Digestive System: Anatomy & Physiology Study Notes

Digestive System Overview

Introduction

The digestive system is a complex series of organs and glands that processes food, extracts nutrients, and eliminates waste. It consists of the gastrointestinal (GI) tract and accessory digestive organs. It takes the food that we eat or drink and breaks them down into smaller components so that they can be absorbed from our digestive system into the blood and transported to our body tissues.

• GI Tract (gastrointestinal tract) : One continuous tube that starts at the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and ends at the anal canal. When food is moving through our GI tract the food gets renamed. When the food enters the oral cavity and mixes with our saliva the food is called a bolus. the bolus is what is swallowed down our pharynx and esophagus. when the bolus reaches the stomach it mixes with our digestive enzymes in our stomach which is then names chyme (a paste like substance).

• Accessory Digestive Organs: NOT part of our GI tract but they contribute different secretions to our food etc.... and help digest those substances. Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Functions of the Digestive System

Major Functions

The digestive system performs six essential functions to maintain homeostasis and nutrient supply.

1. Ingestion: Intake of food and liquids into the mouth.

2. Motility: Movement of food through the GI tract via muscular contractions (propulsion/ peristalsis ( the movement of the materials in your GI tract in a forward direction. kind of like when you squeeze your toothpaste out of your toothpaste tube.) and mixing (back and forth movement to help what we are eating be mixed and combines with the secretions from the GI tract.)

3. Secretion: Release of digestive enzymes, acids, and mucus to aid digestion.

4. Digestion: Mechanical (chewing, mixing) occurs in the oral cavity. and chemical (enzymatic breakdown) processes.

5. Absorption: When our nutrients are moved from the GI Tract into our blood and lymphatic vessels.

6. Elimination: Removal of indigestible substances and waste products.

Tunics

Definition: Our Gi tract from the esophagus to the large intestine is composed out of four layers called tunics. aka layers of the GI tract

1. Mucosa (the deepest or inner most layer)- made out of simple columnar epithelium.

2. Submucosa- made out of connective tissue like areolar tissue.

3. Muscularis- made out of two layers of smooth muscle

4. Serosa (most outer layer)- covered by a Visceral peritoneum. intraperitoneal organs (the stomach, most of the small intestine and part of the large intestine) aka adventitia : retroperitoneal organs (suprarenal glands, aorta and IVC, duodenum, pancreas, urinary bladder, ureters, colon, kidneys, esophagus aka SAD PUCKER) -made out of areolar connective tissue.

Sphincter: when the muscularis layer is thickened at some point which helps close off the lumen along different parts of the GI tract in order to control the movement of those material moving through the GI tract.

Oral Cavity and Associated Structures

Hard & Soft Palates

The palate separates the oral and nasal cavities, facilitating swallowing and speech.

• Hard palate: Bony anterior portion. Made out of two cranium bones known as the maxilla and the palatine. covered by a layer of non keratinized stratified squamous epithelium. Sits in the front.

• Soft palate: Muscular posterior portion (in the back). made and covered with non keratinized stratified squamous epithelium. The uvula elevates when we swallow and closes off the entrance to the nasal pharynx to prevent any ingested materials from entering into that region.

• Includes uvula, palatine tonsil, lingual tonsil, and associated muscles

Salivary Glands

Produce saliva to moisten food, begin starch digestion, and protect oral tissues.

• Parotid gland: Largest, near ear. produces 20-30% of saliva.

• Sublingual gland: Under tongue. Produces 3-5%

• Submandibular gland: Below jaw, produces most saliva

Teeth

Teeth mechanically break down food. Permanent teeth are classified by type and eruption age.

Type	Number	Function	Eruption Age
Incisors	8	Cutting	6-8 yrs
Canines	4	Tearing	9-12 yrs
Premolars	8	Grinding	10-12 yrs
Molars	12	Grinding	6-25 yrs

Pharynx and Esophagus

Pharynx

The pharynx is a muscular tube that serves both respiratory and digestive functions, guiding food from the mouth to the esophagus.

• Voluntary phase: Bolus pushed by tongue

• Pharyngeal phase: Soft palate closes nasopharynx, epiglottis closes larynx

• Esophageal phase: Peristalsis moves bolus to stomach

Esophagus

The esophagus is a muscular tube connecting the pharynx to the stomach, featuring sphincters to regulate passage.

• Superior esophageal sphincter: Prevents air entry

• Inferior esophageal sphincter: Prevents reflux

Peritoneal Relationships

Intraperitoneal vs. Retroperitoneal Organs

Organs are classified by their relationship to the peritoneum.

Type	Description	Examples
Intraperitoneal	Completely surrounded by visceral peritoneum	Stomach, most of small intestine, part of large intestine
Retroperitoneal	Outside parietal peritoneum	Suprarenal glands, aorta, duodenum, pancreas, colon, kidneys, esophagus, rectum

Stomach

Gross Anatomy

The stomach is a muscular organ with distinct regions: cardia, fundus, body, and pylorus.

• Functions: Mechanically digests food, chemically digests proteins, releases intrinsic factor

Histology

The stomach wall contains specialized cells for digestion and protection.

• Surface mucous cells: Secrete alkaline mucus

• Mucous neck cells: Secrete acidic mucus

• Parietal cells: Secrete hydrochloric acid and intrinsic factor

• Chief cells: Secrete pepsinogen

• G cells: Secrete gastrin

Small Intestine

Gross Anatomy

The small intestine is the primary site for chemical digestion and nutrient absorption.

• Regions: Duodenum, jejunum, ileum

• Functions: Completes chemical digestion, absorbs 90% of nutrients and water

Histology

The small intestine wall features circular folds, villi, and microvilli to increase surface area for absorption.

• Mucosa: Contains absorptive cells and goblet cells

• Submucosa: Contains blood vessels and lymphatics

• Muscularis: Inner circular and outer longitudinal layers

• Serosa: Outer covering

Large Intestine

Gross Anatomy

The large intestine absorbs water and electrolytes, forming and storing feces.

• Regions: Cecum, colon (ascending, transverse, descending, sigmoid), rectum, anal canal

• Functions: Absorbs water, forms feces, houses gut flora

Rectum & Anal Canal

Structure and Function

The rectum and anal canal regulate the elimination of feces.

• Rectal valve: Controls passage of feces

• Internal and external anal sphincters: Regulate defecation

Liver

Functions

The liver is a vital organ for metabolism, detoxification, and bile production.

• Produces and releases bile

• Detoxifies blood

• Stores excess nutrients

• Produces plasma proteins

Bile Composition

• Water

• Bicarbonate ions

• Bile salts & pigments

• Cholesterol

• Lecithin

• Mucin

Gallbladder & Biliary Apparatus

Structure and Function

The gallbladder stores and concentrates bile, releasing it into the duodenum via the biliary apparatus.

• Common hepatic duct: Formed by right and left hepatic ducts

• Cystic duct: Connects gallbladder to common bile duct

• Main pancreatic duct: Joins bile duct at hepatopancreatic ampulla

Pancreas

Structure and Function

The pancreas produces digestive enzymes and bicarbonate, essential for digestion in the small intestine.

• Exocrine function: Acinar cells produce digestive enzymes

• Endocrine function: Islets of Langerhans produce insulin and glucagon

• Functions: Produces and releases digestive enzymes and bicarbonate

Summary Table: Digestive System Functions

Organ	Main Function
Mouth	Ingestion, mechanical digestion
Pharynx	Swallowing
Esophagus	Transport of food
Stomach	Mechanical and chemical digestion
Small Intestine	Chemical digestion, absorption
Large Intestine	Water absorption, feces formation
Liver	Bile production, metabolism
Gallbladder	Bile storage
Pancreas	Enzyme and bicarbonate secretion

Additional info: These notes expand on the provided images and text, adding definitions, examples, and tables for clarity and completeness.