Digestive System PDF

The Digestive System Clement Okraku Tettey, PhD.  The primary function of the digestive system is to transfer nutrients, water, and electrolytes from the food we eat into the body’s internal environment Ingested food is essential as: an energy source or fuel, from which the cells can ge...

The Digestive System Clement Okraku Tettey, PhD.  The primary function of the digestive system is to transfer nutrients, water, and electrolytes from the food we eat into the body’s internal environment Ingested food is essential as: an energy source or fuel, from which the cells can generate ATP a source of building supplies for the renewal and addition of body tissues The digestive system performs four basic digestive processes 1. Motility 2. secretion 3. digestion 4. absorption  MOTILITY refers to the muscular contractions that mix and move forward the contents of the digestive tract The smooth muscle in the walls of the digestive tract maintains a constant low level of contraction known as tone Two basic types of phasic digestive motility: 1. propulsive movements 2. mixing movements  Propulsive movements propel or push the contents forward through the digestive tract The contents are moved forward in a given segment at an appropriate velocity Allowing that segment to do its job  Mixing movements: serve a twofold function First, by mixing food with the digestive juices, these movements promote digestion of the food Second, they facilitate absorption by exposing all parts of the intestinal contents to the absorbing surfaces of the digestive tract Contraction of the smooth muscle within the walls of the digestive organs accomplishes movement  SECRETION: A number of digestive juices are secreted into the digestive tract lumen by exocrine glands Each with its own specific secretory product Each digestive secretion consists of water, electrolytes, and specific organic constituents such as; Enzymes bile salts mucus  On appropriate neural or hormonal stimulation, the secretions are released into the digestive tract lumen Endocrine cells located in the digestive tract wall secrete gastrointestinal hormones into the blood That help control digestive motility and exocrine gland secretion  DIGESTION: The biochemical breakdown of the structurally complex foodstuffs of the diet into smaller, absorbable units by the enzymes produced within the digestive system Humans consume three different biochemical categories of energy-rich foodstuffs carbohydrates proteins fats These large molecules cannot cross plasma membranes intact to be absorbed from the lumen of the digestive tract into the blood or lymph  Digestion is accomplished by enzymatic hydrolysis (“breakdown by water”) By adding H2O at the bond site, enzymes in the digestive secretions break down the bonds that hold the small molecular subunits within the nutrient molecules together Thus setting the small molecules free  ABSORPTION Small absorbable units that result from digestion, along with water, vitamins, and electrolytes, are transferred from the digestive tract lumen into the blood or lymph  The digestive system consists of the digestive tract plus the accessory digestive organs The accessory digestive organs include the salivary glands the exocrine pancreas and the biliary system, which is composed of the liver and gallbladder These exocrine organs lie outside the digestive tract and empty their secretions through ducts into the digestive tract lumen The digestive tract wall has four layers A cross section of the digestive tube reveals four major tissue layers From the innermost layer outward they are the ; Mucosa submucosa muscularis externa and serosa  The mucosa lines the luminal surface of the digestive tract Divided into three layers: mucous membrane lamina propria muscularis mucosa  The mucous membrane is an inner epithelial layer that serves as a protective surface It is also modified for secretion and absorption It contains exocrine gland cells for secretion of digestive juices Also contains endocrine gland cells for secretion of blood-borne gastrointestinal hormones  The lamina propria is a thin middle layer of connective tissue It houses the gut-associated lymphoid tissue (GALT) GALT is important in the defense against disease-causing intestinal bacteria  The muscularis mucosa a sparse layer of smooth muscle It is the outermost mucosal layer that lies adjacent to the submucosa  SUBMUCOSA a thick layer of connective tissue contains the larger blood and lymph vessels Both of which send branches inward to the mucosal layer and outward to the surrounding thick muscle layer Also, a nerve network known as the submucosal plexus lies within the submucosa (plexus means “network”)  MUSCULARIS EXTERNA The major smooth muscle coat of the digestive tube consists of two layers: an inner circular layer and an outer longitudinal layer Contraction of these inner circular fibers decreases the diameter of the lumen, constricting the tube Contraction of the fibers in the outer layer, which run longitudinally along the length of the tube, shortens the tube  Together, contractile activity of these smooth muscle layers produces the propulsive and mixing movements Another nerve network, the myenteric plexus, lies between the two muscle layers Together the submucosal and myenteric plexuses, along with hormones and local chemical mediators, help regulate local gut activity  SEROSA The outer connective tissue covering of the digestive tract Secretes a watery, slippery fluid (serous fluid) Which lubricates and prevents friction between the digestive organs and surrounding viscera Regulation of digestive function is complex and synergistic Four factors are involved in regulating digestive system function: 1. Autonomous smooth muscle function 2. Intrinsic nerve plexuses 3. Extrinsic nerves 4. Gastrointestinal hormones Regulation of digestive function is complex and synergistic Autonomous smooth muscle function: Specialized smooth muscle cells serve as pacemaker cells that display rhythmic, spontaneous variations in membrane potential Noncontractile cells known as the interstitial cells of Cajal are the pacemaker cells that instigate cyclic slow-wave activity These pacemaker cells lie at the boundary between the longitudinal and circular smooth muscle layers Interstitial cells of Cajal are connected with smooth muscle cells by gap junctions Regulation of digestive function is complex and synergistic Intrinsic nerve plexuses The intrinsic nerve plexuses are the two major networks of nerve fibers—the submucosal plexus and the myenteric plexus These intrinsic nerve networks primarily coordinate local activity within the digestive tract Often termed the enteric nervous system Various types of neurons are present in the intrinsic plexuses Some innervate the smooth muscle cells and exocrine and endocrine cells of the digestive tract This directly affect digestive tract motility, secretion of digestive juices, and secretion of gastrointestinal hormones Regulation of digestive function is complex and synergistic Some of the neurons are excitatory, and some are inhibitory Some release acetylcholine as a neurotransmitter to promote contraction of digestive tract smooth muscle Regulation of digestive function is complex and synergistic Extrinsic nerves: Nerve fibers from both branches of the autonomic nervous system Originate outside digestive tract They innervate the various digestive organs The autonomic nerves influence digestive tract motility and secretion either by; 1. modifying ongoing activity in the intrinsic plexuses 2. altering the level of gastrointestinal hormone secretion, or 3. acting directly on the smooth muscle and glands Regulation of digestive function is complex and synergistic The sympathetic system dominates in “fight-or-flight” situations Tends to inhibit or slow down digestive tract contraction and secretion The parasympathetic nerve fibers supplying the digestive tract increase smooth muscle motility Also promotes secretion of digestive enzymes and hormone Regulation of digestive function is complex and synergistic Gastrointestinal hormones: These gastrointestinal hormones are carried through the blood To other areas of the digestive tract Where they exert either excitatory or inhibitory influences on; Smooth muscle Exocrine gland cells The Mouth Movements of the tongue are important in guiding food within the mouth during chewing and swallowing The major taste buds are located on the tongue The uvula plays an important role in sealing off the nasal passages during swallowing The Mouth The first step in the digestive process is mastication (chewing) The motility of the mouth that involves the slicing, tearing, grinding, and mixing of ingested food by the teeth The functions of chewing are 1. To grind and break food up into smaller pieces to facilitate swallowing and to increase the food surface area on which salivary enzymes will act 2. To mix food with saliva, and 3. To stimulate the taste buds (This in feedforward fashion, reflexly increases salivary, gastric, pancreatic, and bile secretion to prepare for the arrival of food) The Mouth Saliva is produced largely by three major pairs of salivary glands That lie outside the oral cavity and discharge saliva through short ducts into the mouth Saliva is about 99.5% H2O and 0.5% electrolytes and protein The most important salivary proteins are; 1. amylase 2. mucus and 3. lysozyme The Mouth Functions of Saliva; 1. Saliva begins digestion of carbohydrate in the mouth through action of salivary amylase, an enzyme that breaks polysaccharides down into maltose 2. Saliva facilitates swallowing by moistening food particles and by providing lubrication through the presence of mucus 3. Saliva exerts some antibacterial action 4. Saliva serves as a solvent for molecules that stimulate the taste buds Only molecules in solution can react with taste bud receptors The Mouth 6. Saliva plays an important role in oral hygiene by helping keep the mouth and teeth clean 7. Saliva is rich in bicarbonate buffers, which neutralize acids in food as well as acids produced by bacteria in the mouth, thereby helping prevent dental caries The continuous basal secretion of saliva in the absence of apparent stimuli is brought about by constant low-level stimulation by the parasympathetic nerve endings that terminate in the salivary glands The Mouth Digestion in the mouth involves the hydrolysis of polysaccharides into disaccharides by amylase No absorption of foodstuff occurs from the mouth Pharynx and Esophagus The motility associated with the pharynx and esophagus is swallowing swallowing is the entire process of moving food from the mouth through the esophagus into the stomach The pressure of the bolus stimulates pharyngeal pressure receptors which send afferent impulses to the swallowing center located in the medulla The swallowing center then reflexly activates the muscles involved in swallowing Pharynx and Esophagus Food must be kept from reentering the mouth entering the nasal passages Entering the trachea All of this is managed by the following coordinated activities  Pharynx and Esophagus The position of the tongue against the hard palate keeps food from reentering the mouth during swallowing The uvula seals off the nasal passage from the pharynx so that food does not enter the nose The swallowing center briefly inhibits the nearby respiratory center  Pharynx and Esophagus The swallowing center triggers a primary peristaltic wave that sweeps from the beginning to the end of the esophagus Progression of the wave is controlled by the swallowing center, with innervation by means of the vagus As the peristaltic wave sweeps down the esophagus, the gastroesophageal sphincter relaxes so that the bolus can pass into the stomach After the bolus has entered the stomach this sphincter again contracts Pharynx and Esophagus Esophageal secretion is entirely mucus mucus is secreted throughout the length of the digestive tract by mucus-secreting gland cells in the mucosa Esophageal mucus lessens the likelihood that the esophagus will be damaged by any sharp edges in the newly entering food Esophageal mucus protects the esophageal wall from acid and enzymes in gastric juice if gastric reflux occurs The stomach A J-shaped saclike chamber lying between the esophagus and small intestine Arbitrarily divided into three sections The fundus- part of the stomach that lies above the esophageal opening The body-middle or main part of the stomach The antrum the lower part of the stomach  The stomach performs three main functions: 1. The stomach’s most important function is to store ingested food until it can be emptied into the small intestine 2. The stomach secretes hydrochloric acid (HCl) and enzymes that begin protein digestion 3. Through the stomach’s mixing movements, the ingested food is pulverized and mixed with gastric secretions to produce a thick liquid mixture known as chyme  The four basic digestive processes carried out by the stomach are; 1. Motility 2. secretion 3. digestion and 4. Absorption  Motility The four aspects of gastric motility are 1 filling 2 storage 3 mixing, and 4 emptying  food delivered to the stomach from the esophagus is stored in the relatively quiet body without being mixed The fundic area usually does not store food Food is gradually fed from the body into the antrum, where mixing does take place The strong antral peristaltic contractions mix the food with gastric secretions to produce chyme  Each antral peristaltic wave propels chyme forward toward the pyloric sphincter Usually only a few milliliters of antral contents are pushed into the duodenum with each peristaltic wave Before more chyme can be squeezed out, the peristaltic wave reaches the pyloric sphincter  Causes it to contract more forcefully Sealing off the exit and blocking further passage into the duodenum The bulk of the antral chyme that was being propelled forward but failed to be pushed into the duodenum is halted at the closed sphincter Tumbled back into the antrum, only to be propelled forward and tumbled back again as the new peristaltic wave advances This tossing back and forth thoroughly mixes the chyme in the antrum Stomach distension triggers increased gastric motility The main gastric factor that influences the strength of contraction is the amount of chyme in the stomach Stomach distension triggers increased gastric motility Through a direct Effect of stretch on the smooth muscle Involvement of the intrinsic plexuses The vagus nerve Stomach hormone gastrin FACTORS IN THE DUODENUM THAT INFLUENCE THE RATE OF GASTRIC EMPTYING The four most important duodenal factors that influence gastric emptying are 1. Fat 2. acid 3. hypertonicity and 4. distension  Fats: fat digestion and absorption take place only within the lumen of the small intestine Therefore, when fat is already in the duodenum, further gastric emptying of more fatty stomach contents into the duodenum is prevented Until the small intestine has processed the fat already there Fat is the most potent stimulus for inhibition of gastric motility  Acid: The stomach secretes hydrochloric acid (HCl) Hence highly acidic chyme is emptied into the duodenum It is neutralized by sodium bicarbonate secreted into the duodenal lumen from the pancreas Unneutralized acid irritates the duodenal mucosa and inactivates the pancreatic digestive enzymes Hence, unneutralized acid in the duodenum inhibits further emptying of acidic gastric contents until complete neutralization can be accomplished  Hypertonicity: When absorption becomes slower than digestion, large numbers of molecules remain in the chyme and increase the osmolarity of the duodenal contents Water enters the duodenum from the plasma as the duodenal osmolarity rises Leads to intestinal distension And circulatory disturbances ensue because of the reduction in plasma volume Hence, gastric emptying is reflexly inhibited when the osmolarity of the duodenal contents starts to rise  Distension: Too much chyme in the duodenum inhibits the emptying of even more gastric contents Giving the distended duodenum time to cope with the excess volume of chyme it already contains before it gets any more Gastric digestive juice is secreted by glands located at the base of gastric pits The stomach secretes about 2 liters of gastric juice daily By cells in the lining of the stomach The gastric mucosa, which is divided into two distinct areas: The oxyntic mucosa, which lines the body and fundus and The pyloric gland area (PGA), which lines the antrum  Gastric digestive juice is secreted by glands located at the base of gastric pits The luminal surface of the stomach is pitted with deep pockets Formed by infoldings of the gastric mucosa The first part of these invaginations are called gastric pits at the base of which lie the gastric glands A variety of secretory cells line these invaginations some exocrine and some endocrine or paracrine Gastric digestive juice is secreted by glands located at the base of gastric pits Three types of gastric exocrine secretory cells are found in the walls of the pits and glands in the oxyntic mucosa Gastric digestive juice is secreted by glands located at the base of gastric pits Mucous cells line the gastric pits and the entrance of the glands They secrete a thin, watery mucus Gastric digestive juice is secreted by glands located at the base of gastric pits The chief cells secrete pepsinogen Gastric digestive juice is secreted by glands located at the base of gastric pits The parietal (or oxyntic) cells secrete HCl and intrinsic factor Collectively, they make up the gastric digestive juice. Gastric digestive juice is secreted by glands located at the base of gastric pits Gastric digestive juice is secreted by glands located at the base of gastric pits  Three types of gastric exocrine secretory cells are found in the walls of the pits and glands in the oxyntic mucosa Mucous cells secrete a thin, watery mucus Chief cells secrete the enzyme precursor pepsinogen parietal (or oxyntic) cells secrete HCl and intrinsic factor (important in the absorption of vitamin B12) These exocrine secretions are all released into the gastric lumen Collectively, they make up the gastric digestive juice As a result of this HCl secretion, the pH of the luminal contents falls as low as 2 Importance of Gastric digestive juice Although HCl does not actually digest anything it performs these specific functions that aid digestion: 1. HCl activates the enzyme precursor pepsinogen to an active enzyme, pepsin, and provides an acid medium that is optimal for pepsin activity 2. It aids in the breakdown of connective tissue and muscle fibers, reducing large food particles into smaller particles 3. It denatures protein; that is, it uncoils proteins from their highly folded final form, thus exposing more of the peptide bonds for enzymatic attack 4. Along with salivary lysozyme, HCl kills most of the microorganisms ingested with food Importance of Gastric digestive juice The major digestive constituent of gastric secretion is pepsinogen Pepsinogen is stored in the chief cells’ cytoplasm HCl converts it to the active form of the enzyme; pepsin Once formed, pepsin acts on other pepsinogen molecules to produce more pepsin Pepsin initiates protein digestion by splitting certain amino acid linkages in proteins to yield peptide fragments Importance of Gastric digestive juice Mucus is protective 1: Mucus protects the gastric mucosa against mechanical injury 2: It helps protect the stomach wall from self-digestion because pepsin is inhibited when it comes in contact with the layer of mucus coating the stomach lining 3: Being alkaline, mucus helps protect against acid injury by neutralizing HCl in the vicinity of the gastric lining Importance of Gastric digestive juice Intrinsic factor is important in the absorption of vitamin B12 This vitamin can be absorbed only when in combination with intrinsic factor Binding of the intrinsic factor-vitamin B12 complex with a special receptor triggers the receptor-mediated endocytosis of the complex Vitamin B12 is essential for the normal formation of red blood cells factors which control the secretion of gastric digestive juices Other secretory cells in the gastric glands release endocrine and paracrine regulatory factors involved in the digestion of nutrients in the gastric lumen Endocrine cells known as G cells secrete the hormone gastrin into the blood Enterochromaffin-like (ECL) cells secrete the paracrine histamine D cells secrete the paracrine somatostatin These three regulatory factors from the gastric pits primarily control the secretion of gastric digestive juices  Gastrin is released into the blood in response to protein products in the stomach Gastrin stimulates the parietal and chief cells, promoting secretion of a highly acidic gastric juice Gastrin indirectly promotes HCl secretion by stimulating the ECL cells to release histamine Somatostatin It turns off the HCl-secreting cells and their most potent stimulatory pathway The gastric mucosal barrier protects the stomach lining from gastric secretions Peptic ulcer of the stomach wall The gastric mucosal barrier occasionally is broken and the gastric wall is injured by its acidic and enzymatic contents An erosion, or peptic ulcer, of the stomach wall results Excessive gastric reflux into the esophagus and dumping of excessive acidic gastric contents into the duodenum can lead to peptic ulcers in these locations as well Protein digestion begins in the antrum of the stomach Digestion by the gastric juice itself is accomplished in the antrum of the stomach where the food is thoroughly mixed with HCl and pepsin Beginning protein digestion Carbohydrate digestion continues in the body of the stomach Carbohydrate digestion in the stomach continues under the influence of salivary amylase Even though acid inactivates salivary amylase, the unmixed interior of the food mass is free of acid The stomach absorbs alcohol and aspirin but no food No food or water is absorbed into the blood through the stomach mucosa Two noteworthy non-nutrient substances are absorbed directly by the stomach— ethyl alcohol and aspirin Alcohol can enter the blood through the submucosal capillaries Pancreatic and Biliary Secretions When gastric contents are emptied into the small intestine they are mixed with: Juice secreted by the small-intestine mucosa Secretions of the exocrine pancreas Secretions of the liver that are released into the duodenal lumen  The pancreas is a mixture of exocrine and endocrine tissue The pancreas is an elongated gland that lies behind and below the stomach Contains both exocrine and endocrine tissue The exocrine part consists of acini, which connect to ducts that eventually empty into the duodenum The smaller endocrine part consists of isolated islands of endocrine tissue, the islets of Langerhans The pancreas The exocrine pancreas secretes a pancreatic juice consisting of two components: 1) pancreatic enzymes actively secreted by the acinar cells 2) an aqueous alkaline solution actively secreted by the duct cells that line the pancreatic ducts The aqueous (watery) alkaline component is rich in sodium bicarbonate (NaHCO3)  The pancreas Pancreatic enzymes are stored within zymogen granules after being produced Then are released by exocytosis as needed These pancreatic enzymes are important Because they can almost completely digest food in the absence of all other digestive secretions The pancreas The acinar cells secrete three different types of pancreatic enzymes capable of digesting all three categories of foodstuffs: 1) proteolytic enzymes for protein digestion 2) pancreatic amylase for carbohydrate digestion and 3) pancreatic lipase for fat digestion PANCREATIC PROTEOLYTIC ENZYMES The three major pancreatic proteolytic enzymes are; Trypsinogen Chymotrypsinogen and Procarboxypeptidase Each of which is secreted in an inactive form  PANCREATIC PROTEOLYTIC ENZYMES When trypsinogen is secreted into the duodenal lumen it is activated to its active enzyme form, trypsin By enterokinase an enzyme embedded in the luminal membrane of the cells that line the duodenal mucosa Trypsin then autocatalytically activates more trypsinogen Trypsinogen remains inactive inside the pancreas until it reaches the duodenal lumen where enterokinase triggers the activation process The pancreas also produces a chemical known as trypsin inhibitor Which blocks trypsin’s actions if spontaneous activation of trypsinogen inadvertently occurs within the pancreas PANCREATIC PROTEOLYTIC ENZYMES Chymotrypsinogen and procarboxypeptidase are converted by trypsin to their active forms chymotrypsin and carboxypeptidase respectively within the duodenal lumen Once enterokinase has activated some of the trypsin, trypsin then carries out the rest of the activation process Each of these proteolytic enzymes attacks different peptide linkages The end products that result from this action are a mixture of small peptide chains and amino acids Mucus secreted by the intestinal cells protects against digestion of the small-intestine wall by the activated proteolytic enzymes PANCREATIC AMYLASE Pancreatic amylase contributes to carbohydrate digestion By converting polysaccharides into the disaccharide maltose Amylase is secreted in the pancreatic juice in an active form Because active amylase does not endanger the secretory cells These cells do not contain any polysaccharides PANCREATIC LIPASE Pancreatic lipase is extremely important because it is the only enzyme secreted throughout the entire digestive system that can digest fat Pancreatic lipase hydrolyzes dietary triglycerides into monoglycerides and free fatty acids which are the absorbable units of fat lipase is secreted in its active form because there is no risk of pancreatic self- digestion by lipase PANCREATIC AQUEOUS ALKALINE SECRETION Pancreatic enzymes function best in a neutral or slightly alkaline environment This acidic chyme must be neutralized quickly in the duodenal lumen To allow optimal functioning of the pancreatic enzymes Also to prevent acid damage to the duodenal mucosa The alkaline (NaHCO3-rich) fluid secreted by the pancreatic duct cells into the duodenal lumen neutralizes the acidic chyme Pancreatic exocrine secretion is regulated by secretin and CCK The primary stimulus specifically for secretin release is acid in the duodenum Secretin is carried by the blood to the pancreas where it stimulates the duct cells to increase their secretion of a NaHCO3-rich aqueous fluid into the duodenum The alkaline pancreatic secretion that neutralizes the acid  Pancreatic exocrine secretion is regulated by secretin and CCK Cholecystokinin is important in regulating pancreatic digestive enzyme secretion The main stimulus for release of CCK from the duodenal mucosa is the presence of fat and protein products The circulatory system transports CCK to the pancreas where it stimulates the pancreatic acinar cells to increase digestive enzyme secretion Among these enzymes are lipase and the proteolytic enzymes carbohydrate does not have any direct influence on pancreatic digestive enzyme secretion The biliary system Besides pancreatic juice, the other secretory product emptied into the duodenal lumen is bile The biliary system includes the liver, the gallbladder, and associated ducts Bile salts are derivatives of cholesterol  They are actively secreted into the bile and eventually enter the duodenum Following their participation in fat digestion and absorption most bile salts are reabsorbed into the blood from the terminal ileum From here bile salts are returned by the hepatic portal system to the liver, which resecretes them into the bile The small intestine is the site where most digestion and absorption take place It is arbitrarily divided into three segments— the duodenum, the jejunum, and the ileum Motility secretion digestion, and absorption Small Intestine Motility Small-intestine motility includes; segmentation and migrating motility complex Segmentation both mixes and slowly propels the chyme Small Intestine Secretion Exocrine gland cells in the small-intestine mucosa secrete an aqueous salt and mucus solution called succus entericus (“juice of intestine”) Secretion increases after a meal in response to local stimulation of the small-intestine mucosa By the presence of chyme Small Intestine Secretion The mucus in the secretion provides protection and lubrication Furthermore, this aqueous secretion provides plenty of H2O to participate in the enzymatic digestion of food No digestive enzymes are secreted into this intestinal juice The small-intestine enzymes complete digestion within the brush-border membrane Special hairlike projections on the luminal surface of the small-intestine epithelial cells, the microvilli, form the brush border The brush-border plasma membrane contains three categories of integral proteins that function as enzymes: Small Intestine Digestion Enterokinase, which activates the pancreatic proteolytic enzyme trypsinogen The disaccharidases (maltase, sucrase, and lactase), which complete carbohydrate digestion by hydrolyzing the remaining disaccharides into their constituent monosaccharides The aminopeptidases, which hydrolyze most of the small peptide fragments into their amino acid components Thus, carbohydrate and protein digestion are completed within the confines of the brush border Large Intestine