Chapter 24

I. INTRODUCTION

A. Food contains substances and energy the body needs to construct all cell components. The food must be broken down through digestion to molecular size before it can be absorbed by the digestive system and used by the cells.

B. The organs that collectively perform these functions compose the digestive system.

II. OVERVIEW OF THE DIGESTIVE SYSTEM

A. Organization

1. The two major sections of the digestive system perform the processes required to prepare food for use in the body.

2. The gastrointestinal tract is the tube open at both ends for the transit of food during processing. The functional segments of the GI tract include the mouth, esophagus, stomach, small intestine, and large intestine.

3. The accessory structures that contribute to the food processing include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

B. Digestion includes six basic processes.

1. Ingestion is taking food into the mouth (eating).

2. Secretion is the release, by cells within the walls of the GI tract and accessory organs, of water, acid, buffers, and enzymes into the lumen of the tract.

3. Mixing and propulsion result from the alternating contraction and relaxation of the smooth muscles within the walls of the GI tract.

4. Digestion

a. Mechanical digestion consists of movements of the GI tract that aid chemical digestion.

b. Chemical digestion is a series of catabolic (hydrolysis) reactions that break down large carbohydrate, lipid, and protein food molecules into smaller molecules that are usable by body cells.

5. Absorption is the passage of end products of digestion from the GI tract into blood or lymph for distribution to cells.

6. Defecation is emptying of the rectum, eliminating indigestible substances from the GI tract.

III. LAYERS OF THE GI TRACT

A. The basic arrangement of layers in the gastrointestinal tract from the inside outward includes the mucosa, submucosa, muscularis, and serosa (visceral peritoneum).

B. The mucosa consists of an epithelium, lamina propria, and muscularis mucosa.

1. The epithelium consists of a protective layer of non-keratinized stratified cells, simple cells for secretion and absorption, and mucus secreting cells, as well as some enteroendocrine cells that put out hormones that help regulate the digestive process.

2. The lamina propria consists of three components, including loose connective tissue that adheres the epithelium to the lower layers, the system of blood and lymph vessels through which absorbed food is transported, and nerves and sensors.

a. The lymph system is part of the mucosa-associated lymph tissues (MALT) that monitor and produce an immune response to pathogens passing with food through the GI tract.

b. It is estimated that there are as many immune cells associated with the GI tract as in all the rest of the body.

3. The muscularis mucosa causes local folding of the mucosal layer to increase surface are for digestion and absorption.

C. The submucosa consists of areolar connective tissue. It is highly vascular, contains a part of the submucosal plexus (plexus of Meissner), and contains glands and lymphatic tissue.

1. The submucosal plexus is a part of the enteric nervous system and it also contains parasympathetic and sympathetic postganglionic neurons.

2. It regulates movements of the mucosa, vasoconstriction of blood vessels, and innervates secretory cells of mucosal glands.

D. Muscularis

1. The muscularis of the mouth, pharynx, and superior part of the esophagus contains skeletal muscle that produces voluntary swallowing. Skeletal muscle also forms the external anal sphincter.

2. Through the rest of the tract, the muscularis consists of smooth muscle in an inner sheet of circular fibers and an outer sheet of longitudinal fibers.

3. The muscularis also contains the major nerve supply to the GI tract - the myenteric plexus (plexus of Auerbach), which is also part of the enteric nervous system and contains postganglionic fibers from both divisions of the ANS. This plexus mostly controls GI tract motility.

E. The serosa is the superficial layer of those portions of the GI tract that are suspended in the abdominoplevic cavity.

1. The esophagus is covered by an adventitia.

2. Inferior to the diaphragm, the serosa is also called the visceral peritoneum.

IV. PERITONEUM

A. The peritoneum is the largest serous membrane of the body.

1. The parietal peritoneum lines the wall of the abdominal cavity.

2. The visceral peritoneum covers some of the organs and constitutes their serosa.

3. The potential space between the parietal and visceral portions of the peritoneum is called the peritoneal cavity and contains serous fluid.

4. Some organs, such as the kidneys and pancreas, lie on the posterior abdominal wall behind the peritoneum and are called retroperitoneal.

5. The peritoneum contains large folds that weave between the viscera, functioning to support organs and to contain blood vessels, lymphatic vessels, and nerves of the abdominal organs.

6. Extensions of the peritoneum include the mesentery, mesocolon, falciform ligament, lesser omentum, and greater omentum.

B. Peritonitis is an acute inflammation of the peritoneum.

V. MOUTH

A. Introduction

1. The mouth (oral or buccal cavity) is formed by the cheeks, hard and soft palate, lips, and tongue.

2. The vestibule of the oral cavity is bounded externally by the cheeks and lips and internally by the gums and teeth.

3. 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.

B. Salivary Glands

1. The major portion of saliva is secreted by the salivary glands, which lie outside the mouth and pour their contents into ducts that empty into the oral cavity; the remainder of saliva comes from buccal glands in the mucous membrane that lines the mouth.

2. There are three pairs of salivary glands: parotid, submandibular (submaxillary), and sublingual glands.

3. Saliva lubricates and dissolves food and starts the chemical digestion of carbohydrates. It also functions to keep the mucous membranes of the mouth and throat moist.

4. Chemically, saliva is 99.5% water and 0.5% solutes such as salts, dissolved gases, various organic substances, and enzymes.

5. Salivation is entirely under nervous control.

C. Structure and Function of theTongue

1. The tongue, together with its associated muscle, forms the floor of the oral cavity. It is composed of skeletal muscle covered with mucous membrane.

2. Extrinsic and intrinsic muscles permit the tongue to be moved to participate in food manipulation for chewing and swallowing and in speech.

3. The lingual frenulum is a fold of mucous membrane that attaches to the midline of the undersurface of the tongue.

4. The upper surface and sides of the tongue are covered with papillae. Some papillae contain taste buds.

5. On the dorsum of the tongue are glands that secrete lingual lipase, which initiates digestion of triglycerides.

D. Structure and Function of the Teeth

1. The teeth project into the mouth and are adapted for mechanical digestion.

E. Mechanical and Chemical Digestion in the Mouth

1. Through mastication (chewing), food is mixed with saliva and shaped into a bolus that is easily swallowed.

2. The enzyme salivary amylase converts polysaccharides (starches) to disaccharides (maltose). Lingual lipase is another salivary enzyme and it starts to work after food is swallowed. It starts lipid digestion.

VI. PHARYNX

A. The pharynx is a funnel-shaped tube that extends from the internal nares to the esophagus posteriorly and the larynx anteriorly.

1. It is composed of skeletal muscle and lined by mucous membrane.

2. The nasopharynx functions in respiration only, whereas the oropharynx and laryngopharynx have digestive as well as respiratory functions.

B. Deglutition, or swallowing, moves a bolus from the mouth to the stomach. It is facilitated by saliva and mucus and involves the mouth, pharynx, and esophagus.

1. Deglutition consists of a voluntary state, pharyngeal stage (involuntary), and esophageal stage (involuntary).

2. Receptors in the oropharynx stimulate the deglutition center in the medulla and the lower pons of the brain stem.

VII. ESOPHAGUS

A. The esophagus is a collapsible, muscular tube that lies behind the trachea and connects the pharynx to the stomach.

B. The wall of the esophagus contains mucosa, submucosa, and muscularis layers. The outer layer is called the adventitia rather than the serosa due to structural differences.

C. Physiology of the Esophagus

1. The esophagus contains an upper and a lower esophageal sphincter.

2. During the esophageal stage of swallowing progressive contractions of the muscularis push the bolus onward. There propulsive contractions are termed peristalsis.

VIII. STOMACH

A. Introduction

1. The stomach is a J-shaped enlargement of the GI tract that begins at the bottom of the esophagus and ends at the pyloric sphincter.

2. It serves as a mixing and holding area for food, begins the digestion of proteins, and continues the digestion of triglycerides, converting a bolus to a liquid called chyme. It can also absorb some substances.

B. Anatomy of the Stomach

1. The gross anatomical subdivisions of the stomach include the cardia, fundus, body, and pylorus.

2. When the stomach is empty, the mucosa lies in folds called rugae.

C. Histology of the Stomach

1. The surface of the mucosa is a layer of simple columnar epithelial cells and surface mucous cells.

a. Epithelial cells extend down into the lamina propria forming gastric pits and gastric glands.

b. The gastric glands consist of three types of exocrine glands: mucous neck cells (secrete mucous), chief or zymogenic cells (secrete pepsinogen and gastric lipase), and parietal cells (secrete HCl and intrinsic factor).

c. Gastric glands also contain enteroendocrine cells which are hormone producing cells. G cells secrete the hormone gastrin into the bloodstream.

2. The submucosa is composed of areolar connective tissue.

3. The muscularis has three layers of smooth muscle: longitudinal, circular, and an inner oblique layer.

4. The serosa is a part of the visceral peritoneum.

a. At the lesser curvature, the visceral peritoneum becomes the lesser omentum.

b. At the greater curvature, the visceral peritoneum becomes the greater omentum.

D. Mechanical and Chemical Digestion in the Stomach

1. Mechanical digestion consists of peristaltic movements called mixing waves.

2. Chemical Digestion

a. Chemical digestion consists mostly of the conversion of proteins into peptides by pepsin, an enzyme that is most effective in the very acidic environment (pH 2) of the stomach. The acid (HCl) is secreted by the stomach’s parietal cells. Pepsin is secreted as pepsinogen and is converted to pepsin by HCl. This prevents the pepsin from destroying the chief cells that secrete it.

b. Gastric lipase splits certain molecules in butterfat of milk into fatty acids and monoglycerides and has a limited role in the adult stomach.

3. The stomach wall is impermeable to most substances; however, some water, electrolytes, certain drugs (especially aspirin), and alcohol can be absorbed through the stomach lining.

E. Regulation of Gastric Secretion and Motility

1. Gastric secretion is regulated by nervous and hormonal mechanisms.

2. Stimulation occurs in three overlapping phases: cephalic (reflex), gastric, and intestinal.

3. Cephalic Phase

a. The cephalic phase consists of reflexes initiated by sensory receptors in the head.

b. The cephalic phase stimulates gastric secretion and motility.

3. Gastric Phase

a. The gastric phase begins when food enters the stomach.

b. When the stomach walls are distended or when pH increases because proteins have entered the stomach and buffered some of the stomach acid, the stretch receptors and chemoreceptors are activated resulting in waves of peristalsis and continual flow of gastric juice.

c. Hormonal negative feedback also regulates gastric secretions during the gastric phase.

1) Chemoreceptors and stretch receptors stimulate the ANS to release acetylcholine which stimulates the release of gastrin by G cells.

2) Gastrin stimulates growth of the gastric glands and secretion of large amounts of gastric juice. It also strengthens contraction of the lower esophageal sphincter, increases motility of the stomach, and relaxes the pyloric and ileocecal sphincters.

4. Intestinal Phase

a. The intestinal phase is due to activation of receptors in the small intestine.

b. When partially digested food enters the small intestine, it triggers the enterogastric reflex and the secretion of secretin and cholecystokinin (CCK) by the intestinal mucosa. The effect is inhibition of gastric secretion.

5. Regulation of Gastric Emptying

a. Gastric emptying is the periodic release of chyme from the stomach into the duodenum.

b. Gastric emptying is stimulated by two factors: nerve impulses in response to distention of the stomach and stomach gastrin in response to the presence of certain types of foods.

c. Most food leaves the stomach 2-6 hours after ingestion. Carbohydrates leave earliest, followed by proteins and then fats.

d. Gastric emptying is inhibited by the enterogastric reflex and by the hormones CCK and secretin.

e. Vomiting is the forcible expulsion of the contents of the upper GI tract (stomach and sometimes duodenum) through the mouth. Prolonged vomiting, especially in infants and elderly people, can be serious because the loss of gastric juice and fluids can lead to disturbances in fluid and acid-base balance.

IX. PANCREAS

A. The pancreas is divided into a head, body, and tail and is connected to the duodenum via the pancreatic duct (duct of Wirsung) and accessory duct (duct of Santorini).

B. Pancreatic islets (islets of Langerhans) secrete hormones and acini secrete a mixture of fluid and digestive enzymes called pancreatic juice.

C. Pancreatic Juice

1. Pancreatic juice contains enzymes that digest starch (pancreatic amylase), proteins (trypsin, chymotrypsin, and carboxypeptidase), fats (pancreatic lipase), and nucleic acids (ribonuclease and deoxyribonuclease). Pancreatic juice is released into the duodenum.

2. It also contains sodium bicarbonate which converts the acid stomach contents to a slightly alkaline pH (7.1-8.2), halting stomach pepsin activity and promoting activity of pancreatic enzymes.

D. Pancreatic secretion is regulated by nervous and hormonal mechanisms. Please refer to lecture notes for further detail.

X. LIVER AND GALLBLADDER

A. The liver is the heaviest gland in the body and the second largest organ in the body after the skin.

B. Anatomy of the Liver and Gallbladder

1. The liver is divisible into left and right lobes, separated by the falciform ligament. Associated with the right lobe are the caudate and quadrate lobes.

2. The gallbladder is a sac located in a depression on the posterior surface of the liver.

C. Histology of the Liver and Gallbladder

1. The lobes of the liver are made up of lobules that contain hepatic cells (liver cells or hepatocytes), sinusoids, stellate reticuloendothelial (Kupffer’s) cells, and a central vein.

2. The mucosa of the gallbladder is simple columnar epithelium arranged in rugae. There is no submucosa. The smooth muscle of the muscularis ejects bile into the cystic duct. The outer layer is the visceral peritoneum. Functions of the gallbladder are to store and concentrate bile until it is needed in the small intestine.

3. Jaundice is a yellowish coloration of the sclera, skin, and mucous membranes due to a buildup of bilirubin.

4. The liver receives a double supply of blood from the hepatic artery and the hepatic portal vein. All blood eventually leaves the liver via the hepatic vein.

D. Hepatic cells (hepatocytes) produce bile that is transported by a duct system to the gallbladder for concentration and temporary storage.

1. Bile is partially an excretory product (containing components of worn-out red blood cells) and partially a digestive secretion.

2. Bile’s contribution to digestion is the emulsification of triglycerides. Please refer to lecture notes for further detail.

F. The rate of bile secretion is regulated by nervous and hormonal mechanisms as well as by volume of hepatic blood flow and the concentration of bile salts in the blood.

G. The liver also functions in carbohydrate, lipid, and protein metabolism; removal of drugs and hormones from the blood; excretion of bilirubin; synthesis of bile salts; storage of vitamins and minerals; phagocytosis; and activation of vitamin D.

H. The fusion of individual crystals of cholesterol is the beginning of 95% of all gallstones. Gallstones can cause obstruction to the outflow of bile in any portion of the duct system. Treatment of gallstones consists of using gallstone-dissolving drugs, lithotripsy, or surgery

XII. SUMMARY OF DIGESTIVE HORMONES

A. The effects of the three major digestive hormones - gastrin, secretin, CCK, - are to regulate gastric secretion and motility, as well as secretion of the pancreas, liver, and gallbladder.

1. Gastrin promotes secretion of gastric juice and increases gastric motility.

2. Secretin promotes secretion of bicarbonate ions into pancreatic juice and bile. It inhibits secretion of gastric juice and promotes normal growth and maintenance of the pancreas. It enhances the effects of CCK. Overall, it causes buffering of acid in chime.

3. CCK stimulates secretion of pancreatic juice rich in digestive enzymes and ejection of bile into the duodenum. It also slows gastric emptying.

B. There are other hormones secreted by and having effects on the GI tract. They include motilin, substance P, bombesin, vasoactive intestinal polypeptide, gastrin-releasing peptide, and somatostatin.

XIII. SMALL INTESTINE

A. Introduction

1. The major events of digestion and absorption occur in the small intestine.

2. The small intestine extends from the pyloric sphincter to the ileocecal sphincter.

B. Anatomy of the Small Intestine

1. The small intestine is divided into the duodenum, jejunum, and ileum.

2. Projections called circular folds, or plicae circularies, are permanent ridges in the mucosa that enhance absorption by increasing surface area and causing chyme to spiral as it passes through the small intestine.

C. Histology of the Small Intestine

1. The mucosa forms fingerlike villi which increase the surface area of the epithelium available for absorption and digestion.

a. Embedded in the villus is a lacteal (lymphatic capillary) for fat absorption.

b. The cells of the mucosal epithelium include absorptive cells, goblet cells, enteroendocrine cells, and Paneth cells.

c. The free surface of the absorptive cells feature microvilli, which increase the surface area. They form the brush border which also contains several enzymes.

d. The mucosa contains many cavities lined by glandular epithelium. These cavities form the intestinal glands (crypts of Lieberkuhn).

2. The submucosa of the duodenum contains duodenal (Brunner’s) glands which secrete an alkaline mucous that helps neutralize gastric acid in chyme. The submucosa of the ileum contains aggregated lymphatic nodules (Peyer’s patches).

D. Intestinal Juice and Brush Border Enzymes

1. Intestinal juice provides a vehicle for absorption of substances from chyme as they come in contact with the villi.

2. Some intestinal enzymes (brush border enzymes) break down foods inside epithelial cells of the mucosa on the surfaces of their microvilli.

3. Some digestion also occurs in the lumen of the small intestine.

E. Mechanical Digestion in the Small Intestine

1. Segmentation, the major movement of the small intestine, is a localized contraction in areas containing food.

2. Peristalsis propels the chyme onward through the intestinal tract.

F. Brush Border Enzymes (Note: only the enzymes of the brush border of the small intestine are listed here. Remember that there are other enzymes released by the pancreas that participate in digestion in the small intestine)

1. Carbohydrates are broken down into monosaccharides for absorption.

a. Brush border enzymes for carbohydrates are alpha dextrinase, maltase, sucrase, and lactase. Please refer to lecture notes for further detail.

b. In some individuals, there is a failure of the intestinal mucosal cells to produce the enzyme lactase. This results in lactose intolerance, the inability to digest the sugar lactose found in milk and other dairy products. It may be a temporary or long-lasting condition and is characterized by diarrhea, gas, bloating, and abdominal cramps after ingestion of diary products.

2. Protein digestion

a. Brush border enzymes for protein digestion are aminopeptidase and dipeptidase. Please refer to lecture notes for further detail.

4. Nucleic acid digestion

a. Brush border enzymes for nucleic acid digestion are nucleosidases and phosphatases. Please refer to lecture notes for further detail.

G. Regulation of Intestinal Secretion and Motility

1. The most important mechanism for regulating small intestinal secretion is the action of local reflexes in response to the presence of chyme.

2. Hormones (vasoactive intestinal polypeptide or VIP) also assume a role.

3. Parasympathetic impulses increase motility; sympathetic impulses decrease motility.

H. Absorption in the Small Intestine

1. Absorption is the passage of the end products of digestion from the GI tract into blood or lymph and occurs by diffusion, facilitated diffusion, osmosis, and active transport.

2. Absorption of Monosaccharides

a. Essentially all carbohydrates are absorbed as monosaccharides.

b. They are absorbed into blood capillaries.

3. Absorption of Amino Acids, Dipeptides, and Tripeptides

a. Most proteins are absorbed as amino acids by active transport processes.

b. They are absorbed into the blood capillaries in the villus.

4. Absorption of Lipids

a. Dietary lipids are all absorbed by simple diffusion.

b. Long-chain fatty acids and monoglycerides are absorbed as part of micelles, resynthesized to triglycerides, and formed into protein-coated spherical masses called chylomicrons.

1) Chylomicrons are taken up by the lacteal of a villus.

2) From the lacteal they enter the lymphatic system and then pass into the cardiovascular system, finally reaching the liver or adipose tissue.

5. Absorption of Electrolytes

a. Many of the electrolytes absorbed by the small intestine come from gastrointestinal secretions and some are part of digested foods and liquids.

b. Active transport mechanisms are primarily used for electrolyte absorption.

6. Absorption of Vitamins

a. Fat-soluble vitamins (A, D, E, and K) are included along with ingested dietary lipids in micelles and are absorbed by simple diffusion.

b. Water-soluble vitamins (B and C) are absorbed by simple diffusion.

7. Absorption of Water

a. All water absorption in the GI tract occurs by osmosis from the lumen of the intestines through epithelial cells and into blood capillaries.

b. The absorption of water depends on the absorption of electrolytes and nutrients to maintain an osmotic balance with the blood.

c. Alcohol begins to be absorbed in the stomach. The longer alcohol remains in the stomach, the slower it is absorbed. Blood alcohol levels rise more slowly when fat rich foods are consumed with alcohol.

Please refer to lecture notes for further detail.

XIV. LARGE INTESTINE

A. Anatomy of the Large Intestine

1. The large intestine (colon) extends from the ileocecal sphincter to the anus.

2. Its subdivisions include the cecum, colon, rectum, and anal canal.

3. Hanging inferior to the cecum is the appendix.

a. Inflammation of the appendix is called appendicitis.

b. A ruptured appendix can result in gangrene or peritonitis, which can be life-threatening conditions.

4. The colon is divided into the ascending, transverse, descending, and sigmoid portions.

B. Histology of the Large Intestine

1. The mucosa of the large intestine has no villi or permanent circular folds. It does have a simple columnar epithelium with numerous goblet cells.

2. The muscularis contains specialized portions of the longitudinal muscles called taeniae coli, which contract and gather the colon into a series of pouches called haustra.

C. Mechanical movements of the large intestine include haustral churning, peristalsis, and mass peristalsis.

D. The last stages of chemical digestion occur in the large intestine through bacterial, rather than enzymatic, action. Substances are further broken down and some vitamins are synthesized by bacterial action and absorbed by the large intestine.

E. Absorption and Feces Formation in the Large Intestine

1. The large intestine absorbs water, electrolytes, and some vitamins.

2. Feces consist of water, inorganic salts, sloughed-off epithelial cells, bacteria, products of bacterial decomposition, and undigested parts of food.

3. Although most 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.

F. Defecation Reflex

1. The elimination of feces from the rectum is called defecation.

2. Defecation is a reflex action aided by voluntary contractions of the diaphragm and abdominal muscles. The external anal sphincter can be voluntarily controlled (except in infants) to allow or postpone defecation.

3. Diarrhea refers to frequent defecation of liquid feces. It is caused by increased motility of the intestine and can lead to dehydration and electrolyte imbalances.

4. Constipation refers to infrequent or difficult defecation and is caused by decreased motility of the intestines, in which feces remain in the colon for prolonged periods of time. It may be alleviated by increasing one’s intake of dietary fiber and fluids.