Revised 4/15/08
INTRODUCTION:
DIGESTION is the breakdown of large food molecules into smaller molecules that the body cells can absorb and use.
There are 6 essential digestive processes. Read & learn about them in your textbook on page 765.
1. Ingestion
2. Propulsion of food: swallowing & peristalsis
3. Secretion of water, acid, buffers, and digestive enzymes.
4. Mechanical and chemical (enzymatic) digestion
5. Absorption of nutrients.
6. Defecation of wastes.
ORGANIZATION OF DIGESTIVE SYSTEM ORGANS: FIG. 25.1
The gastrointestinal (GI) tract (alimentary canal) is about 9 meters (30 feet) long in a cadaver and consists of the MOUTH, PHARYNX, ESOPHAGUS, STOMACH, SMALL INTESTINE, & LARGE INTESTINE.
The accessory structures aid the GI tract in the mechanical & chemical digestion of food. The accessory structures include the TEETH, TONGUE, SALIVARY GLANDS, LIVER, GALL BLADDER, & PANCREAS.
THE MOUTH AND ASSOCIATED ORGANS: FIG. 25.4
The ORAL CAVITY (FIG. 25.4) is lined with MUCOUS MEMBRANE. The walls of the mouth have to withstand abrasion, so the mucous membrane consists of NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM (Table 3.1, p. 65).
The ORBICULARIS ORIS (FIG. 11.4, pp. 299-300) muscle forms the bulk of the lips. The lips are attached to the gum by superior and inferior folds of mucous membrane called the LABIAL FRENULUM (FIG. 25.4).
The cheeks form the lateral walls of the oral cavity and contain skeletal muscles such as the BUCCINATOR (FIG. 11.6, p. 306) & MASSETER (FIG. 11.4, p. 300). The lips & cheeks assist with speech & help keep food between the teeth when we chew.
The PALATE forms the roof of the mouth. The HARD PALATE (FIG. 25.4) contains the MAXILLAE & PALATINE bones (FIG. 7.7, p. 174). It serves as a rigid surface against which the tongue can force food during chewing & swallowing. The posterior SOFT PALATE (FIG. 25.4) dips inferiorly at its free edge to form the UVULA. The FAUCES (opening to the throat) is also located here.
The TONGUE (FIG. 25.4) is composed of skeletal muscle covered by mucous membrane. It is anchored at its inferior base to the HYOID bone (FIG. 7.4, page 170). The muscles of the tongue allow the tongue to change shape and to move from side to side and in and out. These movements are involved in speech, maneuvering food for chewing, forming the food into a BOLUS (ball of food), and forcing food to the back of the mouth for swallowing. The tongue is secured to the floor of the mouth by the LINGUAL FRENULUM (FIG. 25.4). The superior and lateral tongue surface is covered with bumps called PAPILLAE. Some of the papillae contain the taste buds, which contain the sensory organs of taste (FIG. 22.2, page 675).
The SALIVARY GLANDS (FIG. 25.5) secrete saliva, a fluid that contains mucus and salivary amylase, a digestive enzyme that breaks down starch into sugar. Saliva (1) keeps the mucous membranes moist between meals, (2) dissolves food chemicals so that they can be tasted by taste buds, (3) moistens food and aids in its compaction into a bolus, & (4) begins the chemical breakdown of starchy foods. Most of the saliva is secreted by the major salivary glands:
1. PAROTID GLANDS: inferior and anterior to the ears.
2. SUBMANDIBULAR GLANDS: inferior to the tongue in the posterior floor of the mouth. You can see the opening of the ducts of the submandibular glands in FIG. 25.4, under the tongue.
3. SUBLINGUAL GLANDS: inferior to the tongue, superior & anterior to the submandibulars.
The TEETH (FIG. 25.4 & 25.7) are embedded in the sockets of the ALVEOLAR PROCESSES of the MAXILLAE and MANDIBLE. The alveolar processes are covered by the GINGIVAE (gums). The teeth tear and grind the food, breaking it down mechanically into smaller pieces (chewing or mastication). The teeth are classified according to shape and function.
1. INCISORS: chisel-shaped; adapted for cutting food.
2. CANINES (CUSPIDS or EYETEETH): adapted for tearing or shredding food.
3. PREMOLARS (BICUSPIDS) and MOLARS have broad crowns with rounded cusps; used for grinding and crushing.
You should be able to identify the teeth in an adult mouth.
Swallowing is the mechanism that moves food from the mouth to the stomach. The tongue pushes on the hard palate, pushing the bolus of food from the ORAL CAVITY through the FAUCES to the OROPHARYNX. The SOFT PALATE & UVULA rise to close off the NASOPHARYNX, so that food doesn't go up into your nose when you swallow. Food moves from the PHARYNX into the ESOPHAGUS as the LARYNX is closed off by the EPIGLOTTIS. The longitudinal and circular smooth muscle layers propel food through the esophagus by alternating waves of contractions (peristalsis). See FIG. 25.9.
THE ESOPHAGUS: FIGS. 25.1, 25.9 & 25.10
The esophagus is a muscular, collapsible tube, 25 cm (10 in) long and posterior to the trachea (FIG. 24.2, p. 734). It passes through the mediastinum (FIG. 1.7, page 13) and then pierces the diaphragm through the opening called the ESOPHAGEAL HIATUS (FIG. 11.13. page 323 shows the esophague going through the diaphragm). Then the esophagus joins the stomach. Mucus secreted by glands in the esophagus prevents friction when food passes through the esophagus during peristalsis. The smooth muscle sphincter called the GASTROESOPHAGEAL (LOWER ESOPHAGEAL or CARDIAC) SPHINCTER (FIG. 25.10) is found where the esophagus enters the stomach. It relaxes during peristalsis, allowing the bolus of food to enter the stomach.
HISTOLOGY OF THE GI TRACT
There are 4 layers in the GI tract: MUCOSA, SUBMUCOSA, MUSCULARIS, and SEROSA. See FIG. 25.2.
The MUCOSA (MUCOUS MEMBRANE) is a moist epithelial tissue membrane that lines the LUMEN of the GI tract from the mouth to the anus. The epithelial layer of the mucous membrane of the MOUTH, PHARYNX, ESOPHAGUS, & ANAL CANAL is composed of NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM (Table 3.1, p. 65). The epithelium of the mucous membrane of the STOMACH, SMALL INTESTINE & LARGE INTESTINE consists of NON-CILIATED SIMPLE COLUMNAR EPITHELIUM (Table 3.1, p. 64) rich in mucus-secreting GOBLET CELLS. The slippery mucus protects the organs from being digested by digestive enzymes and eases the passage of foods along the GI tract. The mucosa of these organs is also involved in secretion of digestive enzymes and absorption of nutrients.
The LAMINA PROPRIA is the connective tissue layer of the MUCOUS MEMBRANE and it consists of AREOLAR CONNECTIVE TISSUE. The lamina propria contains blood capillaries & lymph capillaries. It also contains lymphatic nodules, which are important in the defense against pathogens from the gut.
The MUSCULARIS MUCOSAE consists of circular and longitudinal smooth muscle fibers that throw the lining of the stomach and small intestine into folds that increase the surface area for absorption.
The SUBMUCOSA is a DENSE IRREGULAR CONNECTIVE TISSUE layer that binds the mucosa to the third layer, the muscularis. The submucosa contains a rich supply of blood and lymphatic vessels, as well as digestive glands and a nerve supply that controls digestive gland secretions.
The MUSCULARIS of the MOUTH and PHARYNX is SKELETAL MUSCLE. The muscularis of the ESOPHAGUS, STOMACH, & SMALL & LARGE INTESTINES is SMOOTH MUSCLE. The smooth muscle is arranged as an INNER CIRCULAR LAYER and an OUTER LONGITUDINAL LAYER. The smooth muscle muscularis is important in peristalsis, the alternating waves of contractions and relaxation that move food along the GI tract. The muscularis also contains a nerve supply that controls these smooth muscle contractions.
The SEROSA (SEROUS MEMBRANE) is the protective outermost layer of the stomach, small intestine and large intestine. The serous membrane is also called the VISCERAL PERITONEUM.
PERITONEUM (serous membrane): (FIG. 25.2, 25.3)
The PERITONEUM is the SEROUS MEMBRANE in the ABDOMINOPELVIC CAVITY. The VISCERAL PERITONEUM makes up the serosa that covers the external surfaces of most digestive organs in the abdominopelvic cavity. The PARIETAL PERITONEUM is the serous membrane that lines the walls of the abdominopelvic cavity. Organs that lie posterior to the parietal peritoneum are described as retroperitoneal.
The PERITONEAL CAVITY contains pale yellow serous fluid which lubricates the peritoneum. Peritonitis is an infection of the peritoneum, which can be due to external wounds (stabbing or gunshot) or rupturing of the GI tract, as in a ruptured appendix.
The parietal peritoneum is continuous with the visceral peritoneum by way of double-layered extensions, called MESENTERY (peritoneal folds), that suspend the digestive organs within the peritoneal cavity. Mesentery contains blood vessels, lymphatic vessels and nerves that serve the digestive organs. Know the GREATER OMENTUM & LESSER OMENTUM (FIG. 25.3) and FALCIFORM LIGAMENT (FIG. 25.3 & 25.12).
THE STOMACH (FIGS. 25.1 & 25.10)
The STOMACH is a J-shaped organ, about 25 cm (10 inches) long, just under the diaphragm in the EPIGASTRIC, LEFT HYPOCHONDRIAC & UMBILICAL REGIONS (FIG. 1.9, page 16) of the abdominopelvic cavity.
The stomach serves as a temporary "storage tank" for food and an important site for mechanical breakdown of food and the chemical breakdown of proteins.
GROSS ANATOMY OF THE STOMACH: (FIG. 25.10) Know the regions of the stomach:
1. CARDIA
2. FUNDUS (the rounded top of the stomach)
3. BODY
4. PYLORUS
The smooth muscle sphincter at the junction of the esophagus and stomach is the GASTROESOPHAGEAL (LOWER ESOPHAGEAL) SPHINCTER. The sphincter at the junction of the stomach and small intestine is the PYLORIC SPHINCTER.
When the stomach is empty the MUCOSA (stomach lining) forms large longitudinal folds called RUGAE. Rugae allow the stomach to stretch to comfortably hold 1.5 liters of food, and when really distended, the stomach can hold as much as 4 liters.
HISTOLOGY OF THE STOMACH: (FIG. 25.11)
The epithelium that lines the stomach MUCOSA is composed entirely of nonciliated SIMPLE COLUMNAR EPITHELIUM (Table 3.1, page 64).
Located in the MUCOSA are GASTRIC GLANDS, which open into the stomach through the GASTRIC PITS. The gastric glands produce GASTRIC JUICE: The GASTRIC GLANDS consist of the following cell types:
1. CHIEF CELLS in the gastric glands secrete pepsinogen. Pepsinogen is an inactive digestive enzyme & is converted to its active form, pepsin, under the acidic conditions of the stomach. Pepsin digests proteins into amino acids.
2. PARIETAL CELLS in the gastric glands secrete hydrochloric acid (HCl), which makes the stomach contents very acidic (pH 1.5-3.5). This acidic pH helps destroy bacteria in food; it is also necessary to convert pepsinogen to pepsin (see #1 above).
3. MUCOUS CELLS in the gastric glands secrete a sticky, alkaline mucus, which clings to the stomach wall and shields it from the damaging effects of HCl.
4. PARIETAL CELLS also secrete intrinsic factor, which is necessary for absorption of vitamin B12 by the small intestine. Vitamin B12 is necessary for red blood cell production in the red bone marrow. Failure to absorb B12 results in pernicious anemia.
The stomach mucosa does not absorb many nutrients. It does absorb some water, electrolytes, aspirin, and alcohol.
The stomach MUSCULARIS consists of 3 layers of smooth muscle that churn the food: CIRCULAR, LONGITUDINAL and OBLIQUE.. Once the food is broken down physically and mixed with gastric juices, it has been converted to a creamy, semifluid mass called chyme.
Food moves from the stomach into the small intestine through a ring of circular smooth muscle called the PYLORIC SPHINCTER. As the food churns in the stomach, small amounts of chyme squirt through the sphincter into the small intestine. It takes about 2-6 hours for the stomach to completely empty.
SMALL INTESTINE (FIGS. 25.1 & 25.16)
The SMALL INTESTINE is a convoluted tube extending from the pyloric sphincter of the stomach to the large intestine. The longest section of the GI tract, with an average length of 6.4 m (21 ft) in a cadaver and an average length of about 3 m (10 ft) in a living person (due to tonic contractions of longitudinal smooth muscle). Its diameter is 2.5 cm (1 in).
The small intestine is divided into 3 segments: (FIG. 25.16)
1. The DUODENUM is the first section of the small intestine. It is retroperitoneal and about 25 cm (10 in) long. Most digestion and absorption of nutrients takes place in the duodenum. DUODENAL GLANDS in the SUBMUCOSA of the duodenum produce alkaline mucus that helps neutralize the acidic chyme that comes in from the stomach.
2. JEJUNUM: about 1 m (3 ft) long.
3. ILEUM: about 2 m (6 ft) in length and joins the large intestine at the ILEOCECAL SPHINCTER. (It is this region of the small intestine that contains the Peyer's Patches; see Lymphatic System notes).
The completion of enzymatic digestion takes place in the small intestine. About 90% of all nutrient absorption takes place in the small intestine. The anatomy of the small intestine makes digestion and absorption more efficient. For example, the small intestine is the longest portion of the GI tract; this gives a larger surface area for nutrient absorption. Also, the PLICAE CIRCULARES (circular folds) (FIG. 25.16) increase the surface area exposed to the food. The chyme spirals down the plicae circulares, exposing more nutrients to the intestinal wall, where absorption of nutrients takes place.
HISTOLOGY OF THE SMALL INTESTINE: (FIG. 25.17)
The MUCOSA is made up of SIMPLE COLUMNAR EPITHELIUM (Table 3.1, page 64) with scattered mucus-secreting GOBLET CELLS. The simple columnar epithelial cells possess MICROVILLI (Table 3.1, page 64; FIGS. 25.17 (b) & 25.18). Microvilli are microscopic projections of the plasma membrane, located on the apical (exposed) surface of the mucosal epithelial cells. The microvilli of the epithelial cells of the small intestine are called the BRUSH BORDER because they have a fuzzy appearance in high magnification images. The plasma membrane of the brush border cells contains several digestive enzymes called brush border enzymes. The brush border enzymes digest nutrients at the cell surface and the cell then absorbs the nutrients (see Functions, pages 792-793).
The MUCOSA also contains simple tubular glands called INTESTINAL GLANDS that secrete intestinal juice, a clear yellow fluid of water & mucus with a slightly alkaline pH of 7.6.
Notice how the mucosa of the small intestine (FIGS. 25.17 & 25.18) forms fingerlike projections. These are called VILLI. Villi are approximately 1 mm high, giving the inside lining of the small intestine a velvety appearance. The large number of villi (20-40 villi per square millimeter) increases the surface area for nutrient absorption.
The areolar connective tissue layer of the mucosa is called the LAMINA PROPRIA. The lamina propria contains a dense blood capillary bed, which receives absorbed nutrients from the small intestine. The lamina propria also contains modified lymphatic capillaries called LACTEALS. The lacteals receive the absorbed fats from the small intestine. The fats are carried in the lymph to larger lymphatic vessels, and are ultimately dumped into the bloodstream by the THORACIC DUCT. Where does the lymph go from there?
The MUSCULARIS of the small intestine consists of a circular and longitudinal layer of smooth muscle. These two layers contract and relax in waves, moving the chyme through the small intestine by peristalsis.
The indigestible & unabsorbable material in the chyme enters the large intestine through the ILEOCECAL SPHINCTER (VALVE).
THE PANCREAS (FIG. 25.12)
The PANCREAS is the principal digestive enzyme-producing organ of the digestive system. It is 12-15 cm (5-6 inches) long and 2.5 cm (1 in) thick. Located behind the stomach, the pancreas is also retroperitoneal.
The pancreas is both an endocrine & exocrine gland. The pancreatic endocrine cells that secrete hormones that control blood sugar levels. The pancreatic exocrine cells secrete about 1200-1500 ml of pancreatic juice into ducts each day. Pancreatic juice is an alkaline fluid that contains many digestive enzymes (see Functions, page 784). The pancreatic juice leaves the pancreas in the PANCREATIC DUCT & ACCESSORY DUCT (FIG. 25.12). The pancreatic duct fuses with the COMMON BILE DUCT to form the HEPATOPANCREATIC AMPULLA. The hepatopancreatic ampulla and accessory duct empty into the duodenum of the small intestine.
THE LIVER (FIGS. 25.12 & 25.14)
The liver is the largest gland in the body, weighing about 1.4 kg (3 lbs) in the average adult. The liver is located under the diaphragm, in the RIGHT HYPOCHONDRIAC and EPIGASTRIC regions (FIG. 1.9, p. 16), and it occupies more of the right side of the abdomen than the left. The liver consists of 2 principal lobes, the larger right lobe and the smaller left lobe. The FALCIFORM LIGAMENT is a MESENTERY that separates the right and left lobes and suspends the liver from the diaphragm. The liver is almost completely enclosed by VISCERAL PERITONEUM.
Review the HEPATIC PORTAL SYSTEM from the cardiovascular system section (page 500).
FUNCTIONS OF THE LIVER (see pages 787-788) include the following:
1. The liver is an exocrine gland that secretes bile, a yellow-to-green fluid containing bile salts, bile pigments & cholesterol. The bile is secreted by HEPATOCYTES into the BILE CANALICULI (FIG. 25.14). The bile canaliculi empty into ducts. The bile eventually leaves the liver in the COMMON HEPATIC DUCT (FIG. 25.12). The common hepatic duct merges with the CYSTIC DUCT of the gall bladder to form the COMMON BILE DUCT. The common bile duct and the pancreatic duct merge to form the HEPATOPANCREATIC AMPULLA, which empties the bile into the duodenum of the small intestine.
Bile's digestive function is fat emulsification. In other words, bile breaks up fats and oils into small particles which are easier to digest with enzymes. Bilirubin, a product of hemoglobin breakdown, is excreted in bile and is one of the bile pigments that gives bile its green color. When bilirubin is broken down in the large intestine by bacteria, the by-product gives the waste material (feces) its brown color.
2. The liver converts the simple sugar glucose into glycogen (a complex carbohydrate similar to plant starch), and stores glycogen for later use. When blood sugar levels are low, the liver breaks down stored glycogen into glucose and releases glucose into the bloodstream. This keeps blood sugar levels constant so that body cells always have a steady supply of glucose.
3. The liver produces cholesterol and uses it to make bile salts. Cholesterol is also a component of the cell's plasma membrane & is used to make steroid hormones.
4. Storage of fats.
5. The liver stores vitamins A, B12, C, D, E, and K, as well as iron and copper.
6. The liver converts amino acids into glucose sugar for energy. Ammonia (NH3) is produced as a by-product.
7. Detoxification: The liver converts toxic ammonia to a less toxic waste product, urea. The liver also detoxifies antibiotics such as penicillin, ampicillin and erythromycin and excretes them in the bile. The liver detoxifies other drugs as well, such as alcohol and amphetamines.
8. The liver produces plasma proteins, such as fibrinogen & albumin, that are found in the blood.
9. The KUPFFER'S CELLS that reside in the sinusoids of the liver phagocytize bacteria and worn-out formed elements in the blood (FIG. 25.14).
THE GALLBLADDER (FIGS. 25.12 & 25.13)
The GALLBLADDER is a thin-walled, green sac, approximately 7-10 cm (3-4 in) long. It sits in a fossa on the posterior and inferior surface of the right liver lobe (FIG. 25.13). The wall of the gallbladder contains smooth muscle.
The gallbladder's function is to store bile. Although the liver continuously secretes bile, a sphincter of the hepatopancreatic ampulla guards the entry of bile into the duodenum of the small intestine. The sphincter is closed when bile is not needed for digestion. The bile backs up the CYSTIC DUCT into the gallbladder, where it is stored. Water is reabsorbed from the bile, making the bile more concentrated. During digestion, the smooth muscle in the gallbladder wall contracts and pushes bile into the cystic duct. The hepatopancreatic ampulla sphincter relaxes, releasing the bile into the duodenum.
THE LARGE INTESTINE: FIGS. 25.1 & 25.19
The LARGE INTESTINE is about 1.5 m (5 ft) long & 6.5 cm (2.5 in) in diameter.
The large intestine is divided into 3 STRUCTURAL REGIONS: THE CECUM, COLON AND RECTUM: (FIG. 25.19)
1. The CECUM is a dead-end pouch inferior to the ILEOCECAL SPHINCTER (VALVE). Hanging from its inferior surface is the VERMIFORM APPENDIX. The appendix contains lymphatic tissue and is a MALT structure (see your lymphatic system notes).
2. The cecum merges with the COLON, which has 4 distinct regions: ASCENDING COLON, TRANSVERSE COLON, DESCENDING COLON, & SIGMOID COLON.
3. RECTUM: last 20 cm (8 in) of the GI tract. The last 2-3 cm (1 inch) of the rectum is the ANAL CANAL, which opens to the exterior at the ANUS. The mucosa of the anal canal is composed of NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM, which resists abrasion. Anal sinuses produce mucus, which aids in emptying the anal canal. The INTERNAL ANAL SPHINCTER is composed of circular smooth muscle and the EXTERNAL ANAL SPHINCTER is voluntary skeletal muscle.
After 3-10 hours in the large intestine, chyme has become solid or semisolid as a result of water absorption. The remaining residue is the feces, which contains water, undigested food residues, mucus, sloughed-off dead epithelial cells from the mucosa of the GI tract, and millions of bacteria.
HISTOLOGY OF THE LARGE INTESTINE (FIG. 25.20)
The large intestinal MUCOSA is NON-CILIATED SIMPLE COLUMNAR EPITHELIUM (except for the anal canal), but there are no villi like there are in the small intestine. The columnar cells function primarily in water absorption. Goblet cells are present in the mucosa and the mucus helps ease the passage of feces (solid waste) to the end of the GI tract. Lymphatic nodules are found in the lamina propria of the mucosa.
The mucosa of the large intestine produces no digestive enzymes. Bacteria in the lumen of the large intestine digest any remaining carbohydrates and amino acids left in the chyme, releasing hydrogen, carbon dioxide, methane, and hydrogen sulfide gases into the lumen (yes -that's where that gas comes from!). The bacteria in the lumen of the large intestine are beneficial because they make vitamin K and some B vitamins.
The mucosa of the large intestine absorbs the vitamins synthesized by intestinal bacteria, some electrolytes (salts) and most of the remaining water in the chyme. So, the large intestine is important in maintaining water and electrolyte balance in the body.
The longitudinal smooth muscle layer of the large intestine's MUSCULARIS is thickened into three bands called the TAENIAE COLI. Since the taeniae coli maintain tonic contractions, they cause the intestinal wall to pucker into small pocketlike pouches, called HAUSTRA (FIG. 25.19).