Digestive System Overview

Overview of the Digestive System

• Purpose involves the mechanic and chemical break down of foods into usable components for the body's cells.
• Key processes include ingesting, mixing, transporting, and digesting food, along with absorbing nutrients, and expelling waste.
• The nutrient pathway is as follows: food becomes bolus in the mouth, then chyme in the stomach, and finally is distributed to blood and body tissues.
• Waste is eliminated through defecation.

Organs of the Digestive System

• Organs fall into two categories: those of the gastrointestinal tract and accessory digestive organs.
• The gastrointestinal (GI) tract, also called the digestive tract or alimentary canal, includes:
  • Oral cavity (mouth)
  • Pharynx (throat)
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anal canal
• The tract ends at the anus.
• Muscle contractions in the GI tract wall propel materials.
• Accessory digestive organs assist the GI tract, developing as outgrowths and connecting to it.

Digestive System Functions

• The digestive system carries out six main functions:
  • Ingestion: Intake of solids/liquids into the oral cavity.
  • Propulsion: Movement of materials along the digestive tract.
  • Secretion: Release of mucin, acids, bile, and enzymes.
  • Digestion: Mechanical and chemical breakdown of food.
  • Absorption: Uptake of nutrients into blood/lymph.
  • Elimination: Expulsion of feces via defecation.

Propulsion Mechanisms

• Propulsion involves two types of movements:
  • Peristalsis: Wave-like muscle contractions that move material forward.
  • Segmentation: Mixing and churning movements in the small intestine.
• Segmentation helps dissolve digested particles.

Digestion Processes

• Digestion encompasses the breakdown of large food items.
• Digestion has two aspects:
  • Mechanical digestion: Physically breaking down materials.
  • Chemical digestion: Breaking down materials via enzymes.
• Mastication, or chewing, is the first part of mechanical digestion.

Secretion

• Secretion involves the production and release of fluids.
• Mucin, acid, bile, and digestive enzymes secreted into the GI tract lumen facilitate chemical digestion and passage of materials.
• Mucin secretions protect and lubricate the GI tract wall.

Absorption

• Absorption involves passive movement or active transport of electrolytes.
• Digestion products, vitamins, and water are absorbed across the GI tract epithelium into blood and lymph vessels.

Elimination

• Elimination is the digestive system's final function
• Undigestible materials are compacted to form feces
• Feces are then eliminated from the GI tract through defecation.

Oral Cavity Details

• The oral cavity, or mouth, functions as the GI tract's entrance,
• The mouth is the site for initial mechanical and chemical digestion.
• Boundaries of the oral cavity include:
  • Lips/teeth (anterior)
  • Oropharynx (posterior)
  • Palate (superior)
  • Mylohyoid muscle (inferior)
• Regions of the oral cavity are:
  • Vestibule (cheeks/lips to gums)
  • Oral cavity proper (central to alveolar processes)
• The epitelium consists of Nonkeratinized stratified squamous, for abrasion resistance.

Oral Cavity Boundaries

• The oral cavity is bounded by:
  • Teeth and lips (anteriorly)
  • Oropharynx (posteriorly)
  • Hard and soft palates (superiorly)
  • Mylohyoid muscle covered with a mucous membrane (inferior surface)
• The tongue attaches to the cavity floor

Regions of the Oral Cavity

• The oral cavity has:
  • The vestibule, the space between cheeks/lips and gums
  • The oral cavity proper, located central to the alveolar processes of the mandible and maxillae

Epithelial Lining

• The oral cavity's epithelial lining consists of nonkeratinized stratified squamous epithelium.
• This lining protects against abrasive activities, and is continuously moistened by saliva.

Cheeks

• Lateral walls of the oral cavity.
• They are covered with skin, and contain buccinator muscles.
• Cheeks compress against teeth during chewing.
• Cheeks also hold solid materials in place.

Lips

• Form the anterior wall of the oral cavity.
• They are mainly composed of the orbicularis oris muscle, and covered with keratinized stratified squamous epithelium.
• Lips appear reddish due to superficial blood vessels.
• Closes the oral cavity during chewing to hold food.
• The labial frenulum connects lips to the gingivae (gums) .

Hard Palate

• Forms the anterior two-thirds of the oral cavity's "roof".
• It's a bony structure consisting of:
  • Palatine processes of maxillae.
  • Horizontal plates of palatine bones.
• Covered with dense connective tissue and nonkeratinized stratified squamous epithelium.
• Transverse palatine folds (friction ridges) are present.
• Separates the oral cavity from the nasal cavity.
• Assists in tongue movement during swallowing.

Soft Palate

• The posterior one-third of the palate.
• Primarily skeletal muscle with nonkeratinized stratified squamous epithelium.
• The uvula extends inferiorly from the posterior part.
• It's elevated during swallowing to close off the nasopharynx.
• Prevents food and liquid from entering the nasal cavity, along with the uvula.

Fauces

• An opening between the oral cavity and the oropharynx.
• Houses palatine tonsils between arches.
• Allows the bolus to enter the pharynx.
• Its palatine tonsils monitor antigens in food/drink, initiating immune responses.

Tongue Overview

• Is an accessory digestive organ that is critical for digestion.
• Is highly mobile and muscular, also used for multiple processes.

Tongue Anatomy

• is primarily composed of skeletal muscle with stratified squamous epithelium.
• The superior (dorsal) surface has papillae (small projections).
• Filiform papillae are keratinized, providing texture and abrasion resistance instead of taste buds.
• Other papillae types (fungiform, vallate) are nonkeratinized and contain taste buds.
• The inferior surface connects to the oral cavity floor via the lingual frenulum.
• The posteroinferior surface contains lingual tonsils for immune response.

Tongue Functions in Digestion

• Functions include:
  • manipulation and mixing ingredients with saliva to form a bolus
  • pushing the bolus towards the oropharynx to trigger swallowing.
• Papillae (excluding filiform) identify sensory input.

Additional Roles

• Assists with sound articulation.
• Lingual tonsils monitor ingested materials providing an immune response.

Tongue Histology and Clinical Insight

• The epithelium is keratinized over filiform papillae for abrasion resistance.
• It is nonkeratinized for flexibility and taste.
• Muscle layers are vital for digestion and speaking.
• Ankyloglossia, or "tongue-tied",can impair speech and swallowing.

Salivary Gland Overview

• Are collective producers of saliva, and thus initiate digestion.
• Daily secretion ranges from 1.0–1.5 liters, mostly during mealtime.

Saliva Composition

• 99.5% percent water. This is a primary ingredient, and moistens food and oral structures
• 0.5% solutes, with amylase to initiate carbohydrate digestion.
• also contains lysozyme, antibodies,and ions.
• Saliva's pH typically ranges from 6.4-6.8.

Saliva Functions

• Moistens food into a semisolid bolus for swallowing.
• Has lubricating propertizes
• Initiates chemical digestion of carbohydrates via amylase.
• Helps stimulate taste receptors

Salivary Gland Types

• The two categories are:
  • Intrinsic Salivary Glands:
    • They are unicellular and located within the oral cavity.
    • They secrete lingual lipase to digest lipids.
  • Extrinsic Salivary Glands: -Are multicellular, exocrine, and external to the oral cavity. -There are three pairs: - Parotid: Largest; secretes 25–30% saliva, serous only. Its duct connects to the upper 2nd molar. - Submandibular: Inferior to mandible, secretes most saliva (60 - 70%), a mixed fluid, to ducts at the lingual frenulum. - Sublingual: Smallest; secretes 3–5% mixed saliva. Its multiple ducts connect to the oral floor.

Salivary Gland Histology

• Secretory cells come in two types:
  • Mucous cells: these secrete mucin for mucus production
  • Serous cells: secrete watery fluid with ions, lysozyme, and amylase.
• Proportion varies: Parotid glands secrete serous fluid only, whereas others (submandibular/sublingual) produce mixed fluids.
• Parasympathetic influence stimulates secretion (e.g., "mouth waters" at the sight of food).
• Sympathetic influence inhibits secretion (e.g., dry mouth in fight-or-flight).
• “Dry mouth” (inadequate saliva) can affect the quality of oral hygiene.

Teeth Overview

• Collectively known as the dentition.
• They are responsible for mastication.

Teetch Functions

• Break down food physically during chewing.
• Prepare food for chemical digestion in the stomach and intestines.

Teeth Development and Eruption

• Deciduous Teeth: 20 Erupt 6-30 months after birth, eventually lost and replaced by permanent teeth.
• Permanent Teeth: 32 Erupt anteriorly to posteriorly (except first molars at ~6 years).
• Third Molars: (Wisdom Teeth): Last to erupt (late teens/early 20s), often impacted and requiring extraction.

Permanent Teeth

• Incisors:
  • Anterior, chisel-shaped.
  • Functions for slicing/cutting food.
• Canines (Cuspids):
  • Posterolateral to incisors.
  • Functions for puncturing/tearing.
• Premolars (Bicuspids):
  • Posterolateral to canines.
  • Functions for crushing/grinding.
• Molars:
  • Posterior, large/flat crowns.
  • Functions for grinding and crushing food.
• Each jaw quadrant has 2 incisors, 1 canine, and 2

Pharynx Overview and Anatomy

• A shared space for both digestive and respiratory systems.
• Conducts swallowed materials from the oral cavity to the esophagus.
• Its location is posterior to the oral cavity, extending from the nasopharynx to the laryngopharynx.

Pharynx Epithelium

• The epithelium is lined with nonkeratinized stratified squamous epithelium, for protection from abrasion.

Pharynx Regions

• Includes:
  • Nasopharynx: A respiratory area.
  • Oropharynx: Permits digestive entry.
  • Laryngopharynx: Connects to the esophagus and larynx.

Pharynx Structure and Function

• Pharyngeal Constrictors ( three pairs) form the pharyngeal wall. - Superior Pharyngeal Constrictor: Begins contraction.
  • Middle Pharyngeal Constrictor: Continues the transfer by moving downward.
  • Inferior Pharyngeal Constrictor: Completes transfer to the esophagus.
  • Reduction of pharynx diameter during sequential swallowing.
• It takes about 1 second to push the bolus to the esophagus.
• It coordinates with the epiglottis to prevent the bolus entry into the larynx.

Pharynx Innervation

• Vagus nerves (CN X) provide the innervation.
• This innervation is responsible for control during the swallowing reflex.
• Coordination for swallowing occurs in the medulla oblongata.
• It protects and supports the airway by withstanding mechanical stress.
• Disorders like dysphagia is known as pharyngeal constrictor dysfunction.

Peritoneum, Peritoneal Cavity, and Mesentery

• Has moist serous membrane lining to support GI organs
• This supports organs by;
  • Reducing friction.
  • Allows movement.
  • Stabilizes intraperitoneal structure

Peritoneum Layers

• Includes - Parietal Peritoneum: internal surfact - Visceral Peritoneum: Outer surface

Intra/Retroperitoneal Organs:

• Surrounded by intraperitoneal Organs, examples; -Stomach - Small intestine - Large intestine

• These have mobilitly in position

• Retroperitoneal Organs lay against posterior abdominal, examples; - Most of the duodenum -Pancreas

• Less mobile in position

Functions and Types

• Stabilize and anchors organs.
• Contain blood vessels, lymph vessels, and nerves
• Contains various ligaments and mesenteries - Greater Omentum covers abdominal organs. - Lesser Omentum connects the liver to the stomach.

• Fan-shaped fold that suspends from posterior wall.
• Holds the attached large intestine: - transverse mesocolon - sigmoid mesocolon
• Contains Peritoneal Ligametns: - coronar, connecting liver to the diaphragm. - Falciform, connects anterior wall.

Histology/Description

• Tissue that runs through GI tract, includes: - Epithelium - Lamina propria. - Thin Layer of Smooth Muscle
• Submocosa: Connectice tissue.
• Muscularis contains inner and outer layer
• Adventitia/Serosa made of connective tissue

Description/Function

• 25 cm (10 inches) in length
• Passes throgh diaphragm to connect with the stomach.
• Is in thorax, although only 1.5 cm is in abdomen.

Tissue types in the Esophagus

• Contains Nonkertinized/Straified squampous tissues
• submucosa: Elastic fibers
• Muscularis includes Skeletal and smooth muscle.
• Contain Inferior and superior muscles with circular skeletal muscles
• Function is to transport bolus through peristalsis.

Voluntary Phase

• Occurs during conscious initiation in the oral cavity.
• Tongue and saliva mix to begin mechanical digestion and compresses them into a Bolus
• Pushes food down the mouth to begin swallowing process

Involuntary Phase

• Reflex triggered by bolus towards oralpharynx
• Soft palate elavates, sealing the the pharynx, to prevent fluids toward the nasal area.
• The diameter propels to propel the throat to propell food towards the stomach

Esophageal Phase

• Transports thru the esophagus to the stomach
• Peristalsis to help begin this process
• Bolus moves aproximately 25cm toward the stomach.
• Is involuntary process to swallowing center

Stomach Overview

• J shape
• Located near diiaphram near upper quadrant
• Stores food to turn it into Chyme

Function

• Mechanical digesting to churn/mix boluses
• Chemical digesting by secreting fluids

Types

:

• Consists of cardia, located below esophagus
• Fundus located above
• Body Body is the point for secretion
• Pylorus: has Antrum that allows it to regulate the chime from upper portion