The Digestive System
The Digestive System Overview
Digestive Functions
Recall: Cells have selectively permeable membranes.
**Digestion:
Definition:** Mechanical and chemical breakdown of foods so cell membranes can absorb them.
Types:
Mechanical Digestion: Involves force and movement, with no chemical change.
Chemical Digestion: Enzymes alter molecules to smaller, simpler versions.
Alimentary Canal: A tubular part of the digestive tract connecting the mouth to the anus (also known as gastrointestinal tract).
Origin: Develops from the endoderm layer of embryos.
Accessory Organs: Release substances into the alimentary canal to aid in digestion; includes salivary glands, liver, gallbladder, and pancreas.
Organs of the Alimentary Canal
Order from mouth to anus:
Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Rectum
Anus
Alimentary Canal Structure
Tissue Structure:
Four Key Layers:
Mucosa:
Innermost layer consisting of epithelial tissue, connective tissue, and some smooth muscle; responsible for absorptive and secretory functions.
Submucosa:
Comprised of loose connective tissue, contains glands, and vessels; facilitates movement of materials to and from digestive tissues.
Muscularis:
Made up of circular and longitudinal smooth muscle; functions in movement, where circular muscles shorten diameter and longitudinal muscles shorten length.
Serosa:
Outer layer consisting of epithelial and connective tissue; secretes serous fluid to protect organs.
Alimentary Canal Innervation
Innervation:
Controlled by branches of the autonomic nervous system.
Control of Digestion:
Regulated by both nervous and endocrine systems.
Oversees contraction and secretion.
Sensory Neurons:
Respond to stretch, pH, nutrient presence, and solute concentration.
Nervous System:
Parasympathetic Division: Excites digestion.
Sympathetic Division: Inhibits digestion.
Enteroendocrine Cells:
Located in the stomach and small intestine; secrete chemical messengers that regulate digestive processes.
Muscular Digestion and Movement in the Alimentary Canal
Types of Movement:
Mixing:
Achieved by alternating contractions, primarily occurring in the small intestine.
Segmentation: An alternating contraction and relaxation of smooth muscle segments that aid in mixing.
Propelling:
Achieved through peristaltic movements.
Peristalsis: A wave of contraction that moves progressively along the length of the tube, pushing contents forward.
Initial Digestion: The Mouth and Its Structures
The Mouth:
Entry point for ingestion; first part of the alimentary canal.
Mastication:
The initial mechanical and chemical digestion through chewing.
Cheeks:
Form the lateral walls of the mouth, composed of muscle.
Lips:
Mobile structures containing skeletal muscle and sensory receptors.
Tongue:
A muscular organ with papillae.
Lingual Papillae:
Rough, bumpy projections that house taste buds.
Palate:
The roof of the mouth consisting of:
Hard Palate:
Anterior bony region.
Soft Palate:
Muscular posterior section extending into the uvula.
The Teeth
Teeth Overview:
Not Part of the Skeletal System
Development:
Humans develop both primary (baby) and secondary (permanent) teeth.
Dentition:
Refers to the shape, size, and number of teeth indicative of an omnivorous diet.
Cuspids/Canines:
Function to grasp and tear food, associated with carnivorous diets.
Molars:
Designed for grinding food into smaller particles.
Structure of a Tooth
Tooth Components:
Crown:
The part projecting beyond the gum, covered with enamel.
Enamel:
Hard outer covering made of calcium salts.
Dentin:
Living, bone-like tissue that encases the pulp cavity.
Root:
Anchored to the alveolar processes of the jaw; contains the bulk of the pulp, which consists of blood vessels, nerves, and connective tissue.
Cementum:
A tissue covering the root, which together with the periodontal ligament, facilitates anchoring of the tooth within the jaw.
The Salivary Glands
Major Salivary Glands:
Submandibular Gland:
The largest, located anterior to and below the ears.
Secretions: Rich in salivary amylase; secretory cells are primarily serous.
Sublingual Gland:
The smallest, located on the floor of the mouth beneath the tongue.
Secretions: Thick and stringy mucus; secretory cells are primarily mucous.
Parotid Gland:
Medium-sized, located inside the mandible on the floor of the mouth.
Secretions: More viscous than submandibular; contains both serous and mucous secretory cells.
The Pharynx
Pharynx (Throat) Parts:
Nasopharynx:
Communicates with the nasal passage.
Oropharynx:
Located posterior to the oral cavity, below the nasopharynx; functions in passing both food and air.
Laryngopharynx:
Below the oropharynx and posterior to the larynx; connects to the esophagus.
Swallowing Process:
The bolus is pushed by the tongue into the oropharynx.
A complex swallowing reflex engages, causing the epiglottis to close off the trachea.
Food is propelled through the esophagus into the stomach by peristalsis.
The Esophagus
Structure:
A straight, collapsible tube that provides a passageway for food.
Function:
Propels food from the pharynx to the stomach.
Mucous Glands:
Lubricate the inner walls to aid in passing the bolus.
Lower Esophageal Sphincter:
A smooth muscle positioned above the stomach opening, which primarily remains contracted and is relaxed by peristalsis from the esophagus.
Vomiting:
A reflex initiated primarily from the stomach; relaxes the sphincter, closes the trachea, and squeezes food from the stomach.
The Stomach
Parts of the Stomach:
Cardia:
Small area near the esophageal opening.
Fundus:
A ballooning area that acts as a temporary storage site.
Body:
Main part of the stomach located between the fundus and pylorus.
Pylorus:
Funnel-shaped portion that narrows and connects to the small intestine.
Pyloric Sphincter:
A contractile ring that controls gastric emptying into the small intestine.
Stomach Phases of Gastric Secretion
Gastric Juice:
A mixture of mucus, hydrochloric acid, and enzymes, including pepsin, intrinsic factor, and gastric lipase.
Phases of Secretion:
Cephalic Phase:
Initiated by parasympathetic reflexes stimulated by food-related stimuli, leading to gastric secretions.
Gastric Phase:
Triggered by the presence of food and stomach distension, which releases gastrin and stimulates further gastric secretions.
Intestinal Phase:
When food moves into the small intestine, it triggers a sympathetic reflex that inhibits gastric secretions.
The Small Intestine
Three Parts:
Duodenum
Jejunum: Most vascularized portion.
Ileum
Functionality:
Supported by the mesentery.
Intestinal Villi:
Projections of mucosa into the lumen that enhance absorption, most prominent in the duodenum and part of the jejunum.
Microvilli:
Present on villus cells and significantly increase surface area for absorption.
Chemical Digestion:
Completes chemical digestion and absorbs nutrients.
Secretes enzymes that digest everything except nucleic acids.
The Large Intestine
Overview:
Has a larger diameter than the small intestine.
Comprised of the cecum, colon, rectum, and anal canal.
Function:
Walls lack villi and have limited digestive functions.
Absorbs water and electrolytes.
Intestinal flora break down molecules that escape human enzymes.
Facilitates the passage of feces.
Accessory Organs: The Pancreas
Structure:
Composed of pancreatic acinar cells and pancreatic ducts.
Regulation:
Secretions are regulated by the pH of the duodenum.
Support of Digestive System:
Pancreatic Juice Components:
Pancreatic amylase
Pancreatic lipase
Proteolytic enzymes
Nucleases
Accessory Organs: The Liver
Functional Unit:
Hepatic Lobules: Composed of many hepatocytes.
Hepatic Secretions:
Transported to the common hepatic duct.
Primary Functions:
Detoxification of blood.
Synthesis of fats from excess carbohydrates.
Protein metabolism.
Storage of glycogen, iron, and vitamins.
Bile Secretion:
Composed of water, bile salts, pigments, cholesterol, and electrolytes; aids digestion and enhances fat metabolism and absorption.
Accessory Organs: The Gallbladder
Structure:
A saclike organ associated with the liver, primarily for the storage and regulation of bile release.
Health Implications:
Cholesterol can precipitate out of solution, forming gallstones, which may block bile flow, resulting in pain and jaundice.
Bile Duct Connection:
Links the cystic duct and common hepatic ducts, playing an essential role in bile transportation.