Some other names for the digestive tract are the gastrointestinal (GI) tract and the alimentary canal. The study of the digestive tract is called gastroenterology. Mechanical digestion involves physically breaking the food into small bits, as in chewing in the mouth or “mixing and mashing” in the stomach. Chewing food well will facilitate chemical digestion. Chemical digestion breaks food into simple substances, such as glucose, that can be absorbed; chemical digestion breaks chemical bonds.
The digestive tract is a hollow tube extending from the mouth to the anus Accessory organs of digestionsuch as salivary glands, the liver, gallbladder, and pancreas empty their secretions into the tube at various points (This is discussed in detail on later slides.)
The digestive tract has four layers. Within the muscle layer, the muscles are arranged to produce both mixing and a longitudinal peristaltic movement. The nerves are the enteral nervous system (parasympathetic) and innervate all the muscles for mixing and peristalsis. The glands that secrete the most important digestive enzymes and mucus into the digestive tract are located in the mucosa and submucosa.
Peristalsis, an alternating rhythmic contraction and relaxation of the muscles, moves food from the mouth to the anus. Squeezing the end of a toothpaste tube is an analogy for peristalsis moving food bit by bit through the tract. There is not a single wave of peristalsis; that wouldn’t leave time for digestion. In the esophagus, peristalsis moves food from the proximal to the distal end. A new wave begins at the proximal end of the stomach to move food to the duodenum. Within the duodenum, peristalsis is more of a swaying action to maximize absorption.
The individual tooth has a crown, neck, and root. The tooth is embedded within the bone and gingiva. Patients on chemotherapy are more prone to a painful gingivitis. The three salivary glands produce one liter of saliva per day to facilitate swallowing; they are exocrine glands so ducts carry their secretions to the oral cavity. Occasionally, stones lodge in these ducts; this is called sialolithiasis and causes extreme pain on eating and salivation. Mumps is a swelling of the parotid gland; mumps causes a chipmunk-like appearance because of the location of the parotid gland in front of the ear. In young boys, viral mumps can lead to sterility.
When food passes through the pharynx, it is directed into the esophagus and not the trachea, primarily because the epiglottis closes the entrance to the trachea. Have students put their fingers on the upper throat and swallow. They should detect upward movement. Why is it dangerous to administer fluids orally to a comatose or semicomatose person? The swallowing mechanism is impaired so the epiglottis may not move up to close the entrance to the trachea. The patient may aspirate.
Ask students to predict problems that a patient might experience after a gastrectomy. The major problem is regulation of gastric emptying, not digestion or absorption. In the discussion, it will be important to point out “dumping syndrome.” The banding and stapling procedures for weight loss are intended to make a patient feel full sooner and curb appetite. Gastric secretion and motility are regulated by the vagus nerve, a part of the enteral nervous system.
The lower esophageal sphincter (LES) prevents reflux from the stomach into the esophagus. Why does impaired function of the LES cause heartburn or pyrosis? The contents of the stomach are highly acidic and burn the distal esophagus. The pyloric sphincter relaxes to allow chyme to be ejected from the distal stomach (pylorus) into the duodenum. The empty stomach is the size of a sausage. The rugae, accordion-like pleats, allow the stomach to accommodate a large meal. The greater and lesser curvatures are used as landmarks in gastroscopyfor example, locating a lesion on the lesser curvature.
The muscular layers of the stomach are well developed, enabling the stomach to produce chyme. Under hormonal and nervous control, muscle contractions deliver chyme to the duodenum.
Part C of the illustration shows the exocrine glands of the stomach, which help form gastric juice. Because of the presence of HCl, gastric juice is highly acidic, with a pH of 1 to 4. The most important digestive enzyme, secreted by the chief cells, is pepsin. The parietal cells produce both HCl and intrinsic factor, so a person with pernicious anemia has a higher gastric pH than normal; he or she is often described as achlorhydric. Drugs such as proton pump inhibitors and antacids block or neutralize HCl, raising gastric pH but also increasing susceptibility to stomach infections.
A hiatal hernia develops when the opening in the diaphragm for the esophagus is a little too large. Why can eating a large meal worsen symptoms of hiatal hernia? The pressure within the stomach pushes more of that organ through the hernia. Treatment is frequent small meals. Based on the illustration, what is the difference between a nasogastric (NG) tube and a gastrostomy (G) tube? The NG tube goes in the nose and into the stomach; the G tube is surgically implanted into the stomach. Vagolytic drugs slow the gut’s molitity and secretions. What happens when an infant’s pyloric sphincter cannot relax? This condition, called pyloric stenosis, produces projectile vomiting. Because gastric contents cannot move into the duodenum, the next time the child is fed, the stomach is still full and the child vomits.
Dow Jones Industrials is a mnemonic for the three parts of the small intestine the duodenum, jejunum, and ileum. Most digestion and absorption takes place in the duodenum and the first third of the jejunum. The small intestine receives chyme from the stomach. The common bile duct enters the duodenum, carrying bile and pancreatic enzymes. Material that is not digested and absorbed exits the ileum through the ileocecal valve.
The swaying peristaltic motion in the duodenum allows the end products of digestion to be absorbed by the villi. The end products of carbohydrate and protein digestion enter the capillaries and are delivered to the hepatic portal system The end products of fat digestion enter the lacteals and become part of the lymph.
The cecum receives liquid digestive contents from the ileum. The digestive contents then proceed through the large intestines to the anus. What happens if the appendix ruptures? Fecal material spills throughout the abdominal cavity, causing peritonitis and possibly septic shock if untreated. Ask a student who has had an appendectomy to share the symptoms that made him or her seek medical attention. The student will probably report fever, nausea, and pain radiating to the RLQ.
In the large intestines, fluids and electrolytes are absorbed. Peristalsis in the large intestine is characterized by infrequent episodes of mass movement, rather than continuous peristaltic motion. How does the rate of movement through the large intestine affect the consistency of the stool? If the movement is too slow, more water is absorbed and constipation results. If movement is too fast, less water is absorbed, resulting in diarrhea.
Why may removal of a tumor in the large intestine result in a colostomy? If a large part of the bowel is taken and the edges cannot be sutured together, then the proximal end of colon is rerouted through the abdominal wall. Why is a person with a tumor in the sigmoid colon more likely to become obstructed than a person with a tumor in the ascending colon? Fecal material is more liquid in the ascending colon, so it flows around the tumor through the narrowed lumen. A patient with hemorrhoids is likely to have stools streaked with bright red blood. Why? The stool irritates the hemorrhoid and causes bleeding. In contrast, a person who bleeds from the upper GI tract will have black and tarry stools because the protein in the blood has been digested.
The ducts of these accessory organs eventually empty into the common bile duct. The common bile duct empties into the duodenum, the site of most digestion and absorption. The liver contains two main lobes and is located in the mid and right upper abdominal quadrant. The gallbladder is tucked under the liver; it is attached by ducts to both the liver and common bile duct. The main pancreatic duct empties into the base of the common bile duct.
Because the liver is so metabolically active, most blood from the organs of digestion is delivered to it by the hepatic portal system. Why might a person in hepatic failure bleed excessively? The liver synthesizes clotting factors, so with a damaged liver blood coagulation may be compromised. “Statins” lower blood cholesterol because they stimulate the liver to excrete cholesterol into the bile.
End products of digestion move from the villi into the hepatic portal system for processing. The liver is particularly important for the digestion of protein which releases nitrogen and promotes the production of ammonia (NH3), which is toxic to the brain. Only the liver produces urea through the urea cycle enzymes; this allows nitrogen to be safely removed from the body. The liver helps regulate blood glucose. When blood glucose is high, the liver converts it into glycogen. When blood glucose is low, the liver converts glycogen into glucose.
Ducts carry bile from the liver to the gallbladder, where it is concentrated and stored until it is needed in the duodenum. The cystic duct connects the gallbladder to the common bile duct. The main pancreatic duct carries the most important digestive enzymes and empties into the base of the common bile duct proximal to the ampulla of Vater and sphincter of Oddi. The duodenum is called the meeting place for digestion because the chyme from the stomach, bile from the liver, digestive enzymes from the pancreas, and digestive enzymes from the wall of the duodenum meet there.
Hepatic cells use constituents of the blood to manufacture bile. The green structures in the illustration represent bile-carrying ducts. These tiny ducts are called bile canaliculi; they drain into the larger ducts.
Bile is used in the duodenum, but it is stored in the gallbladder. Dietary fat in the duodenum stimulates the walls of the duodenum to secrete cholecystokinin (CCK). CCK travels by blood to the gallbladder, causing it to contract and eject bile into the biliary tree and duodenum. CCK also goes to the pancreas, stimulating it to release digestive enzymes. In addition, it goes to the stomach, slowing the rate of gastric emptying.
The most important digestive enzymes include an amylase, which digests carbohydrate, a lipase, which digests fat, and several proteases, which digest protein. The proteases, such as trypsin, are secreted in an inactive form (trypsinogen). In the duodenum, bile and enzymes activate all the proteases. If activated trypsin is present in the pancreas, it will digest those tissues. Bile reflux into the main pancreatic duct activates trypsinogen and causes pancreatitis, a very painful and potentially lethal condition.
Gall stones form in the gallbladder, causing cholecystitis. Obstructive jaundice occurs when smaller stones escape the gallbladder and lodge in the common bile duct. This condition is called choledocholithiasis. Cirrhosis and/or liver disease can cause bleeding disorders because the liver produces clotting factors. Impeded portal blood flow causes portal hypertension and bleeding (esophageal varices). Increased portal pressure also causes ascites, a collection of fluid in the abdominal cavity. Paracentesis is used to drain ascites.
The GI tract is concerned with the digestion and absorption of carbohydrates, proteins, and fats. Amylases digest polysaccharides (starches) into disaccharides—sucrose, lactose, and maltose. Disaccharidases digest disaccharides into monosacchrarides—glucose, fructose, and galactose. Of the two amylases, salivary amylase (ptyalin) and pancreatic amylase, the latter is more potent. Because of a lactase deficiency, lactose-intolerant individuals cannot digest milk and milk products. This causes bloating, abdominal cramping, and diarrhea.
The end products of protein digestion are amino acids and small peptides. They are absorbed into the capillaries of the villi and enter the hepatic portal circulation. Most proteases are secreted in their inactive form and are activated in the stomach or duodenum.
Because fats are insoluble in water, their digestion requires an extra step, the secretion of bile. Bile breaks a large fat globule into tiny fat globules; this emulsification allows the enzyme lipase to digest fat more efficiently into fatty acids and glycerol. Bile salts bind to the fatty acids, keeping them water-soluble and facilitating absorption. The fatty acids are absorbed into the lacteals, a lymphatic structure. Bile is necessary for digestion of fat but not carbohydrates or proteins, because fats are insoluble in water.
Food moves from the mouth though the esophagus to the stomach. Peristalsis begins in the esophagus. The stomach mixes food with gastric juice, producing chyme. Peristalsis delivers chyme to the duodenum, where it mixes with bile, pancreatic enzymes, and intestinal enzymes. In the small intestine, peristalsis is a swaying movement to facilitate absorption of digestive end products by the villi. The end products of digestion enter the blood or lacteals. The rest of the GI tract absorbs water and electrolytes; waste is excreted as feces.
A balanced diet includes nutrients in their proper amounts. Most people think of malnutrition as nutrient deficiency, but in the United States, obesity is the most common form of malnutrition, leading to other serious health problems such as heart disease and diabetes.
Examples of complex carbohydrates include vegetables such as string beans and broccoli, as well as whole-grain breads. These are considered low glycemic index foods. Examples of simple sugars include candy, sugared drinks, high fructose corn syrup, and other foods that are considered to have a high glycemic index. Ingestion of simple sugars should be largely avoided, because they spike blood glucose and adversely affect metabolism.
Does the term nonessential amino acid mean that these amino acids are not essential nutrients? No, nonessential amino are necessary for proper nutrition but the body can synthesize them. Incomplete proteins, such as vegetable proteins, can provide the needed amino acids. For example, many dishes combine rice and beans, providing a complete complement of amino acids. Global hunger is primarily a problem of protein deficiency, but this is not necessarily a deficiency of meat.
Dietary fat, especially saturated fat, is correlated with atherosclerosis and coronary heart disease. The typical heart-healthy diet restricts the amount of saturated fat. Infants should not be placed on a fat-free diet. Their central systems need fat so myelin and other membranes can develop.
Vitamins are necessary for normal metabolism and vitamin deficiencies can cause serious diseases. For example, vitamin C deficiency causes scurvy, whereas vitamin D deficiency causes rickets in children and osteomalacia in adults. Vitamin A deficiency causes night blindness. Vitamin K deficiency causes bleeding abnormalities. An adequate intake of vitamins is essential, but an overingestion of fat-soluble vitamins is called hypervitaminosis, which can cause life-threatening conditions. The body stores fat-soluble vitamins, so their overingestion can be problematic. See table 23-2 for further examples of vitamin deficiencies.
Minerals are necessary for normal metabolism; mineral deficiencies or excesses can cause serious diseases. A deficiency of iron can cause iron deficiency anemia and an excess of iron can cause hemochromatocytosis. Iodine deficiency can cause goiter, hypothyroidism, and cretinism. Hypokalemia and hyperkalemia (undersupply or oversupply of potassium) can cause fatal heart dysrhythmias. Sodium imbalances can cause water imbalances; sodium deficiency causes low blood volume and shock, and excess sodium expands blood volume and can induce heart failure. See table 23-3 for further examples of mineral deficiencies.
We don’t know what makes us eat or stop eating. In experimental rats, destruction of the hypothalamic feeding center causes anorexia and starvation. In similar experiments, destruction of the satiety center in the hypothalamus causes overeating and morbid obesity. There are numerous theories about what causes satiety and therefore suppresses appetite. The glucostat hypothesis says that glucose sends inhibitory information to the feeing center. The lipostat hypothesis states that the adipocytes (fat cells) secrete the hormone leptin, an appetite suppressant.
The Human Body in Health and
Illness, 4th edition