A&P Ch 23.1: Digestive Anatomy: GI Tract

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

Peristalsis

Alternate waves of contraction and relaxation

Mechanical digestion

Physical mixing of food with digestive enzymes

Chemical digestion

Catalyzed by enzymes

Absorbtion

Uptake of small molecules though intestinal lining

Defecation

Elimination of feces or stool

Portions of the digestive system: Gastrointestinal (GI) Tract

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

Portions of the digestive system: Accessory organs

Salivary glands, liver, gallbladder & pancreas

What are the serous membranes of the abdomen?

1. Peritoneum
2. Retroperitoneal structures

Parts of the peritoneum:

Visceral and parietal peritoneum

Visceral peritoneum

Adheres to the surface of abdominal organs

Parietal peritoneum

Adheres to the wll of the abdominal cavity

Some portions of the peritoneum have different names:

1. Omentum
2. Mesocolon
3. Mesentary

Omentum

Associated with stomach

Mesocolon

Associated with large intestine

Mesentary

Associated with small intestine

Retroperitoneal structures

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

What are the retroperitoneal structures?

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

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

Mucosa structure:

Innermost layer; lines the hollow lumen

What does the mucosa contain?

Epithelium, lamina propria, & muscularis mucosae

Epithelium

Simple columnar epithelium in stomach and intestines

Lamina propria

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

Muscularis mucosae

Thin layer of smooth muscle around lamina propria

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

Submucosa

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

Muscularis externa

Two layers of smooth muscle being:

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

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

Adventitia

Outer layer of GI tract above diaphragm specifically around esophagus

What is adventitia composed of?

Fibrous connective tissue

Serosa

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

What is serosa composed of?

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

What is the oral cavity responsible for?

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

Pharynx

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

Esophagus

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

Esophageal hiatus

Point where esophagus penetrates diaphragm

Cardiac (gastroesophageal) sphincter

Thickening of the muscularis just above stomach

Functions of the stomach:

1. Stores food
2. Mixes food with gastric secretions
3. Initiates protein digestion
4. Limited absorption (ex. absorbs alcohol + aspirin)

Gross anatomy of the stomach

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

Cardiac region

Surrounds the entrance of esophagus

Fundus

Dome shaped; bulges superiorly & laterally to the cardia region

Body

Large middle portion of the stomach

Pyloric region

Funnel-shaped portion of the stomach leading into duodenum

Pyloric sphincter

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

Rugae

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

Lesser curvature

Concave, medial surface of the stomach

Greater curvature

Convex, lateral surface of the stomach

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

Cells of the gastric glands:

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

Mucous neck cells

Secrete thin, acidic mucus

Parietal cells

Secrete HCI and intrinsic factor

HCI (hydrochloric acid)

Activates pepsinogen

Intrinsic factor

Aids in absorbtion of vitamin B12

Chief cells

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

Pepsin

A protein-digesting enzyme

Enteroendocrine (G) cells

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

Summary of the stomach:

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

What is the hostile environment of the stomach?

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

Mucosal barrier of the stomach:

Protects stomach from it's own harsh internal environment

The mucosal barrier of the stomach is formed by:

Mucus coat and tight junctions between epithelial cells

Mucus coat

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

Tight junctions between epithelial cells

Prevent gastric juice from leaking into underlying tissues

What are the function of the small intestine?

1. Primary digestive organ
2. Primary sight of absorption

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

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

Portions of the small intestine:

Duodenum, jejunum, ileum

Duodenum

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

Brunner's (duodenal) glands in submucosa

Secrete alkaline mucus to neutralize acidic incoming chyme

Jejunum

2nd portion of small intestine

Ileum

Distal segment; carries chyme to large intestine

What are the structural modifications of the small intestine?

Circular folds, villi, and microvilli

Circular folds

Large folds of mucosa and submucosa

Villi

Fingerlike projections of mucosa

Microvilli

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

What is the function of the 3 modifications?

They all increase surface area for absorbtion

Histology of the small intestine

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

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

Secretin and cholecystokinin

Other features of the small intestine:

Intestinal crypts, peyer's patches, capillaries, lacteals

Intestinal crypts

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

Peyer's patches

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

Capillaries

In villi; transport absorbed sugars and amino acids

Lacteals

Lymphatic capillaries in villi that transport absorbed fats

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

Feces

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

What are the 5 regions of the large intestine?

1. Cecum
2. Appendix
3. Colon
4. Rectum
5. Anal canal

Cecum

Pouch below the junction of the large intestine within the ileum

Illeocecal valve

Opening between ileum and cecum

Appendix

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

Colon

Extends from ileocecal valve to rectum

Portions of the colon:

a. Ascending colon

b. hepatic flexure

c. transverse colon

d. Splenic flexure

e. Descending colon

f. Sigmoid colon

Ascending colon

Travels up right side of colon

Hepatic flexure

Bend near liver of colon

Transverse colon

Travels across abdomen from right to left of colon

Splenic flexure

Bend near spleen of colon

Descending colon

Travels down left side of colon

Sigmoid colon

Curves around iliac fossa and connects to the rectum

Rectum

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

What does the rectum contain?

Rectal valves which separate feces from flatus (gas)

Anal canal

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

Anus

Opening of anal canal

Internal anal sphincter

Smooth muscle; under autonomic control