Human Anatomy Digestive System Quiz

Anatomy & Physiology FT 400/PT700

• Course focusing on anatomy and physiology.

Chapter 24: Digestive System Part 1

• Lecture outline covers functions of the digestive tract, overview of major digestive system organs, layers of the GI tract, the peritoneum, neural innervation of the GI tract, and the GI tract (oral cavity, pharynx, esophagus).

Digestive System Anatomy

• Consists of two main parts:
  • Gastrointestinal (GI) Tract: A muscular tube (also called the digestive tract or alimentary canal) that moves food from mouth to anus.
  • Accessory Organs: Produce or store secretions that flow into the GI tract through ducts. These organs aid in digestion.

Functions of the Digestive Tract

• Ingestion: Solid and liquid food enter the oral cavity.
• Mechanical Digestion: Crushing and shredding of food (in oral cavity and stomach), mixing and grinding (also involves swallowing and mixing, peristalsis).
• Chemical Digestion: Enzymatic breakdown to small organic molecules (digestive epithelium absorbs these).
• Secretion: Release of fluids (water, acids, enzymes, buffers, salts) from tract epithelium and accessory organs.
• Absorption: Nutrients are moved across digestive epithelium and into the bloodstream.
• Defecation: Undigested material is compacted into feces and eliminated from the body.

Functions of the Digestive Tract - Mechanical Digestion

• Includes all movements that help digest food
  • Mastication (chewing)
  • Swallowing
  • Mixing
  • Peristalsis: muscular contractions in the wall of the GI tract Physically break down food, propel it along the tract, and help dissolve foods by mixing them with fluids secreted into the tract.

Functions of the Digestive Tract - Chemical Digestion

• Enzymes secreted by accessory organs/cells that line the tract break down food chemically
• Primarily accomplishes this through hydrolysis (using water to break chemical bonds) to achieve the following;
  • Breakdown fats into fatty acids and glycerol.
  • Breakdown carbohydrates into monosaccharides.
  • Breakdown proteins into amino acids.

Digestive Tract Overview

• A ten-meter (33-foot) muscular tube beginning at the mouth and ending at the anus.
• Major Organs include the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine.
• Accessory organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Accessory Organs Overview

• Include organs that produce or store secretions;
  • Salivary glands: Produce saliva with mucins and enzymes.
  • Gallbladder: Stores and concentrates bile from the liver.
  • Pancreas: Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete various hormones.
  • Liver: Almost 200 known functions (e.g., bile production, metabolism, detoxification).

Layers of the GI Tract

• Four primary layers from esophagus to anal canal:
  • Mucosa: Inner lining with epithelium, lamina propria (connective tissue), and muscularis mucosae (smooth muscle).
  • Submucosa: Layer of connective tissue (dense irregular), blood & lymphatic vessels, and exocrine glands (secrete buffers/enzymes).
  • Muscularis Layer: Smooth muscle (inner circular, outer longitudinal) for mechanical digestion.
  • Serosa/Adventitia: Outermost layer (serosa is a covering of mesothelium; adventitia is dense irregular connective tissue)

Layers of the GI Tract - Mucosa

• Three layers: epithelium, lamina propria, muscularis mucosae.
• Epithelium varies throughout tract (e.g., nonkeratinized stratified squamous in the mouth, simple columnar in intestines).
• Lamina propria is connective tissue, contains blood/lymphatic vessels and mucosa-associated lymphatic tissue (MALT).
• Muscularis mucosae is smooth muscle, creating folds to increase surface area.

Layers of the GI Tract - Submucosa

• Dense irregular connective tissue layer with blood vessels, lymphatic vessels, and exocrine glands.
• Exocrine glands secrete buffers and enzymes into the digestive tract.
• Contains a network of neurons called the submucosal (Meissner's) plexus.

Layers of the GI Tract - Muscularis Layer

• Primarily smooth muscle in inner circular and outer longitudinal layers.
• Involved in mechanical processing and movement along the tract.
• Contains the myenteric (Auerbach's) plexus between smooth muscle layers.

Layers of the GI Tract - Serosa

• Outermost layer which is the visceral peritoneum.
• A serous membrane.
• Lines the abdominal cavity.
• Connects the GI tract to adjacent structures via adventitia (dense connective tissue).

Peritoneum

• Largest serous membrane in the body.
• Divided into parietal and visceral peritoneum.
• Parietal lines the inner surface of the peritoneal cavity; visceral covers organs within the cavity (visceral peritoneum is also known as serosa).
• Space between layers contains lubricating serous fluid.

Peritoneum - Continued

• Serous membrane continuously secreting peritoneal fluid into the peritoneal cavity.
• Normal volume is ~50 mL.
• Thin layer separates parietal and visceral surfaces and permits smooth sliding movements without friction.
• Inflammation in this cavity may lead to abdominal swelling, known as ascites.

Peritoneum (Folds)

• Contains three major peritoneal folds: mesentery, omenta, and falciform ligament

Peritoneal Folds - Mesentery

• Double sheets of peritoneal membrane with areolar tissue between mesothelial layers, providing access routes for blood vessels, nerves, and lymphatics.
• Stabilizes attached organs.
• Prevents entanglement of intestines.
• Includes the mesocolon (mesentery of the colon)

Peritoneal Folds - Omenta

• Continuations of the mesentery; includes the greater omentum and lesser omentum.
• The greater omentum is a large pouch extending inferiorly, containing adipose tissue for padding and protection. Also contains blood vessels and lymphatic tissue.
• The lesser omentum suspends the stomach and duodenum from the liver and provides a pathway for blood vessels and other structures entering/leaving the liver (the hepatic portal vein, hepatic artery, and common bile duct).

Peritoneal Folds - Falciform Ligament

• Connects the liver to the anterior abdominal wall.
• Is the only digestive organ attached to the anterior abdominal wall.
• Remnant of the umbilical vein.

The Oral Cavity

• Space containing the tongue, teeth, and gums.
• Lined by stratified squamous epithelium (oral mucosa).
• The oral cavity boundaries include a hard palate, formed by the palatine processes of the maxillary bones and the horizontal plates of the palatine bones, and a soft palate, the muscular posterior section.
• Anterior and Lateral Boundary involves the cheeks and lips (labia).
• The inferior boundary is the body of the tongue.
• The posterior boundary comprises the uvula, and palatine tonsils.
• The oral cavity also contains salivary glands.

The Oral Cavity - Salivary glands

• Parotid glands: Secrete a serous secretion containing large amounts of salivary amylase which starts the chemical digestion of carbohydrates. Accounts for about 25% of total saliva.
• Submandibular glands: Secrete aqueous fluids with buffers and mucins. Secrete salivary amylase. Accounts for about 70% of total saliva.
• Sublingual glands: Produce a mucous secretion acting as a buffer and lubricant; provide ~5% of total saliva.

The Oral Cavity - Saliva

• Mostly water (~99.5%).
• Contains ions (Na+, K+, Cl-, HCO3-, PO42-), dissolved gases, urea, uric acid, mucous, immunoglobulins (IgA), lysozyme, and salivary amylase.
• Acts on starches and continuously flushes oral surfaces.
• Maintains a pH near 7.0 to safeguard against bacterial acid buildup.
• Contains antibodies and lysozyme.
• Mixes with food to form a bolus. Small glands (Ebner's glands) secrete lingual lipase for lipid digestion.

The Oral Cavity - Salivation

• Controlled by the autonomic nervous system.
• Parasympathetic stimulation increases saliva secretion.
• Sympathetic stimulation decreases saliva secretion, which is also influenced by the taste and smell of food (“cephalic phase” of digestion).

Teeth

• Accessory digestive organs.
• Dentin forms most of the tooth (harder than bone).
• Covered by enamel (even harder).
• Deciduous teeth (baby teeth).
• Permanent (secondary teeth)
• Have different components in different regions of the jaw. These vary in size, shape, and function, including occlusal surface, crown, neck, and root.

Teeth - Components of a tooth

• Occlusal surface: Portion of the crown used for crushing, slicing, or chewing.
• Enamel: Covers the dentin of the crown (hardest manufactured substance), composed of calcium phosphate, and requires vitamin D for formation and decay resistance.
• Pulp Cavity: Inner chamber of the tooth containing a mineralized matrix similar to bone.
• Root canal: Narrow tunnel within the root containing passageways for blood vessels and nerves.
• Apical foramen: Opening into the root canal.
• Periodontal ligament: Creates gomphosis, an articulation between root dentin and alveolar bone.

Teeth - Regions of a Tooth

• Crown: Part projecting into oral cavity from gum surfaces.
• Neck: Boundary between crown and root.
• Root: Part below gum line, residing within the bony socket (alveolus).

Digestion in the Mouth

• Mechanical: Chewing (mastication), tongue manipulating food, teeth grinding, mixing with saliva, to form the bolus (soft, flexible mass).
• Chemical: Salivary amylase acting on starches, and lingual lipase secreted by lingual glands acting on triglycerides.

Pharynx

• Membrane-lined cavity behind the nose and mouth.
• Skeletal muscle walls play a crucial role in swallowing (with esophagus). Common pathway for food, liquid, and air.
• Three regions: Nasopharynx (respiration only), Oropharynx, and Laryngopharynx.

The Esophagus

• Hollow, muscular tube (~25 cm long, 2 cm wide).
• Carries food/liquids to stomach.
• Narrowest point behind cricoid cartilage.
• Descends behind trachea.
• Enters abdominal cavity through esophageal hiatus in diaphragm. Hiatal hernia occurs when part of the stomach protrudes into the esophagus.

The Esophagus - Histology

• Muscular layers: Superior third is skeletal muscle, middle third is mixed skeletal and smooth muscle, inferior third is smooth muscle.
• Mucosa is nonkeratinized stratified squamous epithelium for protection.

The Esophagus - Control of movement

• Upper esophageal sphincter: Band of smooth muscle (functions as sphincter). Prevents air entering the esophagus.
• Lower esophageal sphincter (cardiac sphincter): At the inferior end of the esophagus; normally contracted to prevent backflow from the stomach.

Swallowing (Deglutition)

• Three phases: Buccal phase, Pharyngeal phase, Esophageal phase.
• Buccal (voluntary): Compression of bolus against hard palate. Tongue forcing into oropharynx, elevating soft palate to seal nasopharynx.
• Pharyngeal (involuntary): Tactile stimulation of uvula, palatine arches triggering swallowing reflex. Larynx elevated; epiglottis folded; uvula and soft palate raised, moving bolus to esophagus.
• Esophageal (involuntary): Bolus forced into esophagus; peristalsis pushes it downward toward stomach. Lower esophageal sphincter opens, bolus enters the stomach.

Peristalsis

• Wave of smooth muscle contractions in the GI tract wall.
• Propels bolus in one direction along the tract.
• Circular (behind) and longitudinal (ahead) muscles contract sequentially.

GI Pathology

• Discusses periodontal disease (most common cause for tooth loss).
• Results in gingival (gum) inflammation when dental plaque forms between teeth and gums.
• Potentially linked to cardiovascular diseases (like inflammation and atherosclerosis).

Review Questions

• Students are asked to consider and answer review questions related to the components of the GI tract, and accessory digestive organs, and specific layers.