Chapter 23: The Digestive System

Teeth

32 adult teeth, 20 deciduous. Types include: Incisors cut food Canines rip/tear food Premolars crush/grind food Molars crush/grind food

Colon Cancer

3rd leading cause of cancer related deaths. Standard screening: Stool checks and colonoscopy.

Saliva

95.5% water with a pH of 6.8-7. Remaining 4.5%: Mucus - Helps to form the bolus IgA & lysozyme - Inhibits bacteria growth. *Salivary Amylase* - An enzyme that initiates breakdown of carbohydrates. Continues acting until it is inactivated by the stomach acids. Small amount of *Lingual Lipase* - Activated in the stomach and assists with fat breakdown.

Peritoneum

A broad serous membranous sac, made up of squamous epithelial tissue surrounded by connective tissue that lines cavity wall(parietal) and covers abdominal organs(visceral) holding them in place.

Hiatal Hernia

A condition in which part of the stomach pushes through an opening in the diaphragm. The presence increases the risk for GERD.

Circular Fold

A deep ridge in the mucosa and submucosa. *Facilitate absorption*. Their shape causes the chyme to spiral, rather than move in a straight line, through the small intestine. *Spiraling slows the movement of chyme* and provides the time needed for nutrients to be fully absorbed. Also called plica circulare.

Emulsification

A process by which large lipid globules must be broken down into small lipid globules.*Bile* is a mixture secreted by the liver to accomplish this process in the small intestine. The components most critical to emulisification are bile salts and phospholipids.

Cirrhosis of the Liver

A severe medical condition where scar tissue in the liver replaces functional tissue.

Mixing Wave

A unique type of peristalsis that mixes and softens food with gastric juices to create chyme. A form of mechanical digestion by the stomach.

Bile

A yellow-brown or yellow-green alkaline(pH 7.6-8.6) solution that is a mixture of water, bile salts, bile pigments, phospholipids(such as lecithin), electrolytes, cholesterol, and triglycerides.

Gall Bladder

Accessory digestive organ that stores and concentrates bile.

Pepsin

All of the following are components of saliva except: Pepsin IgA antibodies Lingual lipase Salivary amylase

Gastrointestinal Tract

Also called the *alimentary canal*. A one way tube about 25 ft in length(in life). Begins at the mouth and terminates at the anus. Main function of the organs of the alimentary canal is to nourish the body.

Cholecystokinin

An intestinal hormone that stimulates the gallbladder to release bile is... A. Enterokinase B. Secretin C. Cholecystokinin D. GIP E. Gastrin

Greater Omentum

Apron-like structure that lies superficial to the small intestine and transverse colon; a site of fat deposition in people who are overweight. Contains adipose and lymph nodes. Protects, insulates, and provides energy reserve. "Fatty apron"

Colon

Ascending, transverse, descending, sigmoid. The ascending and descending colon, along with the rectum are located in the retroperitoneum.

Celiac Disease

Autoimmune disease. Body attacks villi of small intestine. Can be environmentally triggered.

Hepatocytes ➨ bile canaliculi ➨ common hepatic duct ➨ bile duct(includes cystic duct from gallbladder) ➨ duodenum via hepatopancreatic sphincter

Bile Flow

The digestion of proteins and carbohydrates, *which particularly occurs in the stomach*, is completed in the small intestine with the aid of intestinal and pancreatic juices. Lipids arrive in the intestine largely undigested, and its digestion takes place here facilitated by *bile and the enzymatic lipase*. Intestinal juice combines with pancreatic juice to provide a liquid medium that facilitates absorption. The intestine is also where most water is absorbed, via osmosis. The small intestine's absorptive cells also synthesize digestive enzymes and then place them in the plasma membranes of the microvilli. This distinguishes the small intestine from the stomach; that is, enzymatic digestion occurs not only in the lumen, but also on the luminal surfaces of the mucosal cells. For optimal chemical digestion, chyme must be delivered from the stomach slowly and in small amounts. This is because chyme from the stomach is typically hypertonic, and if large quantities were forced all at once into the small intestine, the resulting osmotic water loss from the blood into the intestinal lumen would result in potentially life-threatening low blood volume. In addition, continued digestion requires an upward adjustment of the low pH of stomach chyme, along with rigorous mixing of the chyme with bile and pancreatic juices. Both processes take time, so the pumping action of the pylorus must be carefully controlled to prevent the duodenum from being overwhelmed with chyme.

Briefly describe the chemical digestion that occurs in the small intestine.

Polyps

Bumps on colon wall, precancerous.

Pancreatic Juice

Clear, composed mostly of water along with some salts, sodium bicarbonate, and several digestive enzymes. Enzymes: amylase, lipase, nuclease, trypsin(trypsinogen + enterokinase), chymotrypsin, carboxypeptidase. The bicarbonate ions help to increase the pH of chyme. Pancreatic enzymes are active in the digestion of sugars, proteins, and fats. The protein-digesting enzymes however are produced in their inactive forms and then activated in the duodenum. If produced in their active form, they would digest the pancreas(what happens in pancreatitis). *Enteropeptidase*, the intestinal brush border enzyme, stimulates the activation of *trypsin* from *trypsinogen* of the pancreas, which in turn changes the pancreatic enzymes to procarboxypeptidase and chymotrypsinogen into their active forms, *carboxypeptidase* and *chymotrypsin*.

Mucosa

Consists of, from innermost to outermost, epithelium, lamina propria, and muscularis mucosa. The epithelium is in direct contact w/ingested food. The lamina propria, is a layer of connective tissue. The muscularis mucosa is a thin, smooth muscle layer.

Submucosa

Dense connective tissue that connects the mucosa with the muscularis. Includes blood and lymphatic vessels, and some submucosal glands.

If defecation is delayed for an extended time, additional water is absorbed, making the feces firmer and potentially leading to *constipation*. Causes could be dehydration, lack of fiber, stress, and/or failure to relieve. This may lead to hemorrhoids, hernias, or diverticulitis. On the other hand, if the waste matter moves too quickly through the intestines, not enough water is absorbed, and *diarrhea* can result. This can be caused by the ingestion of foodborne pathogens. Causes could be microbes, stress, or food intolerances. This may lead to dehydration or malnutrition.

Describe Constipation and Diarrhea

Glands secrete mucus but not digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively b/c of bacteria in the lumen of the colon. *Saccharolytic fermentation* bacteria break down some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create *flatus*(gas) in the colon. *flatulence is excessive flatus*.

Describe chemical digestion in the large intestine.

Another word for swallowing--the movement of food from the mouth to the stomach. The entire process takes about 4-8 seconds for solid or semisolid food, and about 1 second for very soft food and liquids. It involves both the skeletal muscles of the tongue and the muscles of the pharynx and esophagus. There are 3 stages: The voluntary phase, the pharyngeal phase, and the esophageal phase. The ANS controls the latter two phases.

Describe deglutination.

Although the walls of the gastric pits are made up primarily of mucus cells, the gastric glands are made up of different types of cell. The glands of the cardia and pylorus are composed primarily of mucus-secreting cells. Cells that make up the pyloric antrum secrete mucus and a number of hormones, including the majority of the stimulatory hormone, *gastrin*. The much larger glands of the fundus and body of the stomach, the site of the most chemical digestion, produce most of the gastric secretions. These glands are made up of a variety of secretory cells. These include parietal cells, mucous neck cells, and endocrine cells.

Describe gastric pits and gastric glands and identify 3 types of gastric gland cells.

Includes a combination of three types of movements. The presence of food residues in the colon stimulate a slow-moving *haustral contraction*. Involving sluggish segmentation, primarily in the transverse and descending colons. When one haustrum is distended with chyme, its muscle contracts, pushing the residue into the next haustrum. These movements also mix the food residue, which helps the large intestine absorb water. The 2nd type of movement is *peristalsis*, which is slower in the large intestine. The 3rd type is *mass movement*. Strong waves start midway through the transverse colon and quickly force contents toward the rectum. Occurs 3-4 times a day. Distension of the stomach and breakdown products of digestion in the small intestine provoke *gastrocolic reflex*, which increases motility, including mass movements, in the colon. Fiber both softens the stool and increases the power of colonic contractions.

Describe mechanical digestion in the large intestine.

The entry of acidic chyme into the duodenum stimulates the release of *secretin*, which in turns increases pancreatic secretion of pancreatic juice. The presence of proteins and fasts in the duodenum stimulates the secretion of *CCK*, which then stimulates the acini to secrete enzyme-rich pancreatic juice and enhances the activity of secretin. CKK also slows down the release of chyme from the stomach.

Describe pancreatic secretion and how it is regulated by hormones and the parasympathetic nervous system.

Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called *Valsalva's maneuver*, in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis. The process of defecation begins when mass movements force feces from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated by the spinal cord. It contracts the sigmoid colon and rectum, *relaxes the internal anal sphincter, and initially contracts the external anal sphincter*. The presence of feces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed. If you decide to delay defecation, it takes a few seconds for the reflex contractions to stop and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate. *When the external anal sphincter relaxes, defecation results.*

Describe the Defecation Reflex.

The entry of food into the esophagus marks the initiation of peristalsis. As in the previous phase, the complex neuromuscular actions are controlled by the medulla oblongata. Peristalsis propels the bolus through the esophagus and toward the stomach. The circular muscle layer of the muscularis contracts, pinching the esophageal wall and forcing the bolus forward. At the same time, the longitudinal muscle layer of the muscularis also contracts, shortening this area and pushing out its walls to receive the bolus. In this way, a series of contractions keeps moving food toward the stomach. When the bolus nears the stomach, distention of the esophagus initiates a short reflex relaxation of the lower esophageal sphincter that allows the bolus to pass into the stomach. During the esophageal phase, esophageal glands secrete mucus that lubricates the bolus and minimizes friction.

Describe the Esophageal Phase of Deglutination

When most of the chyme has been absorbed, the localized segmentation process is replaced by transport movements. The duodenal mucosa secretes the hormone *motilin*, which initiates peristalsis in the form of migrating motility complex. These complexes force chyme through the small intestine one short section at a time, taking about 90 to 120 minutes to finally reach the end. At this point, the process is repeated, starting in the duodenum. The ileocecal valve, a sphincter, is usually in a constricted state, but when motility in the ileum increases, this sphincter relaxes, allowing food residue to enter the first portion of the large intestine, the cecum. Relaxation of the ileocecal sphincter is controlled by both nerves and hormones. First, digestive activity in the stomach provokes the *gastroileal reflex*, which increases the force of ileal segmentation. Second, the stomach releases the hormone gastrin, which enhances ileal motility, thus relaxing the ileocecal sphincter. After chyme passes through, backward pressure helps close the sphincter, preventing backflow into the ileum. Because of this reflex, your lunch is completely emptied from your stomach and small intestine by the time you eat your dinner. It takes about 3 to 5 hours for all chyme to leave the small intestine.

Describe the Migrating Motility Complex form of mechanical digestion in the small intestine.

Stimulation of receptors in the oropharynx sends impulses to the deglutination center(a collection of neurons that control swallowing) in the medulla oblongata. Impulses are then sent back to the uvula and soft palate, causing them to move upward and close off the nasopharynx. The laryngeal muscles also constrict to prevent aspiration of food into the trachea. At this point, deglutition apnea takes place, which means that breathing ceases for a very brief time. Contractions of the pharyngeal constrictor muscles move the bolus through the oropharynx and laryngopharynx. Relaxation of the upper esophageal sphincter then allows food to enter the esophagus.

Describe the Pharyngeal Phase of deglutination.

Consists only of surface mucus cells, which secrete a protective coat of alkaline mucus. A vast number of *gastric pits* dot the surface of the epithelium, marking the entry to each *gastric gland*, which secretes a complex digestive fluid referred to as gastric juice.

Describe the epithelial lining of the stomach and identify its function.

Food passes from the esophagus into the stomach at the lower esophageal sphincter (also called the *gastroesophageal or cardiac sphincter*). The lower esophageal sphincter relaxes to let food pass into the stomach, and then contracts to prevent stomach acids from backing up into the esophagus. Surrounding this sphincter is the muscular diaphragm, which helps close off the sphincter when no food is being swallowed. When the lower esophageal sphincter does not completely close, the stomach's contents can reflux (that is, back up into the esophagus), causing *heartburn* or *gastroesophageal reflux disease (GERD)*.

Describe the location and function of the gastroesophageal sphincter.

The mucosa between the villi is dotted with deep crevices that each lead into a tubular *intestinal gland(crypt of Lieberkühn)*, which is formed by cells that line the crevices. These produce *intestinal juice*, a *slightly alkaline(pH7.4-7.8) mixture of water and mucus*. Each day about .95 to 1.9 liters of intestinal juice are secreted in response to distension of the small intestine or the irritating effects of chyme on the intestinal mucosa. The submucosa of the duodenum is the only site of the complex mucus-secreting *duodenal glands(Brunner's glands)*, which produce a bicarbonate rich alkaline mucus that buffers the acidic chyme as it enters from the stomach. The roles of the cells in the small intestinal mucosa are detailed in the table.

Describe the structure of the small intestine mucosa.

Also known as the oral or buccal phase. So called because you can control when you swallow food. In this phase, chewing has been completed and swallowing is set in motion. The tongue moves upwards and back against the palate, pushing the bolus to the back of the oral cavity and into the oropharynx.

Describe the voluntary phase of deglutination.

Few enzyme-secreting cells are found in the wall of the large intestine, and there are *no circular folds or villi*. Other than in the anal canal, the mucosa of the colon is *simple columnar epithelium* made mostly of *enterocytes(absorptive cells) and goblet cells*. Far more intestinal glands in the walls of the large intestine which contain the enterocytes and goblet cells. The *goblet cells secrete* mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The *enterocytes absorb* water and salts as well as vitamins produced by intestinal bacteria.

Discuss how the histology of the large intestine differs from that of the small intestine and how it relates to their respective functions.

Bile Salts

Emulsifying agents that are important for the absorption of digested lipids. Reclaimed by the *enterohepatic circulation*. Once bile salts reach the ileum they are absorbed and returned to the liver in the hepatic portal blood. The hepatocytes then excrete the bile salts into newly formed bile.

Peptic Ulcers

Erosion of the digestive tract. Can affect any part of the tract. Causes: "Helicobacter pylori", bacteria(80%), stress, cancers, other disorders. May lead to bleeding or perforation, emergency situations.

*Blood from the hepatic artery and hepatic portal vein carry blood into the liver to be serviced by hepatocytes. Bile formed by hepatocytes flows in the opposite direction*. *The hepatic artery delivers oxygenated blood from the heart to the liver. The hepatic portal vein delivers partially deoxygenated blood containing nutrients absorbed from the small intestine* and actually supplies more oxygen to the liver than do the much smaller hepatic arteries. In addition to nutrients, drugs and toxins are also absorbed. After processing the bloodborne nutrients and toxins, the liver releases nutrients needed by other cells back into the blood, which drains into the central vein and then through the hepatic vein to the inferior vena cava. With this hepatic portal circulation, all blood from the alimentary canal passes through the liver. *Serviced blood leaving the liver exits the hepatic vein which connects with the inferior vena cava*.

Explain the vasculature of the liver and how it works.

Villi

Fingerlike projections, absorbs nutrients, increases surface area(There are microvilli on villi too). Brush border enzymes on microvilli: peptidase, sucrase, maltase, lactase, lipase, enterokinase. Lacteals: Structures w/in villi that take in fat/protein complexes; connects with lymphatic system. Chylomicrons: fatty/protein structures formed by intestinal cells.

Cecum

First part of the large intestine. A sac-like structure that is suspended inferior to the ileocecal valve. Receives the content of the ileum and continues the absorption of water and salts. The *appendix* is a winding tube that attaches to the posterior end of the cecum.

Acinar Cells ➨ tiny merging ducts that form two dominant ducts ➨ larger duct ➡︎ Common bile duct ➨ duodenum

Flow of pancreatic juice from site of production to duodenum.

Rugae

Folds of mucosa and submucosa that churn food into chyme.

Diverticulosis

Formation of colon pouches, weakening of wall.

Gall Stones

Formed from the crystallization of bile salts or cholesterol in the gall bladder. Blockage of cystic duct is painful. Removal of the gall bladder or other ways to break up crystals is the treatment. Be careful with fat and cholesterol intake in your diet.

Enteroendocrine Cells

Found in the gastric glands. Secrete various hormones into the interstitial fluid of the lamina propria. These include gastrin, which is released mainly by enteroendocrine *G cells*.

Small Intestine

Function: Digestion and absorption(90%) 3 sections: Duodenum(24cm), jejunum(2.5m), ileum(3.5m) *Mesentery* stabilizes/holds intestine, provides access for blood vessels. *Ileocecal Valve* sphincter muscle opens into cecum in large intestine. *Peristalsis* moves chyme distally through tube. *Segementation* moves food forward and backward to mix. *Bicarbonate ions*(from pancreas) increase pH of chyme released at duodenum.

Ingestion, secretion, movement, digestion, absorption, secretion.

Functions of the Digestive System

Carbohydrate, protein, and lipid metabolism. Detoxification of blood. Removes damaged/old blood cells. Produces bile. Stores vitamins, mineral, and glycogen. Participates in formation of vitamin D.

Functions of the Liver.

Sensory analysis of material. Mechanical digestion by teeth and tongue(lingual papillae provide taste). Chemical digestion by enzymes. Lubrication by mucus and saliva(parasymp. stimulation = thin saliva, symp. stimulation = thick saliva).

Functions of the Mouth in Digestion

Mucous Neck Cells

Gastric glands in the upper part of the stomach contain _______________________________ that secrete thin, acidic mucus that is much different from the mucus secreted by the goblet cells of the surface epithelium.

Gastric Glands

Goblet cells- secrete mucus Chief cells- secrete *pepsinogen* Parietal cells - secrete *HCl* and *intrinsic factor*

Hepatic Lobule

Hexagonal-shaped structure composed of hepatocytes that radiate outward from a central vein.

Orchestrating the breakdown of food and the assimilation of its nutrients into the body.

How are the accessory digestive organs critical to the function of the digestive system?

Both have a nonpolar (hydrophobic) region as well as a polar (hydrophilic) region. The hydrophobic region interacts with the large lipid molecules, whereas the hydrophilic region interacts with the watery chyme in the intestine. This results in the large lipid globules being pulled apart into many tiny lipid fragments of about 1 μm in diameter. This change dramatically increases the surface area available for lipid-digesting enzyme activity. This is the same way dish soap works on fats mixed with water.

How do bile salts and phospholipids work in emulsification?

Rhythmic waves of peristalsis, which begin in the upper esophagus, propel food towards the stomach. The keeps the smooth muscle constricted behind the bolus and relaxed in front of it to push it down. Mucus secretions keep it lubes. The "swallow reflex" is controlled by the medulla.

How does food move through the esophagus towards the stomach?

Regulates salivation. In the absence of food, parasympathetic stimulation keeps saliva flowing at just the right level for comfort as you speak, swallow, sleep, and generally go about life. Food contains chemicals that stimulate taste receptors on the tongue, which send impulses to the superior and inferior salivatory nuclei in the brain stem. These two nuclei then send back parasympathetic impulses through fibers in the glossopharyngeal and facial nerves, which stimulate salivation. Even after you swallow food, salivation is increased to cleanse the mouth and to water down and neutralize any irritating chemical remnants.

How does the ANS influence saliva composition?

It is smaller in diameter.

If the small intestine is so much longer than the large intestine, why is it called the small intestine?

Colitis

Inflammation of the colon.

Hepatitis

Inflammation of the liver, causes can be viral or chemical.

Pancreatitis

Inflammation of the pancreas; Blockage of duct leads to enzymes destroying the pancreas.

Microvilli

Intestinal brush border. Fixed to the surface are enzymes that finish digesting carbohydrates and proteins. Greatly expand the surface area of the plasma membrane and thus, greatly enhance absorption.

Stomach

J-shaped organ(3-6 hours) - tight junctions between epithelial cells. -epithelium is replaced every few days. *DOES NOT ABSORB NUTRIENTS. ONLY CERTAIN DRUGS AND ALCOHOL CAN BE ABSORBED BY GASTRIC GLANDS*

Liver

Largest gland in the body. All blood from digestive organs stops here first. Location: Inferior to diaphragm in the right upper quadrant of the abdominal cavity. *Structure*: Divided into two primary lobes. Large right and small left. Connected to abdominal wall and diaphragm by 5 peritoneal folds called ligaments. The *falciform ligament* and *round ligament* are remnants of the umbilical vein.

Acinus(plural = acini)

Little grape-like exocrine cell clusters, located at the terminal ends of pancreatic ducts. Secrete enzyme-rich *pancreatic juice*. It is then released into the duodenum via pancreatic duct that connects with the common bile duct.

Hepatocytes

Liver cells that secrete about 1 liter of bile each day.

Kupffer Cells

Liver macrophages.

Chief Cells

Located primarily in the basal regions of gastric glands. Secrete *pepsinogen*, the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin.

Parietal Cells

Located primarily in the middle region of the gastric glands. Produce both *hydrochloric acid(HCl)* and *intrinsic factor*.

Pancreas

Location: In the *retroperitoneum* behind the stomach. Exocrine function: Secreting digestive enzymes(chemical digestion). Has a head, a body, and a tail.

Bolus

Mass of masticated food ready to be swallowed

The movement of intestinal smooth muscles includes both segmentation and a form of peristalsis called migrating motility complexes. The kind of peristaltic mixing waves seen in the stomach are not observed here.

Mechanical Digestion in the Small Intestine.

As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your mouth, and you would be unable to avoid biting yourself as you chew, an action enabled by the motor branches of cranial nerves. Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system. These enteric neurons are grouped into two plexuses. The *myenteric plexus* (plexus of Auerbach) lies in the muscularis layer of the alimentary canal and is responsible for motility, especially the rhythm and force of the contractions of the muscularis. The *submucosal plexus* (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food.

Nerve Supply to the Digestive System

Retroperitoneal

Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as _______________________. A positional term meaning "behind the peritoneum".

Chyme

Partially digested food and fluid. Acidic. The stomach, mixes and churns food with gastric juices to form this and then release it into the duodenum.

The Esophagus

Passes through thorax and diaphragm. Connects to stomach. Conveys good during swallowing. *Lined with non-keratinized stratified squamous epithelium, and smooth and skeletal muscle in upper region. Lower region is smooth muscle*. FYI... the human esophagus is 10" long and a giraffe's is 6' long.

Lesser Omentum

Peritoneal fold that suspends stomach from the inferior border of the liver(also anchors duodenum); provides a pathway for structures connecting to the liver.

Intestinal Phase

Phase of gastric secretion has both excitatory and inhibitory elements. The duodenum has a major role in regulating the stomach and its emptying. When partially digested food fills the duodenum, intestinal mucosal cells release a hormone called *intestinal(enteric) gastrin*, which further excites gastric juice secretion. This activity is brief b/c when the intestine distends with chyme, the enterogastric reflex inhibits secretion. One of the effects of this reflex is to close the pyloric sphincter, which blocks additional chyme from entering the duodenum.

Gastric Phase

Phase of gastric secretion that lasts 3-4 hours, and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach. When food reaches the stomach, stretch receptors are activated. This stimulates parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice. Partially digested proteins, caffeine, and rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn induces parietal cells to increase their production of HCl, which is needed to create an acidic environment for the conversion of pepsinogen to pepsin, and protein digestion. Additionally, the release of gastrin activates vigorous smooth muscle contractions. However, it should be noted that the stomach does have a natural means of avoiding excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells in the stomach react by suspending HCl secretion and increasing mucous secretions.

Serosa

Present only in the region of the alimentary canal *within the abdominal cavity*. Consisted of a visceral peritoneum(simple squamous) overlying a layer of loose connective tissue(areolar). Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia.

Gastrin

Produced in the stomach mucosa mainly by G cells of the pyloric antrum. Peptides and amino acids in the stomach stimulate production. In the stomach: Increases secretion by gastric glands; promotes gastric emptying. In the small intestine: promotes intestinal muscle contraction. In the ileocecal valve: relaxes valve. In the large intestine: Triggers mass movements.

Inguinal Hernia

Protruding through a weak point or tear in the thin muscular abdominal wall. Can restrict blood supply to the bowel, herniated through the defect, creating a medical emergency. Gangrene! Causes: Adults - heavy lifting, straining bowel movements, coughing.

Gastric Bypass Surgery

Restricts appetite. Restricts stomach size significantly. Limits absorption of nutrients, fat, and calories. Allows patients to achieve rapid loss. Side effects: Nausea, weakness, sweating, faintness, and occasionally, diarrhea after eating, anemia, calcium absorption, and gallstones.

HCl(hydrochloric acid)

Secreted by parietal cells in the gastric glands. Responsible for the high acidity(pH 1.5-3.5) of the stomach contents and is needed to activate the protein-digesting enzyme pepsin. Cleaves pepsinogen to pepsin which begins the digestion of proteins into polypeptides and smaller peptides.

Intrinsic Factor

Secreted by the parietal cells of the gastric glands. A glycoprotein that is necessary for the absorption of vitamin B₁₂ in the small intestine.

Segmentation

Separates chyme and then pushes it back together, mixing it and providing time for digestion and absorption. Contents in the small intestine is shoved incrementally back and forth. Does not force chyme through tract. Instead, it combine chyme with digestive juices and pushes food particles agains the mucosa to be absorbed. Duodenum is where most rapid segmentation occurs.

Large Intestine

Terminal part of the alimentary canal. *Primary function* is to finish absorption of nutrients and water, synthesize certain vitamins and electrolytes, forms semi-solid feces, store and eliminate feces from the body. *Structure* runs from the appendix to the anus. More than twice the diameter of the small intestine. Suspended inside cavity by peritoneum but rectum is retroperitoneal. *Sections*: Cecum, colon, rectum, and anus. *Ileocecal valve* located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine.

*Submandibular Glands*- In the floor of the mouth, secrete saliva into the mouth through submandibular ducts. *Sublingual Glands*- Lie below the tongue, use the lesser sublingual ducts to secrete saliva into the oral cavity. *Parotid Glands* - Lie between the skin and the masseter muscle, near the ears. Secrete saliva into the mouth through the parotid duct.

The 3 Salivary Glands and Their Functions.

Muscularis

The muscularis in the small intestine is made up of a double layer of smooth muscle; an inner circular layer and an outer longitidinal layer. The contractions of these layers promote mechanical digestion, expose more food to the digestive chemicals, and move the food along the canal.

Cephalic Phase

The reflex phases of gastric secretion. Relatively brief. Takes place before food even enters the stomach. The smell, taste, sight, or thought of food triggers this phase. Impulses from receptors in your taste buds or nose are relayed to the brain, which returns signals that increase gastric secretion to prepare the stomach for digestion.

Chemical Digestion

Type of digestion that utilizes enzymes used to break down nutrients. e.g. salviary glands, stomach, pancreas, small intestine.

Mechanical Digestion

Type of digestion that utilizes muscular action or movement to tear, grind, and mix to smaller pieces. e.g. teeth, stomach, small intestine.

Mesentry

Vertical band of tissue anterior to the lumbar vertebrae anchoring all of the small intestine except the duodenum. Prevents tangling.

Of particular interest is the hepatopancreatic ampulla (ampulla of Vater). Located in the duodenal wall, the ampulla marks the transition from the anterior portion of the alimentary canal to the mid-region, and is where the bile duct (through which bile passes from the liver) and the main pancreatic duct (through which pancreatic juice passes from the pancreas) join. This ampulla opens into the duodenum at a tiny volcano-shaped structure called the major duodenal papilla. The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic juice from the ampulla into the duodenum.

What are some features and functions of the duodenum?

Mechanical and Chemical

What are the 2 types of digestion?

Hepatocytes, bile canaliculi, and hepatic sinusoids.

What are the 3 main components of the liver?

Cephalic, gastric, intestinal; Yes.

What are the 3 phases of gastric secretion? Can they occur simultaneously?

Circular folds, villi, and microvilli(most abundant in the proximal 2/3 of the small intestine, where the majority of absorption occurs).

What are the 3 structural features that maximize the surface area of the small intestine?

Mucosa, submucosa, muscularis, and serosa(which is continuous with the messentry).

What are the 4 layers of the GI Tract from the lumen moving outwards?

Greater omentum, lesser omentum, mesentery, mesocolon, falciform ligament.

What are the 5 major peritoneal folds?

Teeth, tongue, salivary glands, gallbladder, liver, and pancreas.

What are the accessory digestive organs?

Mucosa, submucosa, muscularis externa, and serosa.

What are the four layers of the stomach wall from inside-out?

Cardia, fundus, body, pylorus.

What are the four regions os the stomach?

Parietal lines the wall. Visceral envelopes various abdominal organs, holding them to the dorsal surface of the body wall. A few ml of water fluid act as a lubricant to minimize friction between the two layers.

What are the functions of the two peritoneal layers?

Pharynx, esophagus, stomach, and small and large intestines.

What are the organs that make up the Gastrointestinal Tract(Alimentary Canal)?

In addition to the typical circular and longitudinal smooth muscle layers, the muscularis has an inner oblique smooth muscle layer. As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles.

What is one of the differences between the muscularis externa of the stomach and the muscularis externa of the rest of the alimentary canal?

Cholecystokinin(CCK) and Gastric Inhibitory Peptide(GIP). These slow gastric motility and secretions. Secretin inhibits gastric secretions. Carbs pass through the quickest and lipids take the longest due to CCK release.

What is secreted by the duodenum once it receives chyme?

Pharynx; Nasopharynx, oropharynx, laryngopharynx.

What is the common passageway for solid food, liquids, and air? What are the three regions?

In the mouth, pharynx, esophagus, and anal canal = non-keratinized, stratified squamous epithelium In the stomach and intestines, it is simple columnar epithelium. The epithelium is interspersed w/goblet cells, which secrete mucus, and enteroendocrine cells, which secret hormones.

What is the composition of the epithelium layer of the mucosa?

Loose connective tissue(Areolar), also numerous blood and lymphatic vessels to transport nutrients. Houses the MALT.

What is the composition of the lamina propria layer of the mucosa?

Beta cell releases insulin to lower blood sugar. Alpha cell releases glucagon to raise blood sugar. These cells are found in the Islet of Langerhans of the pancreas.

What is the endocrine function of the pancreas?

Oropharynx & laryngopharyx; nasopharynx.

What regions of the pharynx are used for breathing and digestion? What region is strictly for breathing?

Portal Triad

Where the branch of the bile duct, hepatic portal vein, and hepatic artery meet on the liver.

Contains enzymes that begin the breakdown of proteins.

Which of the following is NOT true of saliva: A. Moistens food and aids in compacting the bolus. B. Dissolves food chemicals so they can be tasted. C. Contains enzymes that begin the breakdown of proteins. D. Cleanses the mouth. E. Contains chemicals that help repel pathogens.

Periodontal Ligament

Which tooth structure forms the support for the gomphosis? A. Crown B. Root C. Cementum D. Dentin E. Periodontal ligament

It is important for both the production of mature red blood cells and normal neurological functioning; Intrinsic factor.

Why is the intestinal absorption of B₁₂ important? What makes this possible?