A&P Ch 23.1: Digestive Anatomy: GI Tract

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Functions of the digestive system:

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104 Terms

1

Functions of the digestive system:

  1. Ingestion of food

  2. Propulsion of food through GI tract (includes peristalsis)

  3. Mechanical digestion

  4. Chemical digestion

  5. Absorption

  6. Defecation

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Peristalsis

Alternate waves of contraction and relaxation

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Mechanical digestion

Physical mixing of food with digestive enzymes

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Chemical digestion

Catalyzed by enzymes

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Absorbtion

Uptake of small molecules though intestinal lining

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Defecation

Elimination of feces or stool

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Portions of the digestive system: Gastrointestinal (GI) Tract

oral cavity → pharynx (oro & laryngo- pharynx) → esophagus → stomach → small intestine → large intestine

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Portions of the digestive system: Accessory organs

Salivary glands, liver, gallbladder & pancreas

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What are the serous membranes of the abdomen?

  1. Peritoneum

  2. Retroperitoneal structures

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Parts of the peritoneum:

Visceral and parietal peritoneum

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Visceral peritoneum

Adheres to the surface of abdominal organs

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Parietal peritoneum

Adheres to the wll of the abdominal cavity

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Some portions of the peritoneum have different names:

  1. Omentum

  2. Mesocolon

  3. Mesentary

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Omentum

Associated with stomach

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Mesocolon

Associated with large intestine

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Mesentary

Associated with small intestine

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Retroperitoneal structures

These structures are located against posterior wall behind the parietal peritoneum. This keeps them fixed in place.

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What are the retroperitoneal structures?

Aorta & IVC, kidneys, pancreas, duodenum, and some portions of large intestine

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Layers of the GI Tract wall

*4 layers that are present from the esophagus to the rectum.

  1. Mucosa

  2. Submucosa

  3. Muscularis externa

  4. Adventitia or serosa

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Mucosa structure:

Innermost layer; lines the hollow lumen

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What does the mucosa contain?

Epithelium, lamina propria, & muscularis mucosae

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Epithelium

Simple columnar epithelium in stomach and intestines

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Lamina propria

Connective tissue; also capillaries for nourishment & absorption; & lymphatic tissue to defend against pathogens

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Muscularis mucosae

Thin layer of smooth muscle around lamina propria

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What are the function of the mucosa?

Secretion of mucus, digestive enzymes, and hormones ; absorption of the end products of digestion ; protection against pathogens in food

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Submucosa

Connective tissue layer containing blood and lymphatic vessels, nerve fibers, and lymphatic tissue

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Muscularis externa

Two layers of smooth muscle being:

a. inner circular muscularis (INC) \n b. outer longitudinal muscularis (OUL)

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What are the functions of the muscularis externa?

  1. Responsible for muscular contractions (including peristalis) which:

    a. move food along GI tract

    b. mix food with GI tract secretions

    c. physically break down food

  2. Thickens at some points to form sphincters

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Adventitia

Outer layer of GI tract above diaphragm specifically around esophagus

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What is adventitia composed of?

Fibrous connective tissue

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Serosa

Outer layer of GI tract below diaphragm, specifically around stomach and intestines

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What is serosa composed of?

A serous covering that binds/anchors GI tract. Same as visceral peritoneum

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What is the oral cavity responsible for?

Chewing & mixing food with saliva ; initiating chemical digestion of carbohydrates by an enzyme called salivary amylase

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Pharynx

Passageway carrying chewed food (bolus) from oral cavity to esophagus; performs no additional chemical digestion

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Esophagus

Tube that carries food from pharynx to stomach; performs no chemical digestion

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Esophageal hiatus

Point where esophagus penetrates diaphragm

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Cardiac (gastroesophageal) sphincter

Thickening of the muscularis just above stomach

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Functions of the stomach:

  1. Stores food

  2. Mixes food with gastric secretions

  3. Initiates protein digestion

  4. Limited absorption (ex. absorbs alcohol + aspirin)

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Gross anatomy of the stomach

Cardiac region (cardia), fundus, body, pyloric region. pyloric sphincter (rugae, lesser curvature, greater curvature)

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Cardiac region

Surrounds the entrance of esophagus

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Fundus

Dome shaped; bulges superiorly & laterally to the cardia region

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Body

Large middle portion of the stomach

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Pyloric region

Funnel-shaped portion of the stomach leading into duodenum

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Pyloric sphincter

Between stomach and duodenum; a valve that controls movement of food

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Rugae

Large folds of mucosa ( and submucosa) present in stomach wall when empty

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Lesser curvature

Concave, medial surface of the stomach

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Greater curvature

Convex, lateral surface of the stomach

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What does the stomach wall contain?

An extra oblique muscle layer in the muscularis (+ INC & OUL) ; gastric pits, which contain glands that make gastric juice

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Cells of the gastric glands:

Mucous neck cells, parietal cells, chief cells, enteroendocrine (G) cells

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Mucous neck cells

Secrete thin, acidic mucus

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Parietal cells

Secrete HCI and intrinsic factor

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HCI (hydrochloric acid)

Activates pepsinogen

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Intrinsic factor

Aids in absorbtion of vitamin B12

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Chief cells

Secrete pepsinogen, an inactive enzyme that's converted to pepsin by HCI in the lumen

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Pepsin

A protein-digesting enzyme

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Enteroendocrine (G) cells

Secrete a hormone called gastrin into the blood that stimulates secretion of gastric juice

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Summary of the stomach:

Food enters stomach as a bolus and leaves as chyme, a semi-fluid mixture that passes through the pyloric sphincter

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What is the hostile environment of the stomach?

Extremely acidic; contains protein-digesting enzymes; few pathogens can survive

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Mucosal barrier of the stomach:

Protects stomach from it's own harsh internal environment

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The mucosal barrier of the stomach is formed by:

Mucus coat and tight junctions between epithelial cells

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Mucus coat

Thick, bicarbonate-rich mucus is built up on the stomach wall

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Tight junctions between epithelial cells

Prevent gastric juice from leaking into underlying tissues

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What are the function of the small intestine?

  1. Primary digestive organ

  2. Primary sight of absorption

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What does the small intestine being the primary digestive organ mean?

Completes digestion of carbohydrates and proteins ; performs digestion of lipids (fats)

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Portions of the small intestine:

Duodenum, jejunum, ileum

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Duodenum

Receives chyme from stomach ; most chemical digestion occurs here ; also receives secretions from pancreas, liver & gallbladder ; contain Brunner's (duodenal) glands in submucosa

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Brunner's (duodenal) glands in submucosa

Secrete alkaline mucus to neutralize acidic incoming chyme

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Jejunum

2nd portion of small intestine

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Ileum

Distal segment; carries chyme to large intestine

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What are the structural modifications of the small intestine?

Circular folds, villi, and microvilli

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Circular folds

Large folds of mucosa and submucosa

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Villi

Fingerlike projections of mucosa

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Microvilli

"brush border" : tiny projections of the intestinal mucosal cell's plasma membrane

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What is the function of the 3 modifications?

They all increase surface area for absorbtion

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Histology of the small intestine

Have tight junctions between absorptive columnar cells; many goblet cells = secrete mucus ; enteroendocrine cells - secrete 2 hormones

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What 2 hormones do enteroendocrine cells secrete in the small intestine?

Secretin and cholecystokinin

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Other features of the small intestine:

Intestinal crypts, peyer's patches, capillaries, lacteals

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Intestinal crypts

Pockets between the villi that secrete intestinal juice (watery mixture w/ mucus)

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Peyer's patches

Aggregations of lymphoid tissue → prevent bacteria of intestine from entering blood

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Capillaries

In villi; transport absorbed sugars and amino acids

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Lacteals

Lymphatic capillaries in villi that transport absorbed fats

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What are the functions of the large intestine?

  1. Absorption of some water and electrolytes

  2. Defecation (elimination of feces)

  3. Vitamin K and folic acid production - by bacteria within the large intestine

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Feces

Undigested food residues, mucus, sloughed-off epithelial cells, bacteria, and some water

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What are the 5 regions of the large intestine?

  1. Cecum

  2. Appendix

  3. Colon

  4. Rectum

  5. Anal canal

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Cecum

Pouch below the junction of the large intestine within the ileum

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Illeocecal valve

Opening between ileum and cecum

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Appendix

Blind-ended, wormlike extension attached to medial surface of cecum; contains lymphoid tissue

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Colon

Extends from ileocecal valve to rectum

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Portions of the colon:

a. Ascending colon

b. hepatic flexure

c. transverse colon

d. Splenic flexure

e. Descending colon

f. Sigmoid colon

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Ascending colon

Travels up right side of colon

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Hepatic flexure

Bend near liver of colon

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Transverse colon

Travels across abdomen from right to left of colon

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Splenic flexure

Bend near spleen of colon

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Descending colon

Travels down left side of colon

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Sigmoid colon

Curves around iliac fossa and connects to the rectum

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Rectum

Portion of GI tract running between sigmoid colon & anal canal; begins in the region of the sacrum and runs anterior to the sacrum

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What does the rectum contain?

Rectal valves which separate feces from flatus (gas)

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Anal canal

Final inch ( ≈3cm) of large intestine; most distal portion of GI tract

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Anus

Opening of anal canal

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Internal anal sphincter

Smooth muscle; under autonomic control

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