from the pharynx to the stomach after passing through an openingin the diaphragm. The esophagus functions primarily as a transport medium betweencompartments. 5. **Stomach:** 1. The cardiac region, where the contentsof the esophagus empty into thestomach. 2. The fundus,whichisformedbytheuppercurvatureoftheorgan. 3. Thebody, themaincentralregion. 4. Thepylorusoratrium,thelowersectionoftheorganthatfacilitatesthe 1. Cardiacor esophagealsphincterthatdividesthetractabove. 2. Pyloric sphincter or pyloric orifice that divides the stomachfromthesmallintestine 1. **Mucosa**: +-----------------------------------------------------------------------+ | **In which part Gastric gland present?** | | | | Gastric glands are primarily located in the **stomach**. They are | | found in the **mucosal layer** of the stomach lining and are | | responsible for secreting gastric juice, which includes hydrochloric | | acid and digestive enzymes. These glands play a crucial role in the | | digestion of food. | | | | **In which mucosal layer gastic gland present?** | | | | Gastric glands are located in the **gastric mucosa**, specifically in | | the **lamina propria** of the mucosal layer of the stomach. They are | | situated within the gastric pits, which lead into the lumen of the | | stomach. The gastric mucosa consists of several cell types that | | contribute to the secretion of gastric juice and other important | | functions of the stomach. | +-----------------------------------------------------------------------+ - This is the innermost layer and is responsible for producing digestive enzymes and stomach acid (hydrochloric acid). It has three sub-layers: - - **Epithelium**: Produces mucus that protects the stomach lining from acid. - **Lamina propria**: A thin layer of connective tissue containing blood vessels and lymph nodes. - **Muscularis mucosae**: A thin layer of muscle that helps move the mucosa to aid digestion. 2. **Submucosa**: - This layer lies beneath the mucosa and contains connective tissue, blood vessels, lymph vessels, and nerves. It supports the mucosa and allows the stomach to stretch after eating. 3. **Muscularis externa (or muscularis propria)**: - This is the muscle layer of the stomach and consists of three sublayers of smooth muscle: - **Circular muscle layer**: It tightens the stomach and helps move food down to the intestines. - **Longitudinal muscle layer**: This outer layer aids in the movement of food through the digestive tract. 4. **Serosa**: - The outermost layer, it is made of connective tissue and a layer of epithelial cells. The serosa forms the stomach\'s outer lining and produces a slippery fluid that reduces friction with surrounding organs. - The stomach mucosa's epithelialliningconsistsonly of surfacemucus cells,whichsecrete aprotectivecoatofalkalinemucus. - The mucosal lining of the stomach is simple columnar epithelium with numerous tubulargastric glands. **The gastric glands open to the surface of the mucosa through tinyholescalledgastricpits**. - Cellsthatmakeupthepyloricantrumsecretemucusandanumberofhormones,includingthemajority of thestimulatoryhormone,gastrin. - Themuchlarger glandsofthefundusandbodyofthestomach,the siteofmostchemicaldigestion,producemostofthe gastricsecretions. ### **Parietal Cells**: - **Location**: Found in the middle part of the gastric glands. - **Function**: - These cells produce **hydrochloric acid (HCl)**, which makes the stomach super acidic (pH 1.5 to 3.5). This acidity: - Activates **pepsin**, an enzyme that helps break down proteins. - **Kills bacteria** that come with food. - **Denatures proteins**, making it easier for enzymes to digest them. - They also produce **intrinsic factor**, a special protein that helps your body absorb **vitamin B12**, which is crucial for making red blood cells. - ### Chief cells: - **Location**: Found deeper in the gastric glands. - **Function**: - Chief cells produce **pepsinogen**, which is an inactive form of **pepsin**. Once it meets the acid (HCl), pepsinogen turns into **pepsin**, which helps digest proteins. - They also release **lipase**, which helps digest fats (though the stomach\'s fat digestion is not as significant as in the small intestine). ### **Mucous Neck Cells**: - **Location**: Found in the upper part of the gastric glands. - **Function**: - These cells produce a thin, **acidic mucus** (called mucin), which is different from the thick mucus that lines the surface of the stomach. - The mucus they secrete forms a protective barrier to shield the stomach lining from damage caused by the stomach's acidity and the mechanical stress of digestion. ### **Enteroendocrine Cells**: - **Location**: Scattered throughout the gastric glands. - **Function**: - These cells release various **hormones** into the surrounding tissues (the lamina propria) that help regulate digestion. For example: - **Gastrin**, which stimulates more acid production. - **Somatostatin**, which slows down acid production when needed. - Other hormones that help with hunger, digestion, and stomach activity. +-----------------------------------------------------------------------+ | ### Summary: | +=======================================================================+ | - **Parietal cells**: Make stomach acid (HCl) and intrinsic factor | | (for vitamin B12 absorption). | +-----------------------------------------------------------------------+ | - **Chief cells**: Produce pepsinogen (which becomes pepsin to | | digest proteins) and lipase (for fat digestion). | +-----------------------------------------------------------------------+ | - **Mucous neck cells**: Secrete a protective, acidic mucus to | | shield the stomach. | +-----------------------------------------------------------------------+ | - **Enteroendocrine cells**: Release hormones that help control | | digestion. | +-----------------------------------------------------------------------+ +-----------------------------------------------------------------------+ | **\"surface mucus gland secretory product is mucin in an alkaline | | fluid\"** | | | | **Surface Mucus Glands**: These glands are found in various parts of | | the body, such as the stomach and respiratory tract. They produce | | mucus, which serves several important functions, like protecting and | | lubricating surfaces. | | | | 1. **Mucin**: Mucin is a type of protein that forms the main | | component of mucus. It has a gel-like consistency and helps trap | | particles, pathogens, and dirt, keeping tissues moist and | | protected. | | | | 2. **Alkaline Fluid**: The secretion from these glands is often | | alkaline (basic) rather than acidic. This alkalinity helps | | neutralize stomach acid in the digestive system and protects the | | lining of the stomach and intestines. In the respiratory system, | | alkaline mucus helps trap and clear irritants more effectively. | | | | **Mucus neck cells secrete mucin in an acidic fluid\" can** | | | | 1. **Mucus Neck Cells**: These cells are located in the gastric | | glands of the stomach. They play a crucial role in the secretion | | of mucus, particularly in the neck region of these glands. | | | | 2. **Mucin**: Similar to other mucus-producing cells, mucus neck | | cells secrete mucin, a glycoprotein that forms the basis of | | mucus. Mucin helps protect the stomach lining and aids in | | lubrication. | | | | 3. **Acidic Fluid**: The secretion from mucus neck cells is often | | mixed with an acidic environment due to the presence of gastric | | acid (hydrochloric acid) produced by parietal cells in the | | stomach. This acidic fluid helps in the digestion of food and | | also maintains a suitable pH for the activity of digestive | | enzymes. | +-----------------------------------------------------------------------+ **GASTRICSECRETION** The secretion of gastric juice is controlled by both nerves and hormones. Stimuli in the brain, stomach, and small intestine activate or inhibit gastric juice production. 1. **The cephalic phase** (reflex phase) of gastric secretion, which is relatively brief, takes place before food enters the stomach. The smell, taste, sight, or thought of food triggers this phase. Impulses from receptors in your taste buds or the nose are relayed to your brain, which returns signals that increase gastric secretion to prepare your stomach for digestion. 2. **The gastric phase** of secretion lasts 3 to 4 hours, and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach. This stimulates parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice. 3. **The intestinal phase of gastric secretion** This phase occurs when food moves from the stomach into the small intestine (duodenum). It regulates how much gastric juice the stomach produces and how quickly it empties. The duodenum is the first part of the small intestine. When partially digested food (chyme) enters it, the duodenum plays a key role in signaling the stomach. When the duodenum senses the presence of chyme, it releases a hormone called intestinal gastrin. This hormone stimulates the stomach to produce more gastric juice, which helps continue the digestion process. The release of intestinal gastrin boosts gastric juice secretion, but this effect doesn't last long. When the duodenum fills up with chyme, it sends signals back to the stomach to slow down its activity. This reflex inhibits gastric juice secretion and slows down stomach emptying, allowing the small intestine more time to digest the food. 1. **Smallintestine** a. **Duodenum** The shortest region is the 25.4-cm (10-in) **duodenum**, which begins at thepyloricsphincter.Justpastthepyloricsphincter,itbendsposteriorlybehindtheperitoneum, becoming retroperitoneal, and then makes a C-shaped curve around the headof the pancreasbefore ascending anteriorly again to return to the peritoneal cavity andjointhejejunum. b. **Jejunum** The **jejunum** is about0.9 meters (3 feet) long (in life) and runs from theduodenum to the ileum.In jejunum majority of digestion and absorption occurs. It has alining which is designed to absorb carbohydrates and proteins. The inner surface of thejejunum, its mucous membrane, is covered in projections called villi, which increase thesurface areaoftissueavailable toabsorbnutrients fromthegutcontents. c. **ileum** The **ileum** is the longest part of the small intestine, measuring about 1.8 meters (6feet) in length. It is thicker, more vascular, and has more developed mucosal folds thanthejejunum.Theileumjoinsthececum,thefirstportion of thelargeintestine,atthe **ileocecalsphincter**(orvalve).  **HISTOLOGY OF SMALLINTESTINE** The wall of the small intestine is composed of the same four layers typically present in the alimentary system. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli. These adaptations are most abundant in the proximal two-thirds of the small intestine, where most absorption occurs. ### **Intestinal Glands (Crypts of Lieberkühn)**: - These are small, tubular glands found in the lining of the small intestine. - They produce **intestinal juice**, which is a mixture of water and mucus. This juice has a slightly alkaline pH (around 7.4 to 7.8), which helps neutralize stomach acid. ### 2. **Function of Intestinal Juice**: - Each day, the small intestine secretes about **0.95 to 1.9 liters (1 to 2 quarts)** of intestinal juice. - The secretion is stimulated by: - **Distention**: When the small intestine stretches as it fills with food. - **Irritating effects of chyme**: The partially digested food that enters from the stomach can irritate the intestinal lining, prompting the secretion. ### 3. **Duodenal Glands (Brunner's Glands)**: - Located in the submucosa of the **duodenum** (the first part of the small intestine), these glands are unique because they secrete a special kind of mucus. - This mucus is rich in **bicarbonate**, which helps to neutralize the acidic chyme coming from the stomach. This is important because it protects the intestinal lining and creates a better environment for enzymes to work in the small intestine. **Mechanical and Chemical Digestion in Small Intestine** The movement of intestinal smooth muscles includes both segmentation and a form of peristalsis called migrating motility complexes **Segmentation** The contraction of intestinal circular smooth muscles that mixes chyme is called **segmentation**. The small intestine is the site where segmentation occurs. - The content move through segmentation, incrementally back and forth, as the rings of smooth muscle repeatedly contract and then relax. Segmentation in the small intestine does not force chyme through the tract. Instead, it combines the chyme with digestive juices and pushes food particles against the mucosa to be absorbed. The duodenum is where the most rapid segmentation occurs. - The duodenal mucosa secretes the hormone **motilin**, which initiates peristalsis in the form of a **migrating motility complex** - These complexes, which begin in the duodenum, force chyme through a short section of the small intestine and then stop. These complexes move slowly down the small intestine, forcing chyme on the way, taking around 90to 120 minutes to finally reach the end of the ileum. 2. **Large intestine** a. **Cecum** The cecum is the beginning of the colon. The small intestine feeds into the cecumthrough a small channel in the side of it (the ileocecal valve), so the end of the cecum isactually closed like a pouch. it is also the widest portion of the large intestine. This is thereservoirwherefoodfromthe small intestine arrivesinthelargeintestine b. **Colon**Asfoodproceedstotheascendingcolon,ittravelsupwardandeventuallysideways across the transverse colon. Any remaining water and electrolytes are absorbedin the ascending and transverse colon so that the food waste that arrives in the descendingcolon is mostly solid. The colon secretes mucus to bind and lubricate the food waste tohelpitpass throughsmoothlyasitis dehydrated. c. **Sigmoid colon**, A terminal section of the large intestine that connects the descendingcolon to the rectum; its function is to store fecal wastes until they are ready to leave thebody. d. **Rectum**Bythetimethesigmoidcolondeliversthefood wastetotherectum,itresemblesthe faceces.. When faeces enters the rectum, it triggers the urge to defecate. This is thenaturalcontinuationofthemassmusclemovements ofthecolon. e. **Anus**Theanusisthecanalyourwastewilltravel through toleaveyourbody. **Figure5LargeIntestine** **Accessaryorgans**: Anorganthat helpswithdigestion butisnot partofthedigestivetract. - Salivaryglands - Liver - Gallbladder - Pancreas - Mesenteries 6. **Salivaryglands**threepairsofsalivaryglandscommunicate with the oral cavity Salivation occursin response to the taste, smell or even appearance offood. This occurs due to nerve signals that tell thesalivaryglandstosecretesalivatoprepareandmoistenthemouth.Eachpairof salivary glandssecretessalivawithslightlydifferentcompositions 7. **Liver** - The liveris a vital organ located in the upper rightpart ofthe abdomen. It lies just below the diaphragm to the right ofthe stomach The liver plays an important role in digestion bysecretingbile. - Theliverisareddish-brown,wedge-shapedstructure.In 1. The liver is connected to two large blood vessels: the hepatic artery and the portal vein.The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal veincarries blood that is rich in digested nutrients from the GI tract and wastes filtered fromthe blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries,which lead into the liver lobules. The nutrients from the GI tract are used to build manyvitalbiochemicalcompounds,andthe wastesfromthe spleenare degradedandexcreted. 2. The main digestive function of the liver is the production of bile. **Bile** is a yellowishalkaline liquid that consists of water, electrolytes, bile salts, and cholesterol, among othersubstances, many of which are waste products. The liversynthesizes glycogen from glucose and stores the glycogen as required to help regulatebloodsugarlevels 3. **Gallbladder** 8. **Pancreas** - Amylase, whichhelpsdigeststarchandother carbohydrates. - Trypsinandchymotrypsin,whichhelpdigestproteins. - Lipase,whichhelpsdigestlipids. - Deoxyribonucleasesandribonucleases, whichhelpdigestnucleicacids. 9. **Mesentery**The **mesentery** isadoublefoldofperitoneal tissue that suspends the small intestineand large intestine from the posterior abdominalwall.Anchoringtheminplace,whilststillallowingsomemovement. - **CavitiesorToothDecay** - **GumDisease(Gingivitis)** - **Xerostomia** - **Dysphagia** - Aburningsensationinyourchest(heartburn),usuallyaftereating, whichmightbeworseatnightorwhilelyingdown. - Backwash(regurgitation)offoodorsourliquid. - Upperabdominalorchestpain. - Troubleswallowing(dysphagia)  - Mostulcersarelocatedinthesmallintestine. These ulcers are calledduodenalulcers - Stomachulcersarecalledgastriculcers. - Ulcers inthethroatarecalledesophagealulcers. - The *H. pylori* bacteria stick to the layerofmucus inthedigestivetractandcauseinflammation (irritation), which cancausethisprotectivelining to breakdown. - NSAIDScanwearawayatthe mucuslayerinthe digestivetract. - Burping - Nausea - Vomiting - Notfeelinghungry - Vomitingblood - viralorbacterialinfection - radiationinduced - medicationinduced - alcoholordruginduced