The Digestive System

Functions to break down food substances into forms that

can be absorbed

Types of Digestion Mechanical

Chemical

Digestive Processes Ingestion Digestion Absorption Elimination

Divisions of the Digestive System Gastrointestinal tract

Oral cavity, pharynx, esophagus, stomach, large and small intestines

Accessory structures Teeth, tongue, salivary glands,

liver, pancreas, and gallbladder

Layers of the GI Tract

Mucosa - innermost layer Epithelium - stratified squamous

(pharynx and esophagus) and simple columnar

Lamina propria - areolar connective tissue

Muscularis - smooth muscle layer causing small folds

Layers of GI Tract Submucosa - loose connective tissue

containing blood vessels, nerves, glands, and lymphatics

Muscularis - two layers of smooth muscle inner circular layer and outer

longitudinal one some skeletal muscle

Serosa - outer mesothelium Peritoneum (parietal and visceral) Extensions: mesentery, mesocolon,

greater and lesser omentum, falciform ligament

Organs and Structures

Mouth Vestibule Tongue Teeth Salivary

glands

Mouth or Buccal Cavity Vestibule- area bounded by the lips

and cheeks externally/internally by the gums and teeth

Tongue - striated skeletal muscle that manipulates food, help form words, and serves as a sense organ

Teeth - breaks food into smaller pieces increasing surface area for digestion

Teeth Found in alveolar sockets Primary (deciduous or baby) teeth

Usually consists of 20 teeth) Permanent teeth (32 teeth) Incisors - chisel-shaped Canines (Cuspids) - fang-like Premolars (bicuspids) - grinding and

crushing Molars - grinding and crushing

Tooth Structure Crown - exposed portion above gum

Covered in enamel Dentin - fills interior of tooth Neck - hidden by gumline Root - embedded in jawbone, number of

roots varies Cementum - attaches tooth to periodontal

ligament

Salivary Glands

Parotid glands Submandibular glands Sublingual glands Functions:

cleanses, dissolves, moistens, digests 97% water Slightly acidic Contains electrolytes, salivary amylase,

mucin, lysozymes, and immunoglobulins

Esophagus Hollow muscular tube that functions to

carry food to the stomach through an opening in the diaphragm called the esophageal hiatus

Upper third composed of skeletal musle; lower regions made of smooth muscle

Hiatal hernia

Digestive Processes of the mouth, pharynx, & esophagus

Mastication - chewing Deglutition - swallowing

chewed food formed into bolus tongue blocks mouth, soft palate

blocks nasopharynx, epiglottis blocks trachea

Peristalsis - rhythmic contractions of circular muscles causing food to move down GI tract

Stomach Serves as a reservoir for food and to

mix food with gastric juice Regions: cardia, fundus, body, and

pylorus Greater and lesser curvatures Pyloric sphincter Rugae Additional oblique smooth muscle layer

Gastric gland cells Mucous goblet cells - secrete mucosal layer Parietal cells (oxyntic) - secrete HCl and

intrinsic factor Chief cells (zymogenic) - secrete pepsinogen

Pepsinogen converted to pepsin which breaks proteins into peptides

Enteroendocrine cells (G-cells)- secrete hormones: gastrin, histamine, endorphins, serotonin,

cholecystokinin, somatostatin Stomach ulcers - caused by Heliobacter

pylori

Digestive Processes in the Stomach

Mechanical & chemical digestion turns food into chyme

Pepsin breaks down proteins into peptides

Rennin (in children) breaks down milk Intrinsic factor - required for absorption

of vitamin B12 in the intestines

Gastric motility & emptying

Stomach contractions produce chyme, increase mixing of food

Entire stomach can hold up to 4L Approx. 3 ml squirts through pylorus

with each contraction Stomach contracts 3X/min Usually empties within 4 hours but may

be delayed depending on contents

Pancreas Secretes pancreatic juice formed by acini Pancreatic juice

sodium bicarbonate that serves to buffer HCl proteases such as trypsin, chymotrypsin,

and carboxypeptidase pancreatic amylase pancreatic lipases pancreatic nucleases

Liver and Gallbladder

Largest internal organ in the body Consists of right and left lobe separated by the

falciform ligament; caudate and quadrate lobes on the posterior surface

Digestive function includes the production of bile; protein, carbohydrate & lipid metabolism; removal of drugs & hormones

Gallbladder - stores and concentrates bile

Small Intestines

Duodenum, Jejunum, and Ileum

Small Intestine Duodenum - (10 inches) receives

chyme from the stomach and digestive secretions from the pancreas and liver ; major digestion occurring here

Secretin Cholecystokinin (CCK) Gastric inhibitory peptide (GIP)

Jejunum and Ileum 8 + 12 = 20 feet long Region where most absorption occurs Plica circulares = folds Villi lined with simple columnar epithelium Each villus contains an arteriole, venule,

capillary bed and a lacteal Microvilli form brush border Peristalsis and segmentation occurs

Enzymes of the small intestine

Dextrinase break down Glucoamylase oligosaccharides Maltase break Sucrase down Lactase disaccharides Carboxypeptidase breaks Aminopeptidase down Dipeptidase proteins

Large Intestine 5.5 feet long Functions:

reabsorption of remaining water and electrolytes

absorption of vitamins B and K (produced by the bacterial flora)

elimination of feces (may take 12-24 hours)

Large Intestine

Cecum Ileocecal valve Appendix Ascending, transverse, and descending

colon Sigmoid colon Taeniae coli / Haustra Rectum / Anus Hemorrhoids/Diverticulitis

Products of Digestion

Carbohydrates Lipids Proteins

Carbohydrates Monosaccharides - single sugars

Glucose Fructose Galactose Deoxyribose Ribose

Carbohydrates Disaccharides - double sugars

Sucrose Lactose Maltose

Polysaccharides - starches (many sugars) Amylose Cellulose Glycogen

Formation of a disaccharide

+

Formation of a fat

Carbohydrate Digestion Carbohydrates are broken down into

disaccharides and then into monosaccharides

Starch (Amylose)+ amylase --> Maltose Maltose + Maltase --> 2 Glucose molecules Lactose + Lactase --> Galactose & Glucose Sucrose + Sucrase --> Glucose & Fructose

Lipid Digestion Lipids (fats) + Lipase ---> Glycerol & Fatty

Acids

This process is made more efficient by the emulsifying action of bile.

Protein Digestion Protein + Pepsin --> large polypeptides Protein + Rennin --> large polypeptides Protein + Trypsin --> small polypeptides Protein + Chymotrypsin --> small

polypeptides Protein + Carboxypeptidase --> sm. Plypeptds Aminopeptidases - to single amino acids Carboxypeptidase - to single amino acids Dipeptidase - to single amino acids

Overview of Digestive Diseases Digestive diseases range from the

occasional upset stomach to the more life-threatening colon cancer and encompass disorders of the gastrointestinal tract, the liver, the gallbladder, and the pancreas.  

What Causes a Digestive Disease?  Digestive disease may develop congenitally

or from multiple factors such as stress, fatigue, diet, or smoking. Abusing alcohol imposes the greatest risk for digestive diseases, particularly increasing the risk of esophageal, colorectal, and liver cancers.

Who Develops a Digestive Disease?  Each year 62 million Americans are

diagnosed with a digestive disorder. The incidence and prevalence of most

digestive diseases increase with age. Women are more likely than men to

report a digestive condition, particularly non-ulcer dyspepsia and irritable bowel syndrome (IBS).

How Are Digestive Diseases Diagnosed? 

Most digestive diseases are very complex, with subtle symptoms. Because of this, patients may undergo extensive and expensive diagnostic tests which may include a blood test, an upper or lower GI series, an ultrasound, and endoscopic examinations of the colon, esophagus, stomach, or small intestine, or more sophisticated tests such as a CAT (computerized axial tomography) scan or MRI. 

Significant Digestive Conditions 

Gallstones affect 20 percent of women and 10 percent of men, or approximately 20 million adult Americans.

are solid masses, primarily of cholesterol that develop in the gallbladder or less often in the bile ducts leading from the liver to the small intestine. 

Symptoms may include mid- or right-upper abdominal pain that may lead to complications such as acute cholecystitis and pancreatitis. Gallstones are rarely associated with gallbladder cancer. 

Treatment options include surgery by open cholecystectomy or laparoscopic cholecystectomy, watchful waiting, or oral bile acid therapy in patients who cannot tolerate surgery.

Gastroesophageal Reflux Disease and Related Disorders  Gastroesophageal reflux disease affects nearly one-third of the American

population. GERD is the backward flow of the stomach's contents into the esophagus.  Heartburn is the most common symptom of GERD.

Doctors also believe that diet and lifestyle habits, hiatal hernia, obesity, and pregnancy contribute to GERD. Certain foods,including chocolate, fried or fatty foods, and alcohol may weaken the LES, permitting reflux and heartburn. 

Antacids such as Tums and Gaviscon neutralize stomach acid for relatively short periods of time.  

Histamine 2 (or H2-blockers), which suppress acid, are also prescribed to relieve symptoms of GERD. The H2-remedies currently available include cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid), and ranitidine HCl (Zantac).

Omeprazole (Prilosec, Losec, or Antra), are pump inhibitors that dramatically inhibit an enzyme, H+(hydrogen), K+(potassium)-ATPase, from producing stomach acid; lansoprazole (Prevacid) has also recently been approved.  

Inflammatory Bowel Disease 

Inflammatory bowel disease (IBD) refers to two chronic intestinal disorders: Crohn's disease and ulcerative colitis. IBD affects between 2 to 6 percent of Americans or an estimated 300,000 to 500,000 people. The causes of Crohn's disease and ulcerative colitis are not known. A leading theory suggests that some agent, perhaps a virus or bacterium, alters the body's immune response, triggering an inflammatory reaction in the intestinal wall. The onset for both diseases peaks during young adulthood. An individual with either disease may suffer persistent abdominal pain, bowel sores, diarrhea, fever, intestinal bleeding, or weight loss. 

 

Crohn's Disease Crohn's disease primarily involves the

small bowel and the colon. It may cause the intestinal wall to thicken, which may narrow the bowel channel and block the intestinal tract.  

Although surgery relieves chronic symptoms, Crohn's disease often recurs at the location where the healthy parts of the bowel were rejoined. The length of time that a Crohn's patient is in remission is not predictable. 

Ulcerative Colitis Ulcerative colitis (UC) is an

inflammatory disorder affecting the inner lining of the large intestine. The inflammation originates in the lower colon and spreads through the entire colon. Blood in the stool is the most common and distinct symptom of ulcerative colitis.  

Irritable Bowel Syndrome 

Irritable bowel syndrome (IBS) is a common functional disorder of the intestines estimated to affect 5 million Americans. The cause of IBS is not yet known. Doctors refer to IBS as a functional disorder because there is no sign of disease when the colon is examined. However, doctors believe that people with IBS experience abnormal patterns of colonic movement. The IBS colon is highly sensitive, overreacting to any stimuli such as gas, stress, or eating high-fat or fiber-rich foods. 

Peptic Ulcer Disease 

Peptic ulcer disease, estimated to affect 4.5 million people in the United States, is a chronic inflammation of the stomach and duodenum.  

Peptic ulcers result from the breakdown of the lining of the stomach and duodenum caused by increased stomach acid and pepsin and Helicobacter pylori (H. pylori). One type of ulcer occurs in the stomach, the other in the duodenum, the first part of the small intestine. Duodenal ulcers are much more common than stomach ulcers, which have a greater risk of malignancy.