DIGESTIVE SYSTEM

The process of digestion allows foods to be broken down into molecules that are small enough to enter the body cells.  The passage of these smaller molecules through the plasma membranes of cells lining the stomach and intestines and then into the blood and lymph is called absorption.

Two groups of organs compose the digestive system.  The GI (gastrointestinal) tract/alimentary canal is composed of the following organs: the mouth, most of the pharynx, esophagus, stomach, small intestine and the large intestine.  The accessory digestive organs are the teeth, tongue, salivary glands, liver, gallbladder, and the pancreas.

Overall, the digestive system performs six basic processes:

1.      Ingestion (eating)

2.      Secretion (water, acid, buffers, and enzymes)

3.      Mixing and propulsion (motility)

4.      Digestion (mechanical and chemical processes {hydrolysis} break down ingested food into small molecules)

5.      Absorption (entrance of the products of digestion into the epithelial cells lining the lumen of the GI tract, and then pass into blood or lymph and circulate to cells throughout the body)

6.      Defecation (feces leave the body through the anus)

Layers of the GI tract:

The four layers from deep to superficial are the mucosa, submucosa, muscularis and serosa.

1.      Mucosa: the inner lining of the GI tract.  Simple columnar epithelium functions in secretion and absorption, lines the stomach and intestines, and are sealed together by tight junctions.  The lamina propria contains many blood and lymphatic vessels.  The MALT are prominent lymphatic nodules that help to protect against disease.  The muscularis mucosae increases the surface area of the stomach and small intestine by forming folds.

2.      Subucosa: binds the mucosa to the muscularis.  The submucosal plexus/plexus of Meissner is the “brain of the gut” and the neurons are part of the enteric nervous system (and mostly responsible of the GI secretions).

3.      Muscularis: Smooth muscle contractions break down and move food, mixes and propels it along the tract.  The myenteric plexus/plexus of Auerbach, is the second enteric nervous system plexus and controls the frequency and strength of contraction of the muscularis.

4.      Serosa: The superficial layer of those portions of the GI tract that are suspended in the abdominopelvic cavity.  Composed of simple squamous epithelium and areolar connective tissue.

Peritoneum

The peritoneum is the largest serous membrane of the body.  The peritoneum is divided into the parietal peritoneum, which lines the wall of the abdominopelvic cavity, and the visceral peritoneum, which covers some of the organs in the cavity and is their serosa.  The slim space between the parietal and visceral portions of the peritoneum is called the peritoneal cavity, which contains serous fluid.  In certain diseases, the peritoneal cavity may become distended by the accumulation of several liters of fluid, a condition called ascites.

The kidneys and pancreas are retroperitoneal.  The peritoneum contains large folds that weave between the viscera.  The folds bind the organs to each other and to the walls of the abdominal cavity.  The greater omentum is the largest peritoneal fold, drapes over the transverse colon and coils of the small intestine like a “fatty apron”.  The greater omentum normally contains a considerable amount of adipose tissue (beer belly).  The many lymph nodes of the greater omentum contribute macrophages and antibody-producing plasma cells that help combat and contain infections of the GI tract.  The falciform ligament attaches the liver to the anterior abdominal wall and diaphragm.  The lesser omentum suspends the stomach and duodenum from the liver.  Another fold of the peritoneum, called the mesentery, is fan-shaped and binds the small intestine to the posterior abdominal wall.  The mesocolon binds the large intestine to the posterior abdominal wall.  The mesentery and mesocolon hold the intestines loosely in place.

Mouth

The mouth/oral cavity/buccal cavity is surrounded by the lips/labia.  The inner surface of each lip is attached to its corresponding gum by a midline fold of mucous membrane called the labial frenulum.  The vestibule of the oral cavity is a space bounded externally by the cheeks and lips and internally by the gums and teeth.  The oral cavity proper is a space that extends from the gums and teeth to the fauces, the opening between the oral cavity and the pharynx or throat.  The hard palate is the anterior portion of the roof of the mouth.  The soft palate forms the posterior portion of the roof of the mouth.

The uvula hands from the free border of the soft palate.  During swallowing, the soft palate and uvula are drawn superiorly, closing off the nasopharynx and preventing swallowed foods and liquids from entering the nasal cavity.  The palatine tonsils are situated between the arches, and the lingual tonsils are situated at the base of the tongue.  The mouth opens into the oropharynx through the fauces.

Salivary Glands

A salivary gland is any cell or organ that releases a secretion called saliva into the oral cavity.  The major salivary glands are the three pairs of the parotid, submandibular and sublingual glands.  Chemically, saliva is 99.5% water and 0.5% solutes.  Salivary amylase is a digestive enzyme that acts on starch.  The water in saliva provides a medium for dissolving foods so that they can be tasted and digestive reactions can begin.  Saliva is only slightly acidic with a pH of 6.35-6.85.  Mucus lubricates the food so it can easily be moved about in the mouth, formed into a ball, and swallowed.

Secretion of saliva (salivation) is controlled by the autonomic nervous system.  Parasympathetic stimulation promotes continuous secretion of a moderated amount of saliva.  Sympathetic stimulation results in dryness of the mouth.

Mumps is an inflammation and enlargement of the parotid glands.

Tongue

The tongue is an accessory digestive organ that forms the floor of the oral cavity.  The lingual frenulum attaches to the floor of the mouth and aids in limiting the movement of the tongue posteriorly.

Teeth

The teeth are also called the dentes.  A typical tooth has three major regions.  The crown is the visible portion above the level of the gums.  Embedded in the socket are one to three roots.  The neck is the constricted junction oft the crown and root near the gum line.  The teeth are composed primarily of dentin, a calcified connective tissue that gives the tooth its basic shape and rigidity.  The pulp contains blood vessels, nerves, and lymphatic vessels.  Narrow extensions of the pulp cavity, called root canals, run through the root of the tooth.  The dentin of the crown is covered by enamel which is the hardest substance in the body and the richest in calcium salts., protects the tooth from the wear and tear of chewing.  The dentin of the root is covered by cementum, another bone-like substance, which attaches the root to the periodontal ligament.  There are 20 teeth in a complete deciduous set and 32 teeth in a complete permanent set.

Mechanical and Chemical Digestion in the Mouth

Mechanical digestion in the mouth results from chewing, or mastication, in which food is manipulated by the tongue, ground by the teeth, and mixed with saliva.  As a result, the food is reduced to a soft, flexible, easily swallowed mass called a bolus.  Salivary amylase initiates the breakdown of starch.  Most of the dietary carbohydrates we eat are starches, but can only be absorbed into the bloodstream as broken down monosaccharides.

Pharynx

When food is first swallowed, it passes from the mouth into the pharynx, a funnel-shaped tube that extends from the external nares to the esophagus posteriorly and the larynx anteriorly.  Swallowed food passes from the mouth into the oropharynx and laryngopharynx, the muscular contractions of which help propel food into the esophagus and then into the stomach and then to the pyloric valve.  The movement of food from the mouth into the stomach is achieved by the act of swallowing, or deglutititon (the degulutition center is located in the medulla oblongata and the returning impulses cause the soft palate and uvula to move upward to close off the nasopharynx).  Swallowing occurs in three stages:

1.      The voluntary stage in which the bolus is passed into the oropharynx

2.      The pharyngeal stage which is the involuntary passage of the bolus through the pharynx into the esophagus

3.      The esophageal stage which is the involuntary passage of the bolus through the esophagus into the stomach

Esophagus

The esophagus is a collapsible muscular tube that lies posterior to the trachea.  It is about ten inches long.  The esophagus begins at the laryngopharynx and pierces the diaphragm through an opening called the esophageal hiatus, and ends in the superior portion of the stomach.  The esophagus secretes mucus and transports food into the stomach. It does not produce digestive enzymes, and does not carry on absorption.  The elevation of the larynx during the pharyngeal stage of swallowing causes the upper esophageal sphincter to relax, and the bolus enters the esophagus.  During the esophageal stage of swallowing, peristalsis, a progression of coordinated contractions and relaxations of the circular and longitudinal layers o f the muscularis, pushes the food bolus onward.  Peristalsis occurs in other tubular structures, including other parts of the GI tract and the ureters, bile ducts, and uterine tubes; in the esophagus it is controlled by the medulla oblongata.  The passage of solid or semisolid food from the mouth to the stomach takes 4-8 seconds.  Very soft foods and liquids pass through in about one second.  The lower esophageal sphincter relaxes during swallowing and thus allows the bolus to pass from the esophagus into the stomach.

Stomach

The stomach is typically a J-shaped enlargement of the GI tract directly inferior to the diaphragm in the epigastric, umbilical, and left hypochondriac regions of the abdomen.  The stomach connects the esophagus to the duodenum, the first part of the small intestine.  The main function of the stomach is to serve as a mixing chamber and holding reservoir.  Empty, it is about the size of a large sausage.  In the stomach, digestion of starch continues, digestion of proteins and triglycerides begins, the semisolid bolus is converted to a liquid, and certain substances are absorbed.

The stomach has four main regions: the cardia, fundus, body, and pylorus.   The cardia surrounds the superior opening of the stomach.  The rounded portion superior to and to the left of the cardia is the fundus.  Inferior to the fundus is the large central portion of the stomach, called the body.  The region of the stomach that connects to the duodenum is the pylorus.  It has two parts, the pyloric antrum which connects to the body of the stomach, and the pyloric canal, which leads into the doudenum.  When the stomach is empty, the mucosa lies in large folds, called rugae, that can be seen with the unaided eye.  The pylorus communicates with the duodenum of the small intestine via a sphincter called the pyloric sphincter.  The concave medial border of the stomach is called the lesser curvature, and the convex lateral border is called the greater curvature.

The mucose is a layer of simple columnar epithelial cells.  Epithelial cells extend down into the lamina propria, where they form columns of secretory cells called gastric glands that line many narrow channels called gastric pits.  Secretions from several gastric glands flow into each gastric pit and then into the lumen of the stomach.  The gastric glands contain three types of exocrine gland cells that secrete their products into the stomach lumen: mucous neck cells, chief cells, and parietal cells.  Both surface mucous cells and mucous neck cells secrete mucus.  Parietal cells produce intrinsic factor, which is needed for absorption of vitamin B12.  The chief cells secrete pepsinogen and gastric lipase.  The secretions of the mucous, parietal, and chief cells from gastric juice.  Gastric glands include a type of entroendocrne cell, the G cell, which is located mainly in the pyloric antrum and secretes the hormone gastrin into the bloodstream.  The muscularis has three (rather than two) layers of smooth muscle: an outer longitudinal layer, a middle circular layer, and an inner oblique layer.

Several minutes after food enters the stomach, gentle, rippling, peristaltic movements called mixing waves pass over the stomach every 15-25 seconds.  This reduces it to a soupy liquid called chyme.  As food reaches the pylorus, each mixing wave forces several milliliters of chyme into the duodenum through the pyloric sphincter.  Most of the cyhme is forced back into the body of the stomach, where mixing continues.  The next wave pushes the chyme forward again and forces a little more into the duodenum.  These forward and backward movements of the gastric contents are responsible for most mixing in the stomach.  Even though parietal cells secrete hydrogen ions (H+) and chloride ions (Cl-) separately into the stomach lumen, the net effect is secretion of hydrochloric acid (HCl).  Generation of bicarbonate ions (HCO3-) builds up in the cytosol and exits the parietal cell in exchange for Cl- via Cl-/HCO3- antiporters in the basolateral membrane next to the lamina propria.  Bicarbonate ion diffuses into nearby blood capillaries.  This is the “alkaline tide”.

The strongly acidic fluid of the stomach kills many microbes in food, and HCl partially denatures (unfolds) proteins in food and stimulates the secretion of hormones that promote the flow of bile and pancreatic juice.  Enzymatic digestion of proteins also begins in the stomach.  The only proteolytic (protein-digesting) enzyme in the stomach is pepsin, which is secreted by chief cells.  Pepsin is most effective in the very acidic environment of the stomach (pH2), and becomes inactive at higher pH.

Regulation of Gastric Secretion and Motility

Both neural and hormonal mechanisms control the secretion of gastric juice and the contraction of smooth muscle in the stomach wall.  Events in gastric digestion occur in three overlapping phases: the cephalic, gastric, and intestinal phases.

The cephalic phase of gastric digestion consists of reflexes initiated by sensory receptors in the head.  Even before food enters the stomach, the sight, smell, taste, or thought of food initiates this reflex.  The cerebral cortex and the feeding center in the hypothalamus send nerve impulses to the medulla oblongata.  The medulla then transmits impulses to parasympathetic preganglionic neurons in the vagus nerves (CN X) which stimulate parasympathetic postganglionic neurons in the submucosal plexus.  Impulses from parasympathetic neurons also increase stomach motility.  Emotions such s anger, fear, and anxiety may slow digestion in the stomach because they stimulate the sympathetic nervous system, which inhibits gastric activity.

In the gastric phase of digestion, the sensory receptors in the stomach initiate both neural and hormonal mechanisms to ensure that gastric secretion and motility continue.  Food of any kind distends (stretches) the stomach and stimulates stretch receptors in its walls.  Chemoreceptors in the stomach monitor the pH of the stomach chyme.  Nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from parietal cells, chief cells, and mucous cells.  Hormonal negative feedback also regulates gastric secretion during the gastric phase.  Note that gastrin increases motility of the stomach.

The intestinal phase of gastric digestion is due to activation of receptors in the small intestine.  Whereas reflexes initiated during the cephalic and gastric phases stimulate stomach secretory activity and motility, those occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the stomach and prevent overloading of the duodenum with more chyme than it can handle.  When chyme containing fatty acids and glucose leaves the stomach and enters the small intestine, it triggers enteroendocrine cells in the small intestinal mucosa to release into the blood two hormones that affect the stomach: secretin and cholecystokinin/CCK.  Secretin mainly decreases gastric secretions, whereas CCK mainly inhibits stomach emptying.

Regulation of Gastric Emptying

The periodic release of chyme from the stomach into the duodenum is regulated by both neural and hormonal reflexes.  Stimuli such as distention of the stomach and the presence of partially digested proteins, alcohol, and caffeine initiate gastric emptying.  These stimuli increase the secretion of gastrin and generate parasympathetic impulses in the vagus nerves.  Gastrin and nerve impulses stimulate contraction of the lower esophageal sphincter, increase motility of the stomach, and relax the pyloric sphincter.  The net effect of these actions is gastric emptying.

Neural and hormonal reflexes also help ensure that the stomach does not release more chyme than the small intestine can process.  The neural reflex (enterogastric reflex) and the hormone CCK inhibit gastric emptying.  Stimuli such as distention of the duodenum and the presence of fatty acids, glucose, and partially digested proteins in the duodenal chyme inhibit gastric emptying.  These stimuli then initiate the enterogastric reflex.  Nerve impulses propagate from the duodenum to the medulla oblongata, where they inhibit parasympathetic stimulation and stimulate sympathetic activity in the stomach.  The same stimuli also increase secretion of CCK.  Increased sympathetic impulses and CCK both decrease gastric motility.  The net effect of these actions is inhibition of gastric emptying.  Within two to four hours after eating a meal, the stomach has emptied its contents into the duodenum.

Pancreas

From the stomach, chyme passes into the small intestine.  Because chemical digestion in the small intestine depends on activities of the pancreas, liver, and gallbladder, we first consider the activities of these accessory digestive organs and their contributions to digestion in the small intestine.  The pancreas is a retroperitoneal gland that is about 5-6 inches long and consists of a head, a body, and a tail and is connected to the duodenum usually by two ducts.  The pancreas is made up of small clusters of glandular epithelial cells, about 99% of which are arranged in clusters called acini and constitute the exocrine portion of the organ.  The cells within acini secrete a mixture of fluid and digestive enzymes called pancreatic juice.  The remaining 1% of the cells are organized into clusters called pancreatic islets (islets of Langerhans), the endocrine portion of the pancreas.  These cells secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide.  Sodium bicarbonate makes pancreatic juice slightly alkaline (pH 7.1-8.2).  The enzymes in pancreatic juice include pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase and elastase.  The principal triglyceride-digesting enzyme in adults is called pancreatic lipase.

Liver and Gallbladder

The liver is the heaviest gland of the body, weighing about three pounds in an average adult, and after the skin it is the second-largest organ of the body.  It is inferior to the diaphragm and occupies most of the right hypochondriac and part of the epigastric regions of the abdominopelvic cavity.  The gallbladder is a pear-shaped sac that is located in a depression of the posterior surface of the liver.  It is 3-4 inches long and typically hangs from the anterior inferior margin of the liver.

The liver is divided into two principal lobes, the large right lobe and a smaller left lobe, by the falciform ligament.  On the free border of the faciform ligament is the ligamentum teres (round ligament) which is a fibrous cord that is a remnant of the umbilical vein of the fetus. (and extends from the liver to the umbilicus).  Inside of the liver sinusoids are fixed phagocytes called Kupffer cells, which destroy worn-out white blood and red blood cells, bacteria, and other foreign matter in venous blood draining from the gastrointestinal tract.  Following the ducts, we find that the common hepatic duct joins the cystic duct from the gallbladder to form the common bile duct.  The functions of the gallbladder are to store and concentrate bile (up to tenfold) until it is needed in the small intestine.  In the concentration process, water and ions are absorbed by the gallbladder mucosa.

Branches of both the hepatic artery and the hepatic portal vein carry blood into liver sinusoids, where oxygen, most of the nutrients, and certain toxic substances are taken up by the hepatocytes.  Because blood from the gastrointestinal tract passes through the liver as part of the hepatic portal circulation, the liver is often a site for metastasis of cancer that originates in the GI tract.

Bile, a yellow, brownish, or olive-green liquid has a pH of 7.6-8.6 and consists mostly of water and bile acids, bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions.  Bile salts, which are sodium salts and potassium salts of bile acids, play a role in emulsification, the breakdown of large lipid globules into a suspension of droplets about 1 micrometer in diameter, and in the absorption of lipids following their digestion.  The phagocytosis of aged red blood cells liberates iron, globin, and bilirubin (derived from heme).  The iron and globin are recycled, and some of the bilirubin is converted to conjugated bilirubin.  Conjugated bilirubin is then secreted into the bile and is eventually broken down in the intestine.  One of its breakdown products (stercobilin) gives feces their normal brown color.  In the regulation of bile secretion, acidic chyme entering the duodenum stimulates other enteroendocrine cells to secrete the hormone secretin into the blood, and CCK causes contraction of the wall of the gallbladder, which squeezes stored bile out of the gallbladder.

Functions of the Liver

Besides secreting bile, which is needed for absorption of dietary fats, the liver performs many other vital functions. 

·        Carbohydrate metabolism: the liver is especially important in maintaining a normal blood glucose level. 

·        Lipid metabolism

·        Protein metabolism: hepatocytes deaminate (remove the amino group, NH2, from) amino acids so that the amino acids can be used for ATP production or converted to carbohydrates or fats.  The resulting toxic ammonia (NH3) is then converted into the much less toxic urea, which is excreted in urine.

·        Processing of drugs and hormones

·        Excretion of bilirubin

·        Synthesis of bile salts

·        Storage of glycogen, vitamins A, B12, D, E, and K, and minerals iron and copper.

·        Phagocytosis (Kuppfer cells)

·        Activation of vitamin D.

If bile contains either insufficient bile salts or lecithin or excessive cholesterol, the cholesterol may crystallize to form gallstones.

Small Intestine

The major events of digestion and absorption occur in a long tube called the small intestine.  Its length alone provides a large surface area for digestion and absorption, and that area is further increased by circular folds, villi, and microvilli.  The small intestine begins at the pyloric sphincter of the stomach, coils through the central and inferior part of the abdominal cavity, and eventually opens into the large intestine.  It averages one inch in diameter and its length is about ten feet in a living persona and bout 21 feet in a cadaver due to the loss of smooth muscle tone after death.  The small intestine is divided into three regions.  The duodenum (meaning 12 finger length) is retroperitoneal and the shortest region.  It starts at the pyloric sphincter of the stomach and about ten inches later merges with the jejunum.  The jejunum (meaning empty at death) extends about three feet to the ileum.  The ileum (twisted) is about six feet long and joins the large intestine at the ileocecal sphincter.  Projections called circular folds enhance absorption by increasing surface area and causing the chyme to spiral, rather than move in a straight line, as it passes through the small intestine. 

Special features of both the mucosa and the submucosa facilitate the process of digestion and absorption.  The mucosa forms a series of finger-like villi, projections that vastly increase the surface area of the epithelium available for absorption and digestion and gives the intestinal mucosa a velvety appearance.  Each villus has a core of lamina propria.  Embedded in this connective tissue are an arteriole, a venule, a blood capillary network, and a lacteal, which is a lymphatic capillary.  Nutrients absorbed by the epithelial cells covering the villus pass through the wall of a capillary or a lacteal to enter blood or lymph, respectively.

The epithelium of the mucosa consists of simple columnar epithelium that contains absorptive cells, goblet cells, enteroendocrine cells, and Paneth cells.  Microvilli form a fuzzy line, called the brush border, extending into the lumen of the small intestine, and greatly increase the surface area (on absorptive cells which digest and absorbs nutrients).  Goblet cells secrete mucus.  Groups of lymphatic nodules referred to as aggregated lymphatic follicles (Peyer’s patches) are also present in the ileum.  Intestinal juice is slightly alkaline (pH 7.6).  Together, pancreatic and intestinal juices provide a liquid medium that aids the absorption of substances from chyme as they come in contact with the microvilli.

The two types of movements of the small intestine (segmentatons and a type of peristalsis called migrating motility complexes) are governed mainly by the myenteric plexus.  Segmentations are localized, mixing contractions that occur in portions of intestine distended by a large volume of chyme.    They mix chyme with the digestive juices, they do not push the intestinal contents along the tract.  The chyme sloshes back and forth.  This movement is similar to alternately squeezing the middle and then the ends of a capped tube of toothpaste.  After most of a meal has been absorbed, which lessens distention of the wall of the small intestine, segmentation stops and peristalsis begins.  Altogether, chyme remains in the small intestine for 3-5 hours.  The completion of the digestion of carbohydrates, proteins, and lipids is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine.

Ingested molecules of sucrose, lactose, and maltose (three disaccharides) are not acted on until they reach the small intestine.  Three brush-border enzymes digest the disaccharides into monosaccharides.  Sucrase breaks sucrose into a molecule of glucose and a molecule of fructose.  Lactase digest lactose into a molecule of glucose and a molecule of galactose.  Maltase splits maltose and maltotriose into two or three molecules of glucose, respectively.  Digestion of carbohydrates ends with the productions of monosaccharides, as mechanisms exist for their absorption.  Protein digestion starts in the stomach, where proteins are fragmented into peptides by the action of pepsin.  Enzymes in pancreatic juice (trypsin, chymotrypsin, carboxypeptidase, and elastase) continue to break down proteins into peptides.  Trypsin, chymotrypsin and elastase all cleave the peptide bond between a specific amino acid and its neighbor.  Carboxypeptidase breaks the peptide bond that attaches the terminal amino acid to the carboxy (acid) end of the peptide.  The most abundant lipids in the diet are triglycerides, which consist of a molecule of glycerol bonded to three fatty acid molecules.  Enzymes that split triglycerides and phospholipids are called lipases.

The most important mechanisms that regulate small intestinal secretion and motility are enteric reflexes that respond to the presence of chyme: vasoactive intestinal polypeptide (VIP) also stimulates the production of intestinal juice.  Segmentation movements depend mainly on intestinal distention, which initiates nerve impulses to the enteric plexuses and the central nervous system.  Enteric reflexes and returning parasympathetic impulses from the CNS increase motility while sympathetic impulses decrease intestinal motility.  Migrating motility complexes strengthen when most nutrients and water have been absorbed, that is, when the walls of the small intestine are less distended.  With more vigorous peristalsis, the chyme moves along toward the large intestine as fast a 10 cm/sec.  The first remnants of a meal reach the beginning of the large intestine in about four hours.

All the chemical and mechanical phases of digestion from the mouth through the small intestine are directed toward changing food into forms that can pass through the epithelial cells lining the mucosa and into the underlying blood and lymphatic vessels.  These forms are monosaccharides (glucose, fructose, and galactose) from carbohydrates; single amino acids, dipeptides, and tripeptides from proteins; and fatty acids, glycerol, and monoglycerides from triglycerides.  Passage of these digested nutrients from the GI tract into the blood or lymph is called absorption.  Monosaccharides pass from the lumen through the apical membrane via facilitated diffusion or active transport.  Glucose and galactose are transported into epithelial cells of the villi via secondary active transport that is coupled to the active transport of sodium.

Within the epithelial cells, many monoglycerides are further digested by lipase to glycerol and fatty acids.  The fatty acids and glycerol are then recombined to form triglycerides, which aggregate into globules along with phospholipids and cholesterol and become coated with proteins.  These large spherical masses are called chylomicrons.  Chylomicrons leave the epithelial cell via exocytosis.  Because they are so large and bulky, chlyomicrons cannot enter blood capillaries in the small intestine.  Instead, they enter the much leakier lacteals.  From there they are transported by way of lymphatic vessels to the thoracic duct and enter the blood at the left subclavian vein.  The fat-soluble vitamins A, D,E, and K are included with ingested dietary lipids in micelles and are absorbed via simple diffusion.  Vitamins B and C are water-soluble and also absorbed via simple diffusion.  Vitamin B12, however, combines with intrinsic factor produced by the stomach, and the combination is absorbed in the ileum via an active transport mechanism.

Large Intestine

The large intestine is the terminal portion of the GI tract and is divided into four principal regions.  The overall functions of the large intestine are the completion of absorption, the production of certain vitamins, the formation of feces, and the expulsion of feces from the body.  The large intestine, which is about five feet long and 2.5 inches in diameter, extends from the ileum to the anus.  It is attached to the posterior abdominal wall by its mesocolon.  Structurally, the four major regions of the large intestine are the cecum, colon, rectum, and anal canal.  The opening from the ileum into the large intestine is guarded by a fold of mucous membrane called the ileocecal sphincter (valve), which allows materials from the small intestine to pass into the large intestine.  Hanging inferior to the ileocecal valve is the cecum.  Attached to the cecum is a twisted, coiled tube, measuring about three inches in length, called the appendix or vermiform appendix. 

The open end of the cecum merges with a long tube called the colon, which is divided into ascending, transverse, descending, and sigmoid portions.  Ascending colon->right (hepatic) flexure-> transverse colon-> splenic flexure->descending colon->sigmoid colon.  The rectum, the last 8 inches of the GI tract, lies anterior to the sacrum and coccyx.  The terminal one inch of the rectum is called the anal canal.

The large intestine has no villi or permanent circular folds in the mucosa.  The epithelium contains mostly absorptive and goblet cells.  The absorptive cells function primarily in water absorption, wheras the goblet cells secrete mucus that lubricates the passage of the colonic contents.  Tonic contractions of the bands gather the colon into a series of pouches called haustra which give the colon a puckered appearance.

The passage of chyme from the ileum into the cecum is regulated by the action of the ileocecal sphincter.  One movement characteristic of the large intestine is haustral churning.  In this process, the haustra remain relaxed and become distended while they fill up.  When the distension reaches a certain point, the walls contract and squeeze the contents into the next haustrum.  Peristalsis also occurs, although at slower rate (3-12 contractions per minute) than in more proximal portions of the tract.  A final type of movement is mass peristalsis, a strong peristaltic wave that begins at about the middle of the transverse colon and quickly drives the contents of the colon to the rectum.  Because food in the stomach initiates this gastrocolic reflex in the colon, mass peristalsis usually takes place three or four times a day, during or immediately after a meal.

The final stage of digestion occurs in the colon through the activity of bacteria that inhabit the lumen.  By the time chyme has remained in the large intestine 3-10 hours, it has become solid or semisolid because of water absorption and is now called feces.  Although 90% of all water absorption occurs in the small intestine, the large intestine absorbs enough to make it an important organ in maintaining the body’s water balance.  The large intestine also absorbs ions, including sodium and chloride, and some vitamins.

Mass peristaltic movements push fecal material from the sigmoid colon into the rectum.  The resulting distention of the rectal wall stimulates stretch receptors, which initiates a defecation reflex that empties the rectum.  The internal anal sphincter responds to parasympathetic stimulation and is involuntary.  The external anal sphincter is voluntarily controlled.  In infants, the defecation reflex (response to distention of the rectal wall by sensory nerve impulses to the sacral spinal cord) causes automatic emptying of the rectum because voluntary control of the external anal sphincter has not yet developed.

Review the Homeostatic Imbalances listed at the end of the chapter.

DIGESTION REVIEW QUESTIONS

1.      What is another name for the alimentary canal?  What type of chemical reaction is catalyzed by the digestive enzymes in the digestive juices of the alimentary canal?

2.      What are the organs of the GI tract?  What are the accessory organs of the digestive system?

3.      What attaches the small intestine to the posterior abdominal wall?

4.      What are the salivary glands?  What are the functions of salivary secretions?

5.      What is another name for salivary enzyme amylase?  What does it function to digest?

6.      What is a bolus?  How is it formed?  What form of digestion is mastication?

7.      What closes off the nasal cavity during swallowing?

8.      What is the correct order of food movement through the tubing?  Two answers.

9.      What is peristalsis?  When does it occur? 

10.  What is different about the muscularis of the stomach versus most organs of the GI tract?   In areas of the GI tract specialized for absorption of nutrients, what is the type of epithelium seen in the mucosa?

11.  What are rugae?  Where is the pyloric sphincter located?

12.  What secretes intrinsic factor?  What is it needed for?

13.  What is gastrin and where is it produced?

14.  What is the purpose of bicarbonate ions diffusing into blood capillaries of the stomach after a meal? 

15.  What is the pH of gastric juice?  What is the cephalic phase of gastric digestion and what occurs? 

16.  What stimulates gastric emptying?  What is CCK and what does it do? 

17.  What is the primary digestive function of the stomach?

18.  How long after consumption is partially digested food usually passed from the stomach to the small intestine?

19.  What does increased activity of the sympathetic nervous system do to the GI system?  And the parasympathetic nervous system?

20.  What is produced by the acini of the pancreas?  And the islets of Langerhans?

21.  What are the functions of the gallbladder?  The function of bile?  What union forms the common bile duct?  What gives bile its greenish color?  What makes fecal material brown in color? 

22.  What controls the emptying of bile from the gallbladder?   What are gallstones usually made of?

23.  What would happen if the hepatocytes failed to function?  Why does the liver produce urea?

24.  What is the major stimulus for secretion of secretin?

25.  Where does most absorption of nutrients occur?

26.  What is produced by the hydrolytic reactions catalyzed by trypsin and chymotrypsin?  And carboxypeptidase?

27.  Specific disaccharides are hydrolyzed by enzymes found in what liquid?  What is the intestinal enzyme that functions to digest fat?

28.  How do monosaccharides enter the capillaries of the villi from epithelial cells?  How is glucose transported into epithelial cells of the villi?

29.  What is the appendix attached to?

30.  What does the large intestine absorb the most of?  The primary chemical digestion in the large intestine results from the action of what enzymes?

Blood Heart Immune Labs Metabolism Reproduction Respiratory Urinary Vessels Digestion