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Physiology 2130
Ingrid L.Stefanovic

Module 15 – Digestive System Intro  Main function: break down organic nutrients so that they can be absorbed into the body o Organic nutrients:  Carbs, proteins, fats, water, vitamins, minerals Digestive System – The Meal  Meat within hamburger is made up of mostly protein with a little fat (lipid)  Hamburger bun and fries are made up of carbs (fries also contain fat)  Soda consists mostly of water, carbs, and some other dissolved substances Digestive System – Anatomy and Basic Function  Mouth – food is broken up by chewing (mastication) and is mixed with saliva  Salivary glands – produce saliva to moisten and begin digesting some food particles  Esophagus – straight muscular tube that connects the mouth and pharynx to the stomach  Stomach – stores, mixes, and digests some food and delivers food to the small intestine  Liver – produces and secretes bile  Gallbladder – stores and concentrates bile  Pancreas - secretes digestive enzymes, hormones, and bicarbonate  Small intestine – allows digestion and absorption of most food particles  Large intestine – stores and concentrates undigested material and absorbs salt and water  Rectum – site where defecation reflex is triggers 1 Digestive System – Basic Processes  Secretion – release of digestive fluids into the lumen of the digestive tract o This fluid may include water, mucus, acid, electrolytes, enzymes, bile sales, and digestive enzymes  Digestion – food is broken down into smaller molecules by the digestive enzymes so they can be taken up by the body  Absorption – these small molecules are taken up by the circulatory system and are distributed throughout the body  Motility – movement of ‘food’ through the digestive system by the contraction of smooth muscle that lines the walls of the tract  Excretion – removal of the unwanted products from the body after almost all of the wanted material is absorbed Digestive System – The Mouth  Digestion of your meal begins in the mouth, where it is broken down by chewing (or mastication) and is mixed with saliva to form a bolus  The saliva consists mostly of water (99.5%) with ions and proteins (0.5%) and is secreted by 3 glands  These glands (parotid, submandibular, sublingual) can produce up to 2L of saliva each day  Saliva helps to lubricate the bolus of food and begins digestive carbs bc it contains the enzyme amylase  The production and secretion of saliva is under the control of the autonomic nervous system 2 Digestive System – Swallowing  Once the bolus of food has been formed and sufficiently lubricated with the saliva, it will be pushed to the back of the mouth by the tongue and the swallowing reflex will be initiated  The uvula of the soft palate closes over the nasopharynx  The larynx is lifted by muscles in the neck, and the epiglottis bends back over the glottis, covering the larynx  The bolus moves down the esophagus through the cardiac orifice/lower esophageal sphincter and into the stomach by a smooth muscle contraction called peristalsis 3 Stomach – Structure  Stomach is divided into 3 general areas: o Upper, dome-shaped fundus o Middle body – comprises the largest part of the stomach o Lower antrum/pyloric region  Pyloric sphincter, located at the distal portion, regulates the emptying of the stomach into the first part of the small intestine, the duodenum  The stomach (when empty) is thrown into folds (rugae), which increase the surface area and allow for expansion of the stomach as it fills with blood Stomach – Function  Stomach liquefies, mixes, and stores each bolus of food from the meal  This mixture, called chyme, is slowly released into the small intestine where most of the digestion and absorption takes place  In doing so, it regulates the amount of food entering the small intestine so that it can be fully digested then absorbed  Some digestion does take place in the stomach o Proteins begin to be digested in the stomach by the enzyme peptin o Very little absorption takes place in the stomach o Only certain substances like alcohol and aspirin can cross the lining of the stomach to be absorbed into the bloodstream  The mixing of chyme is achieved by peristaltic contraction of the stomach walls that begin at the fundus and end at the antrum o These contractions help to move the chyme through the pyloric sphincter into the small intestine where most of the digestion and absorption take place o This muscle activity that causes the movement of substances through the digestive tract is called motility Stomach – Function of Gastric Secretions  The stomach secretes a variety of substances including: o Mucus – secreted by cells in the body and antrum of the stomach, protects the lining of the stomach from the extremely acid enviro that is produced by HCl o Hydrochloric Acid (HCl) – secreted by cells in the stomach body, begins to denature (unfold) the complex protein and connective tissue molecules found in the meat of the hamburger. HCl also kills bacteria and converts the inactive enzyme pepsinogen to its active form called pepsin o Pepsinogen – produced by the cells in the body of the stomach, when converted to its active form pepsin by the HCl, it begins to digest large protein molecules o Gastrin – secreted by the cells in the antrum of the stomach, which stimulates the secretion of HCl o Intrinsic factor – helps with the absorption of vitamin B12 4 Pancreas – Structure  Not directly part of the small intestine however part of it does drain into it, it is nevertheless, very important  Lies just below the stomach and has both digestive/exocrine and endocrine functions  Exocrine products of the pancreas are secreted into a long pancreatic duct o This duct merges with the common bile duct (from the liver and gallbladder) just before entering the duodenum Pancreas – Functions  Pancreas produces and secretes: o Carbohydrate-digesting enzyme amylase o Protein-digesting enzyme trypsin, chymotrypsin, proteases o Fat-digesting enzyme lipase  The pancreas also secretes sodium bicarbonate into the duodenum to neutralize the acid from the stomach 5 Small Intestine – Structure  Stomach contents empty through the pyloric sphincter into the small intestine  This is the longest section of the digestive tract (9m long)  Ends at the ileocecal sphincter where it empties into the ascending colon of the large intestine  Divided into the 3 segments: o Duodenum – first segment (shortest) o Jejunum – second segment o Ileum – longest segment (50% of SI)  The inner wall of the small intestine is thrown into folds  The folds contain fingerlike projections called villi(villus-singular)  The end result is a large surface area with which the food comes into contact  The villi contain a capillary network & lymphatic lacteal to absorb the digested material  A layer of epithelial cells, which secrete digestive enzymes, covers the villi  These cells have microvilli that face out into the lumen of the intestine, forming a brush border Small Intestine – Function  Almost all digestion and absorption takes place in the first 2 segments of the SI – the duodenum and jejunum  The digestion of the chyme occurs by digestive enzymes from the pancreas and epithelial cells  Bile, from the liver, helps digest fat although it is not a digestive enzyme  Note: there are very specific digestive enzymes for each part of our meal. Some enzymes digest the protein of the meat, some digest the carbs of the friends and some digest the fat in the meat and fries  SI is also the site where more of the digested material gets absorbed into the circulatory and lymphatic systems  The process of absorption is greatly enhanced by the large surface area produced by the villi lining the walls of the intestine, the large blood supply to the digestive tract, and the motility or movement of food in the intestine 6 SI – Carb Digestion  Carbs come in a variety of forms  Some carbs are made up of a single building block called monosaccharides, like glucose, fructose, and galactose  Other carbs called disaccharides, consist of 2 monosaccharides, these include maltose (germinating barley), lactose (milk), sucrose (sugar)  Carbs made up of more then 2 units are called polysaccharides, most common are starch and glycogen  In order to absorb the larger carbs, polysaccharides must all be broken down into monosaccharides. Once in this form they can be absorbed by transport systems in the walls of the SI  Digestion of carbs begins in the mouth with the salivary enzyme amylase o This enzyme breaks up the large polysaccharides into smaller polysaccharides and maltose o Once the food reaches the stomach, the digestion of the carbs essentially stops bc of the acidic enviro (low pH) which denatures (partially breaks up) the salivary amylase  When the food reaches the SI, digestion of the carb begins again bc the pancreas secretes amylase into the duodenum  The pancreas also secretes bicarbonate ions which neutralize the acid from the stomach  The neutralized enviro permits the pancreatic amylase to perform its digestive function  Each disaccharide requires a specific enzyme to break it down to its component monosaccharides. These enzymes are located in the brush border of the microvilli located on the intestinal epithelial cells o The enzyme lactase breaks down lactose (found in milk)  galactose and glucose o The enzyme maltase breaks down maltose (starch)  2 glucose monosaccharides o The enzyme sucrose breaks down sucrose (table sugar)  glucose and fructose  Now the carbs are ready for absorption 7 Si – Carb Digestion – Lactose Intolerance  Some people are unable to drink or eat foods containing milk bc they are lactose intolerant o Lactose = carb found in milk  When these people ingest food containing milk, they can get serious cramps, bloating, gas, and diarrhea o Cause = inability to digest lactose bc of the inability to produce the enzyme lactase  If the lactose is not digested, it cannot be absorbed into the body and it will stay in the digestive tract, which can lead to diarrhea  If carbs are present in the large intestine, bacteria will feed on it and produce gas, causing bloating and cramps  People who suffer from lactose intolerance can purchase synthetic lactase, which is generally taken before a meal that may contain lactose SI – Carb Absorption  The process of carb absorption is almost identical to the process of glucose reabsorption in the kidneys  The intestinal epithelial cells contain Na+/K+ pumps on their basal side that establish a concentration gradient for Na+ (high on outside, low on inside)  This gradient powers the Na+/glucose co-transporter (type of secondary active transport mechanism) located on the luminal side of the cell. This transporter moves glucose into the cell as Na+ moves in, down their concentration gradient. Once glucose is inside the cell it will then diffuse out through the basal side by facilitated diffusion SI – Protein Digestion  Protein consist of long chains of AA linked together o There are 20 different AA, 11 nonessential AA that can be produced by the body and 9 essential AA that must come from the diet  Just like carbs, the different groups of AA require different enzymes to break them apart and the proteins must be broken down into AA building blocks before they can be absorbed by transport systems in the SI  Digestions of proteins begins in the stomach o Here, hydrochloric acid (HCl) converts the inactive pepsinogen  active enzyme pepsin o The HCl also helps to uncoil the long, twisted strands of proteins. This unfolding of the protein gives the pepsin access to the long protein chains in order to digest them into smaller chains (polypeptides) o These polypeptides then pass through the pyloric sphincter into the small intestine where they continue to be digested and absorbed into the body 8  Pancreatic enzymes that will continue protein digestion must have an enviro with a neutral pH in order to work optimally. Therefore the chyme from the acidic stomach must be neutralized (this is achieved by bicarbonate which is secreted from the pancreas  As a result of the neutral pH enviro, the pepsin, which was secreted by the stomach to begin the process of digestion, now becomes inactivated o Now the enzymes trypsin & chymotrypsin from the pancreas continue the job in the SI that was begun by pepsin in the stomach  A class of enzymes called proteases can continue digesting proteins in AA o These enzymes are produced in the pancreas and are secreted into the SI o Some proteases are found along the brush border of the intestinal epithelial cells  There are two different classes of protease enzymes, each of which is responsible for breaking apart AA located in different parts of the protein o Endopeptidases break the bonds btwn AA in the inner part of the protein o Exopeptidases break the bonds btwn AA at the ends of the protein SI – Protein Absorption  Now that the proteins are broken into single AA with a few remaining very small polypeptides (dipeptides or tripeptides), absorption can take place  Much like carbs, the absorption of AA is through secondary active transport requiring the presence of a Na+ concentration gradient  As Na+ move into the intestinal epithelial cell and down their concentration gradient, they power a co-transporter that also moves the AA into the cell  The remaining small peptides are absorbed by endocytosis Review  Carbs begin to be digested in the mouth by the enzyme amylase that is found in the saliva  Little carb digestion and some protein digestion takes place in the stomach. Here, HCl coverts pepsinogen  active enzyme pepsin. Pepsin breaks the long strands of protein into small polypeptides  In the SI, carbs are broken down by pancreatic amylase to sucrose, maltose, and lactose. These disaccharides are broken into monosaccharides by sucrose, maltase, and lactase. The monosaccharides are absorbed into the body by a secondary active transport system using a Na+/glucose co- transporter  Also, in the SI, pancreatic enzymes (endopeptidase and exopeptidase) break the polypeptides into AA. These AA are also absorbed into the body by Na+/AA co-transporters relying on the Na+ concentration gradient 9 SI – Fat and Lipids  Fats (lipids) include substances such as triglycerides, phospholipids, cholesterol, and long-chain fatty acids  These substances are not water soluble and do not mix well with water o Therefore, a complex set of steps is necessary to digest fats and then absorb them into our ‘watery enviro’  The churning action of the stomach helps break up the large drops of fat into smaller ones – emulsification  The process of emulsification is extremely important bc it is much easier for the lipid-digesting enzymes to do their work on very small droplets than on very large ones  The main problem is keeping the small droplets from forming back into large ones o Therefore, it is very important to keep the lipids emulsified throughout the whole digestion process SI – Fat Digestion – Bile  Bile is produced in the liver and is transported to the gallbladder where it is stored and concentrated  Bile is not a digestive enzyme; rather, it is a substance that contains, among other things, water, bile salts (most abundant component), cholesterol, fatty acids, and many ions  The gallbladder releases bile into the duodenum of the SI during a meal  The bile salts keep the lipid droplets emulsified, preventing them from forming back into large droplets/stabilizes small fat droplets SI – Fat Digestion – Colipase  At this stage, there is one small problem bc the lipid droplets are coated in bile salts, the enzyme lipase cannot reach and digest the lipids  This problem is solved by another secretion of the pancreas called colipase  Colipase is also not a digestive enzyme, it is a protein that allows the lipase to get access to the lipid interior of the fat droplet. Now the pancreatic lipase can digest the lipids 10 SI – Fat Digestion  The fat droplets can contain different types of lipids, including phospholipids and cholesterol  The pancreatic lipase attacks the phospholipids and removes 2 fatty acid chains, leaving monoglycerides behind  As the lipase slowly digests the lipid interior of the fat droplets, the droplets get smaller and smaller o Eventually they form small sphere-shaped structures called micelles that consist of a single layer of bile salts surrounding a very small lipid droplet o These micelles help to ferry the lipid
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