chapter 17

Chapter 17: Digestive System

17.1 General Characteristics of the Digestive System

  • Digestive System: Composed of the alimentary canal and accessory organs.
      - Alimentary Canal Components:
        - Mouth
        - Pharynx
        - Esophagus
        - Stomach
        - Small Intestine
        - Large Intestine
        - Anal Canal
      - Accessory Organs:
        - Salivary Glands
        - Liver
        - Gallbladder
        - Pancreas

Structure of the Alimentary Canal Wall

  • Layers:
      - Mucosa:
        - Composed of a simple columnar epithelium, villi, microvilli, and intestinal glands.
        - Contains goblet cells that secrete mucus.
      - Submucosa: Contains nerve plexuses, blood vessels, and lymphatic nodules.
      - Muscularis: Composed of circular and longitudinal muscle layers.
      - Serosa: Outer lining of the alimentary canal.

Movements of the Alimentary Canal Wall

  • Mixing Movements: Peristalsis and segmentation involve both muscle layers to promote mixing and movement of contents.

Innervation of the Alimentary Tube

  • Submucosal Plexus: Regulates secretory functions of the gastrointestinal (GI) tract.

  • Myenteric Plexus: Coordinates GI motility.

  • Nervous System Influence:
      - Parasympathetic: Enhances digestive activities.
      - Sympathetic: Inhibits digestive activities.

  • Enteroendocrine Cells: Located in the stomach and small intestine, they secrete hormones to regulate GI function.

17.2 Mouth

  • Initial Segment of Alimentary Canal:
      - Function: Mechanical breakdown of food through mastication; also facilitates speech and sensory reception.

Cheeks and Lips

  • Role: Assist in holding food within the oral cavity during mastication.

Tongue

  • Description: Thick, muscular organ located on the floor of the mouth.
      - Lingual Frenulum: Connects the tongue to the floor of the mouth.
      - Papillae: Projections on the surface that contain taste buds and help in food manipulation.
      - Anchorage: Connected to the hyoid bone.
      - Lingual Tonsils: Lymphatic tissue masses located at the root of the tongue.

Palate

  • Roof of Oral Cavity:
      - Hard Palate: Anterior section, formed by bone.
      - Soft Palate: Posterior section, muscular and flexible.
      - Palatine Tonsils: Located on the sides of the tongue.
      - Pharyngeal Tonsils (Adenoids): Located in the posterior wall of the pharynx.

Teeth

  • Development: Form in the alveolar processes of the maxillary and mandibular bones.
      - Function: Break food into smaller pieces initiating mechanical digestion.
      - Types:
        - Primary Teeth: 20 (deciduous, baby teeth).
        - Secondary Teeth: 32 (permanent, adult teeth).

Types of Teeth

  • Incisors: Sharp edges for cutting.

  • Canine (Cuspids): Pointed for tearing.

  • Premolars (Bicuspids): Flat surfaces for grinding.

  • Molars: Larger flat surfaces for crushing food.

Tooth Structure

  • Crown: Part of the tooth that extends above the gum line.

  • Root: Embedded in the jawbone.
      - Enamel: Hard outer covering of the crown.
      - Dentin: Living tissue beneath enamel.
      - Root Canal: Contains nerves and blood vessels.
      - Cementum: Covers the root and connects it to surrounding structures.
      - Periodontal Ligament: Attaches tooth to the alveolar bone.

17.3 Salivary Glands

  • Function: Produce saliva for digestion and oral health.
      - Saliva: Moistens food and contains enzymes for digestion.
        - Enzymes: Initially break down carbohydrates.
        - Bicarbonate Ions: Help to maintain pH balance in the mouth.

Salivary Secretions

  • Different types of salivary glands consist of:
      - Serous Cells: Secrete a watery fluid rich in enzymes.
      - Mucous Cells: Secrete mucus.

Innervation of Salivary Glands

  • Sympathetic Activation: Leads to a small quantity of thick saliva.

  • Parasympathetic Activation: Results in a larger quantity of watery saliva. This response can be triggered by the thought or smell of appealing food.

Major Salivary Glands

  • Parotid Gland: Ducts into the oral cavity from the cheek area.

  • Submandibular Gland: Located beneath the mandible with ducts under the tongue.

  • Sublingual Gland: Located under the tongue for saliva secretion.

17.4 Pharynx and Esophagus

  • Pharynx Function: Facilitates swallowing and is divided into three regions:
      - Nasopharynx: Air passage, located behind the nasal cavity.
      - Oropharynx: Located behind the oral cavity, serves both air and food.
      - Laryngopharynx: Located behind the larynx, leads to the esophagus.

Muscles of Pharyngeal Wall

  • Superior Constrictor: Forms the upper part of the pharynx.

  • Middle Constrictor: Aids in swallowing.

  • Inferior Constrictor: Squeezes food down into the esophagus.

Swallowing Mechanism

  • Process Overview:
      - (a) Tongue pushes food into the pharynx.
      - (b) Soft palate raises, blocking nasal passage, and epiglottis closes to prevent food from entering the airway.
      - (c) The contraction of pharyngeal constrictor muscles pushes food into the esophagus.
      - (d) Peristaltic waves push food to the stomach.

Esophagus

  • Anatomy: Mucous glands in the submucosa; muscularis consists of three sections:
      - Superior third: Skeletal muscle.
      - Middle third: Both skeletal and smooth muscle.
      - Inferior third: Smooth muscle.

  • Function: Lower esophageal (cardiac) sphincter regulates food entry into the stomach and prevents reflux.

17.5 Stomach

  • Role in Digestion:
      - Serves as the primary site for storage and processing of food with enzymatic breakdown.

  • Regions:
      - Fundus: Upper part of the stomach above the entry point.
      - Body: Main central region.
      - Cardia: Area surrounding the opening from the esophagus.
      - Pylorus: Connects stomach to the small intestine.

Parts of the Stomach

  • Pyloric Sphincter: Regulates the passage of chyme into the small intestine.

  • Rugae: Folds in the stomach lining that allow expansion.

Gastric Secretions

  • Gastric Pits: Small depressions in the stomach lining that contain various cell types.

  • Types of Secretory Cells:
      - Mucous Neck Cells: Secrete mucus to protect the lining.
      - Chief Cells: Produce pepsinogen, which is converted to pepsin, an enzyme essential for protein digestion.
      - Parietal Cells: Secrete hydrochloric acid (HCl) for protein digestion and intrinsic factor for vitamin B12 absorption.
      - Enteroendocrine Cells: Secrete hormones that regulate functions of the digestive system.

Regulation of Gastric Secretions

  • Neural Control: Involves the parasympathetic nervous system to stimulate secretion in response to food presence.

Three Stages of Gastric Secretions

  • Cephalic Phase: Triggered by the sight, taste, smell, or thought of food leading to secretion of gastric juice.

  • Gastric Phase: Presence of food in the stomach activates a response for additional gastric juice secretion via gastrin hormone release.

  • Intestinal Phase: Initiated as food enters the small intestine, leading to slight gastric juice secretion due to intestinal gastrin.

Gastric Absorption

  • The primary site for nutrient absorption is the small intestine; however, the stomach also absorbs:
      - Some water
      - Certain salts
      - Certain lipid-soluble drugs
      - Some alcohol

Mixing and Emptying Actions

  • Pyloric Sphincter Role: They manage the entry of chyme into the small intestine during digestion.

Enterogastric Reflex

  • Triggered when chyme stretches the walls of the small intestine, leading to decreased stomach emptying and increased intestinal filling.