Digestive System: Structure and Function of the Alimentary Canal Unit 4 Week 12 LO's

Oral Cavity

Boundaries of the Oral Cavity

The oral cavity is the entryway to the digestive tract, bounded by several anatomical structures that facilitate ingestion and initial processing of food.

• Anterior: Lips

• Lateral: Cheeks

• Superior: Hard palate (front) and soft palate (back)

• Inferior: Tongue and floor of mouth

• Posterior: Opens to the oropharynx (fauces)

Hard Palate vs Soft Palate vs Uvula

• Hard Palate: Formed by the maxilla and palatine bones; separates the oral and nasal cavities and provides a rigid surface for chewing.

• Soft Palate: Composed of muscle; moves during swallowing to close off the nasopharynx, preventing food from entering the nasal cavity.

• Uvula: A muscular extension of the soft palate; assists in closing the nasopharynx during swallowing.

Tongue: Structures and Functions

The tongue is a muscular organ essential for taste, food manipulation, and speech.

• Papillae Types:

  • Filiform: Provide friction; do not contain taste buds.

  • Fungiform: Contain taste buds; scattered across the tongue.

  • Circumvallate: Large papillae at the back of the tongue; contain many taste buds.

  • Foliate: Located on the sides; less important for taste in humans.

• Functions:

  • Taste perception

  • Manipulation of food and formation of the bolus

  • Speech articulation

Deciduous vs Permanent Teeth

• Deciduous (Baby) Teeth: 20 in total; smaller, whiter, and lack premolars.

• Permanent Teeth: 32 in total; larger, stronger, include premolars and wisdom teeth.

Salivary Glands and Ducts

Salivary glands secrete saliva, which begins the process of digestion and lubricates food.

• Parotid Gland: Located near the ear; parotid duct opens near upper molars.

• Submandibular Gland: Located under the jaw; duct opens at the floor of the mouth.

• Sublingual Gland: Located under the tongue; has many small ducts opening into the floor of the mouth.

Pharynx

Regions and Functions

The pharynx is a muscular tube that serves as a passageway for both air and food, divided into three regions:

• Nasopharynx: Conducts air only.

• Oropharynx: Conducts both air and food.

• Laryngopharynx: Conducts air and food; directs food to the esophagus.

Esophagus

Location

• Located posterior to the trachea.

• Passes through the diaphragm at the esophageal hiatus.

• Terminates at the stomach.

Functions

• Transports food to the stomach via peristalsis (coordinated muscular contractions).

• Secretes mucus for lubrication.

Specializations

• Mucosa: Lined with stratified squamous epithelium for protection against abrasion.

• Muscularis:

  • Upper third: Skeletal muscle (voluntary control)

  • Middle third: Mixed skeletal and smooth muscle

  • Lower third: Smooth muscle (involuntary control)

Structure-Function Relationships

• Tough lining protects against mechanical damage from food.

• Muscle type transitions from voluntary to involuntary, reflecting the swallowing process.

Stomach

Gross Anatomy and Location

• Located in the left upper quadrant (LUQ) of the abdomen, inferior to the diaphragm.

• Regions: Cardia, Fundus, Body, Pylorus.

Functions

• Stores ingested food.

• Performs mechanical digestion by churning food.

• Performs chemical digestion using acid and enzymes.

• Initiates protein digestion.

Specializations

• Gastric Pits and Glands: Invaginations in the mucosa containing specialized cells.

• Cell Types:

  • Parietal cells: Secrete hydrochloric acid (HCl)

  • Chief cells: Secrete pepsinogen (inactive enzyme)

  • Mucous cells: Secrete mucus for protection

Muscularis Special Feature

• Three muscle layers (unique to the stomach):

  • Oblique

  • Circular

  • Longitudinal

  Function: Enhances mixing and mechanical breakdown of food.

Sphincters

• Lower Esophageal (Cardiac) Sphincter: Prevents reflux of stomach contents into the esophagus.

• Pyloric Sphincter: Regulates emptying of stomach contents into the small intestine.

Rugae

• Folds in the stomach lining that allow expansion and increase surface area.

Small Intestine

Location

• Extends from the stomach to the large intestine; occupies the central abdomen.

Functions

• Main site for digestion and absorption of nutrients.

Segments

• Duodenum: Receives bile (from liver/gallbladder) and digestive enzymes (from pancreas).

• Jejunum: Primary site for nutrient absorption.

• Ileum: Absorbs vitamin B12 and bile salts.

Specializations for Absorption

• Plicae Circulares: Circular folds that increase surface area.

• Villi: Finger-like projections that further increase surface area.

• Microvilli: Microscopic projections on epithelial cells; form the "brush border."

• Function: All these structures maximize nutrient absorption.

Other Structures and Functions

• Brunner’s Glands: Secrete alkaline mucus to neutralize stomach acid (mainly in duodenum).

• Crypts of Lieberkühn: Secrete intestinal juice containing digestive enzymes.

• Peyer’s Patches: Aggregated lymphoid nodules for immune defense (mainly in ileum).

Ileocecal Valve

• Located between the ileum and cecum; prevents backflow from the large intestine.

Large Intestine, Rectum, and Anal Canal

Gross Anatomy and Location

• Frames the abdomen; segments include:

• Cecum → Ascending colon → Transverse colon → Descending colon → Sigmoid colon → Rectum → Anal canal

Flexures

• Hepatic Flexure: Right colic flexure (near the liver)

• Splenic Flexure: Left colic flexure (near the spleen)

Functions

• Absorbs water and electrolytes

• Forms and stores feces

• Houses beneficial bacteria (microbiota) that produce vitamins

Specializations

• Taenia Coli: Three longitudinal muscle bands that aid in segmentation and movement of feces.

• Haustra: Pouch-like segments formed by the taenia coli.

• Villi: Fewer and shorter than in the small intestine.

Structure-Function Relationships

• Slower movement allows for maximal water absorption.

• Muscle bands facilitate segmentation and movement of fecal material.

Internal vs External Anal Sphincters

• Internal Anal Sphincter: Smooth muscle; involuntary (autonomic control).

• External Anal Sphincter: Skeletal muscle; voluntary (somatic control).

• Key Point: Internal sphincter is automatic; external sphincter is under conscious control.

Additional info:

• The digestive tract is also known as the alimentary canal and is responsible for the breakdown and absorption of nutrients essential for life.

• Peristalsis is a coordinated, wave-like contraction of smooth muscle that propels food through the digestive tract.