Digestive System: Structure and Function (ANP1115 Lecture 10 Study Notes)

Digestive System Overview

General Anatomical Features

The digestive system is responsible for providing the body with a steady supply of nutrients, water, and inorganic ions. It consists of the alimentary canal (gastrointestinal tract) and accessory digestive organs. The tract itself is a continuous tube open at both ends, making its lumen continuous with the external environment.

• Main organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine

• Accessory structures: Teeth, tongue, salivary glands, pancreas, liver, gallbladder

• Accessory organs assist with mechanical digestion or secrete substances (e.g., saliva, bile, enzymes) to aid digestion.

Major Digestive Processes

Six key processes occur in the digestive system, each with specific locations and functions:

• Ingestion: Taking food into the mouth

• Propulsion: Moving food through the tract (includes swallowing and peristalsis)

• Mechanical breakdown: Chewing, churning, and segmentation to increase surface area

• Digestion: Enzymatic breakdown of food into absorbable molecules

• Absorption: Transport of digested end products into blood or lymph

• Defecation: Elimination of indigestible substances as feces

Motility Patterns

Two primary motility patterns move and mix food in the digestive tract:

• Peristalsis: Adjacent segments of the tract alternately contract and relax, propelling food distally.

• Segmentation: Nonadjacent segments contract and relax, mixing food and breaking it down mechanically.

Peritoneum and Mesenteries

Peritoneum

The peritoneum is a double-layered serous membrane associated with the abdominal organs:

• Visceral peritoneum: Covers external surfaces of digestive organs

• Parietal peritoneum: Lines the body wall of the abdominopelvic cavity

• The space between layers contains serous fluid to reduce friction

Mesenteries

A mesentery is a double layer of peritoneum that extends from the body wall to digestive organs. It:

• Provides a route for blood vessels, lymphatics, and nerves

• Holds organs in place

• Stores fat

Retroperitoneal and Intraperitoneal Organs

Most digestive organs are intraperitoneal (suspended by mesenteries), but some (e.g., pancreas, duodenum) are retroperitoneal, having lost their mesentery during development.

Blood Supply and Splanchnic Circulation

The digestive system requires an extensive blood supply for nutrient absorption and transport. The splanchnic circulation includes arteries branching from the aorta (hepatic, splenic, left gastric, mesenteric arteries) and the hepatic portal circulation, which delivers nutrient-rich blood from the digestive organs to the liver for processing.

Histological Organization of the Alimentary Canal

Four Basic Layers (Tunics)

The wall of the alimentary canal consists of four main layers:

• Mucosa: Innermost layer; functions in secretion, absorption, and protection

• Submucosa: Connective tissue with blood vessels, lymphatics, nerves, and glands

• Muscularis externa: Smooth muscle responsible for peristalsis and segmentation

• Serosa: Outermost layer; visceral peritoneum (or adventitia in some regions)

Mucosa

• Epithelium: Stratified squamous (mouth, esophagus, anus) or simple columnar (stomach, intestines); contains goblet cells that secrete mucus

• Lamina propria: Loose connective tissue with capillaries and MALT (mucosa-associated lymphoid tissue)

• Muscularis mucosae: Thin layer of smooth muscle for local movements

Submucosa

• Dense connective tissue with blood and lymphatic vessels, nerves, and glands

• Contains the submucosal plexus (nerve network regulating glands and smooth muscle)

Muscularis Externa

• Typically two layers: inner circular and outer longitudinal smooth muscle (stomach has an additional oblique layer)

• Responsible for peristalsis and segmentation

• Contains the myenteric plexus (nerve network controlling motility)

• Thickened regions form sphincters to control passage of materials

Serosa

• Outer connective tissue covering (visceral peritoneum)

• In the esophagus and retroperitoneal organs, this layer is called the adventitia

Neural Control of Digestive Function

Enteric Nervous System

The digestive tract has its own nervous system, the enteric nervous system, consisting of the submucosal and myenteric plexuses. It regulates secretory activity, motility, and blood flow through both short (local) and long (CNS-mediated) reflexes.

The Mouth and Associated Structures

Anatomy of the Oral Cavity

The mouth (oral cavity) is the entry point for food and is lined with stratified squamous epithelium for protection. The roof is formed by the hard and soft palates, and the floor by the tongue. The soft palate rises during swallowing to close off the nasopharynx.

• Lips and cheeks: Contain skeletal muscle (orbicularis oris, buccinators) to hold food and aid speech

• Labial and lingual frenula: Folds of tissue connecting lips and tongue to the gums

The Tongue

The tongue is a muscular organ essential for manipulating food, mixing it with saliva, and forming a bolus for swallowing. It contains intrinsic muscles (change shape) and extrinsic muscles (change position).

Papillae and Taste Buds

• Filiform papillae: Conical, keratinized, provide friction

• Fungiform papillae: Mushroom-shaped, contain taste buds

• Vallate papillae: Large, V-shaped row at back of tongue, contain taste buds

• Foliate papillae: On sides of tongue, contain taste buds

Salivary Glands and Saliva

Types and Functions of Salivary Glands

There are three pairs of major (extrinsic) salivary glands and numerous minor (intrinsic) glands:

• Parotid glands: Mostly serous cells (watery secretion)

• Submandibular glands: Mixed serous and mucus cells

• Sublingual glands: Primarily mucus cells

• Intrinsic buccal glands keep the mouth moist between meals

Composition and Function of Saliva

• Mostly water (97–99.5%), slightly acidic (pH 6.75–7.00)

• Contains electrolytes, salivary amylase, lingual lipase, mucin, lysozyme, IgA, and metabolic wastes

• Functions: cleanses mouth, dissolves food for taste, moistens and compacts food, begins starch digestion

Control of Salivation

• Stimulated by parasympathetic nervous system (cranial nerves VII and IX)

• Inhibited by sympathetic stimulation and dehydration (causing dry mouth)

Teeth and Mastication

Dentition and Dental Formula

Humans have two sets of teeth:

• Primary (deciduous) teeth: 20, erupt between 6–24 months

• Permanent teeth: 32, replace primary teeth between ages 6–12

• Teeth are classified as incisors, canines, premolars, and molars based on shape and function

Dentition	Dental Formula (one side)	Total Teeth
Primary	2I, 1C, 2M	20
Permanent	2I, 1C, 2PM, 3M	32

Tooth Structure

• Crown: Exposed part, covered by enamel (hardest substance in body)

• Root: Embedded in jawbone, covered by cement, attached to periodontal ligament

• Dentin: Bone-like material under enamel, maintained by odontoblasts

• Pulp cavity: Contains blood vessels and nerves

Deglutition (Swallowing) and the Esophagus

Deglutition

Swallowing involves coordinated activity of 22 muscle groups and two main phases:

• Buccal phase: Voluntary, tongue pushes bolus into pharynx

• Pharyngeal-esophageal phase: Involuntary, controlled by the swallowing center in the medulla and lower pons, moves food through pharynx and esophagus to stomach

The Esophagus

• Muscular tube (~25 cm), collapsed when empty

• Transitions from skeletal to smooth muscle along its length

• Joins stomach at the gastroesophageal sphincter

• Lined with stratified squamous epithelium for protection

Stomach: Structure and Function

Functional Anatomy

The stomach acts as a temporary storage tank, initiates protein digestion, and converts food to chyme. It is divided into regions: cardia, fundus, body, and pyloric region. The stomach wall contains rugae (folds) that allow expansion.

Microscopic Anatomy

• Surface epithelium: Goblet cells produce mucus and a bicarbonate-rich layer for protection

• Gastric pits: Lead to gastric glands that secrete gastric juice

• Cell types:

  • Mucous neck cells: Secrete acidic mucin

  • Parietal cells: Secrete HCl (activates pepsinogen, denatures proteins) and intrinsic factor (vitamin B12 absorption)

  • Chief cells: Secrete pepsinogen (precursor to pepsin) and gastric lipase

  • Enteroendocrine cells: Secrete hormones (gastrin, histamine, serotonin, somatostatin)

Protection of the Stomach

• Thick, bicarbonate-rich mucus layer

• Tight junctions between epithelial cells

• Rapid epithelial cell turnover (renewed every 3–6 days by stem cells)