The Digestive System

The Digestive System

Introduction

• The lecture discusses the components, functions, and processes of the digestive system.
• It covers the functional divisions, accessory organs, and the detailed structure of the gastrointestinal tract (GIT) wall.

Learning Objectives

• State the key components and functions of the digestive system.
• Understand the different functional divisions of the digestive system.
• State the functions of the accessory digestive organs.
• Understand the process of digestion.
• Define in detail the layers of the gastrointestinal wall and relate the structure of each layer to the overall function of the gastrointestinal tract (GIT).
• Describe the arrangement of tissues and components within the GIT wall of the oesophagus, stomach, small and large intestine; relating structure to function.
• Describe the specialised cell types found within the stomach and the small intestine.
• Understand the types of movement available in the small intestine.

Components of the Digestive System

• Major (hollow) organs (Digestive/Gastrointestinal Tract):
  • Oral Cavity & Pharynx
  • Oesophagus
  • Stomach
  • Small Intestine
  • Large Intestine, including rectum & anus
• Accessory Organs:
  • Teeth & Tongue
  • Salivary Glands
  • Liver
  • Gallbladder
  • Pancreas

Key Functions of the Digestive System

• Converts food into simplest components for absorption or excretion.
  • Ingestion: Entry of food/liquids into digestive tract via oral cavity
  • Mechanical Digestion: Crushing/shearing of ingested food
  • Chemical Digestion: Enzymatic breakdown of food into absorbable substances
  • Absorption: Movement of nutrients into the bloodstream
  • Elimination: Indigestible food compacted into faeces and excreted

Functional Divisions of the Gastrointestinal Tract (GIT)

• The Upper GIT (food breakdown):
  • Mouth / oral cavity
  • Oesophagus
  • Stomach
  • Small intestine (duodenum only)
• The Lower GIT (absorption, waste compaction and excretion):
  • Small intestine (jejunum and ileum)
  • Large intestine, including rectum & anus

Functions of the Accessory Digestive Organs

• Teeth & Tongue:
  • Crushing/shearing of food
  • Shaping of food into bolus
• Salivary glands:
  • Secretion of enzyme (amylase) to begin breaking down carbohydrates
• Liver:
  • Secretion of bile to breakdown fats
  • Detoxification of the blood
  • Storage of iron, glucose and fat-soluble vitamins
• Gallbladder:
  • Storage of bile
• Pancreas:
  • Secretion of pancreatic enzymes to breakdown protein, fat and carbohydrates

Overview of Digestion

• Oral cavity:
  • Mechanical digestion (mastication) and mixing of food with saliva.
  • Enzymes in saliva begin the process of chemical digestion.
• Oesophagus:
  • A fibromuscular tube that transfers food (bolus) into the stomach.
• Stomach:
  • Chemical digestion: Bolus is mixed with gastric juices (hydrochloric acid and digestive enzymes).
  • Mechanical churning: Facilitates digestion.
  • Bolus is converted into chyme.
• Small intestine (duodenum):
  • Chemical digestion: Aided by digestive juices from the pancreas and gallbladder.
• Small intestine (jejunum and ileum):
  • Where the bulk of chemical digestion and nutrient absorption occurs.
• Large intestine:
  • Cecum, ascending colon, transverse colon, descending colon.
  • Reabsorption of water & some nutrients.
  • Compacts waste (faeces) for elimination.
• Rectum & anus:
  • Stores and eliminate waste (faeces).

General Structure of the GIT Wall

• From inside → out:
  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa/Adventitia

The Mucosa

• Epithelial mucous membrane and connective tissue.
• Innermost structure of the GIT wall.
• The mucosa consists of three sublayers:
  • Epithelium
  • Lamina propria (loose connective tissue)
  • Muscularis mucosa

The Mucosa: Epithelium

• Epithelium in the GIT:
  • Stratified squamous (for protection): Mouth, oesophagus, rectum & anus
  • Simple columnar (absorption): Stomach, small intestine & large intestine
• Specialised epithelial structures in the GIT:
  • Goblet cells: Secrete mucous to keep the epithelium moist
  • Microvilli: In the small intestine to increase surface area for nutrient absorption

The Mucosa: Lamina Propria

• Lamina propria (loose connective tissue)
  • Route by which nutrients are absorbed
  • Supports epithelium
  • Contains:
    • Cells, e.g. fibroblasts and macrophages
    • Blood vessels
    • Sensory nerve endings
    • Lymphatic vessels
    • Some mucous glands
  • Mucosa-associated lymphatic tissue (MALT): protection against infection
    • Specifically gut-associated, so GALT

The Mucosa: Muscularis Mucosa

• Muscularis mucosa
  • Thin layer of smooth muscle
  • Believed to cause folds to form in the mucosal layer
  • Increases local movements, thus increasing absorption of nutrients with exposure to “new” surface

The Submucosa

• A layer of loose connective tissue that provides physical support to the mucosa
• Connects the mucosa to the underlying muscularis externa
• Contains:
  • Large blood vessels
  • Submucosal glands
  • Large lymphatic vessels
  • Submucosal (Meissner’s) plexus
  • Sensory neurons and autonomic nerve fibres that innervate the glands
  • Stimulates secretion from glands

The Muscularis Externa

• Multiple muscle layers
• Organized in circular and longitudinal layers (except stomach)
• Contraction of these muscle layers is essential for mechanical processing and moving materials along the GIT
• Composition depends on location within the GIT:
  • Oesophagus: Contains inner circular and outer longitudinal layers of muscle. Upper third is skeletal muscle (voluntary), while the rest is smooth muscle (involuntary)
  • Stomach: Contains inner oblique, middle circular and outer longitudinal layers of smooth muscle
  • Small and large intestines & rectum: Contains inner circular and outer longitudinal layers of smooth muscle
  • Anus: Contains inner circular and outer longitudinal layers of skeletal muscle (voluntary)
• Also contains the myenteric plexus (Auerbach plexus)
  • A network of sensory neurons + autonomic nerve fibres located between the circular and longitudinal muscle layers
  • Coordinates digestive muscle activity (i.e. controls motility)

The Serosa / Adventitia

• Depending on the region of the GIT, the outermost layer of the GIT is either called serosa or adventitia:
  • Serosa:
    • Outermost layer of GIT organs within the abdominopelvic cavity, continuous with the peritoneum, i.e. distal oesophagus, stomach and intestines
    • Loose connective tissue
    • Covered in serous fluid to prevent friction and allow movement
  • Adventitia:
    • Outermost layer of GIT organs that lie outside of the abdominopelvic cavity, i.e. majority of the oesophagus and the anus
    • Dense (fibrous) connective tissue that anchors the organ in place

Structures of the GIT Wall: Linked to Function

• Function: Protection from abrasion
  • Stratified squamous epithelium in the oesophagus and anus
• Function: Vigorous mixing (mechanical disruption)
  • Oblique muscle layer within the stomach
• Function: Efficient absorption
  • Simple columnar epithelium with microvilli in the small intestine

Oesophagus

• Muscular tube that transports food (bolus) from mouth to stomach.
• Runs posteriorly to the trachea
• Approx. 25cm long
• Contains upper and lower oesophageal sphincters
  • Upper: At junction of pharynx and oesophagus. Prevents airflow into oesophagus and also reflux of food into airway.
  • Lower: At gastro-oesophageal junction. Controls entry of bolus into stomach and prevents backflow of gastric juice.
• Mucosa:
  • Stratified squamous epithelium for protection
  • Contains large folds that keep the lumen closed unless swallowing is occurring, and allow for expansion when swallowing
• Submucosa:
  • Large mucus glands for lubrication and to facilitate transport
• Muscularis Externa
  • Upper third is skeletal muscle (swallowing is voluntary)
  • Middle third is a combination of skeletal and smooth muscle
  • Lower third is smooth muscle
• Adventitia*
  • Connective tissue layer anchoring majority of the oesophagus to neighbouring structures
  • *except distal portion, which is serosa

Stomach

• An expandable muscular organ that stores food and mechanically and chemically breaks down food
• Four anatomical regions: Cardia, Fundus, Body, Pylorus
• Contains rugae:
  • Folds of the mucosa & submucosa
  • Allows expansion of the stomach
• Contains two sphincters:
  • Lower gastro-oesophageal sphincter. Controls entry of bolus into stomach and prevents backflow of gastric juice
  • Pyloric sphincter: Distal end of the stomach. Prevents early discharge or stomach contents
• Mucosa
  • Simple columnar epithelium
  • Produces a layer of mucous for protection against gastric juices, and defence
  • Contains deep folds that form gastric glands (contain gastric pits)
  • Contains numerous secretory cells that produce gastric secretions
• Submucosa
• Muscularis Externa
  • Inner oblique layer: Responsible for creating the churning motion to aid mechanical breakdown of food
  • Middle circular layer
  • Outer longitudinal layer: Responsible for moving the bolus towards the pylorus (through muscular shortening)
• Serosa

Stomach: Gastric Glands

• Gastric glands open into the stomach through gastric pits in the mucosa
• They secrete most of the acid and enzymes required for chemical digestion in the stomach
• Contain specialised cells: The composition of these cells depends on the gland’s location within the stomach

Specialised Cells: Mucous Cells

• Secrete mucous and bicarbonate ions (alkaline)
• Protects stomach wall from damaging effects of gastric acid (hydrochloric acid: HCl)
• Prominent in the gastric pit and neck of gastric glands

Specialised Cells: Parietal Cells

• Secrete hydrogen (H^{+}) and Chloride (Cl^{-}-) ions that combine to form HCl
• Kills microbes & denatures proteins
• Secrete intrinsic factor, which is necessary for Vitamin B12 absorption (in the small intestine)
• Located in upper regions of gastric glands

Specialised Cells: Chief Cells

• Secretes pepsinogen (inactive), which is converted to pepsin (active) by HCl
• Pepsin degrades proteins
• Secretes gastric lipase, which breaks down lipids
• Located in lower regions of gastric glands

Specialised Cells: Enteroendocrine Cells

• Produces and releases the hormone gastrin
• Increases stomach motility
• Stimulates HCl/enzyme production
• Relaxes the pyloric sphincter
• Located at the base of gastric glands

Stomach: Nutrient Absorption

• The stomach doesn’t play a big role in absorption of food, but it can absorb:
  • Water (especially when dehydrated)
  • Electrolytes
  • Some drugs (especially aspirin)
  • Alcohol

Small Intestine

• Longest portion of the GIT (approx. 3-5 metres)
• Site of most enzymatic digestion & absorption
• Three anatomical regions:
  • Duodenum: Approx. 5%
  • Jejunum: Approx. 40%
  • Ileum: Approx. 60%
• Mucosa
  • Simple columnar epithelium
  • Produces a layer of mucous for protection against acidic chyme, and defence
  • Contains a series of finger-like projections called villi
• Submucosa
• Muscularis Externa
  • Inner circular layer
  • Outer longitudinal layer
• Serosa

Small Intestine: Types of Movement

• There are two (2) types of movement that occur within the small intestine:
  • Segmentation
    • Local mixing of chyme with intestinal juices
    • Repeated, involuntary contraction/relaxation (sloshing back & forth) of the inner circular muscularis externa layer
    • Increases exposure time of chyme to absorptive surface
  • Peristalsis
    • Involuntary contraction/relaxation of both muscularis externa layers
    • Propels chyme onwards through the tract

Small Intestine: Large Surface Area

• Large surface area is achieved because of the following structures:
  • Plica circularis (si.)
    • Permanent folds within the mucosa and submucosa
    • Present only until the mid-section of the ileum
    • Can’t stretch out like rugae (stomach)
  • Villi
    • The surface of the plicae circulares (pl.) contains small vascularised projections of mucosa called villi
  • Microvilli
    • Cell surface feature sometimes referred to as a ‘brush border’
    • These structures dramatically increase the surface area for absorption

Small Intestine: Villi & Intestinal Crypts

• The mucosal surface contains small vascularised projections called villi
• Between villi, intestinal crypts are seen
• The intestinal mucosa contains specialised cells to assist with the absorption, digestion and motility of GIT contents

Specialised Cells: Enterocytes

• Tall columnar cells
• Apex of cell contains brush border of microvilli
• Absorb nutrients
• Secrete enzymes to aid in breakdown of macronutrients for easier absorption
• Found across the entire surface of a villus

Specialised Cells: Goblet Cells

• Produce mucins, which function to protect against highly acidic contents and to provide lubrication of the lining of the GIT
• Interspersed between enterocytes

Specialised Cells: Enteroendocrine Cells

• Secretes variety of hormones
• Aids in GIT motility, gastric emptying & mechanical digestion
• Located within the intestinal crypts

Specialised Cells: Paneth Cells

• Contain the enzyme lysozyme
• Antibacterial activity
• Located in base of intestinal crypts (between villi)

Large Intestine

• Approx. 1.5 metres in length
• Site of absorption of water & salt, and chyme conversion into faeces
• Approx. 90% of water within chyme is reabsorbed
• Extensive action of gut microflora (chemical digestion via bacteria)
• Four anatomical regions:
  • Caecum
  • Colon (four regions)
  • Rectum
  • Anal canal/anus
• Mucosa
  • Simple columnar epithelium (caecum and colon)
  • Stratified squamous epithelium (rectum and anus)
  • Mucosa drops down into intestinal glands that contain a lot of goblet cells for lubrication
• Submucosa
• Muscularis Externa
  • Inner circular layer
  • Outer longitudinal (incomplete) layer
    • Arranged into ‘ribbons’ called taeniae coli
    • Taeniae coli contract and gather the colon into a series of pouches known as haustra
• Serosa*
  • *except anus, which is adventitia

Large Intestine: Absorption & Faeces Formation

• Site of final absorption of water and salt (within the ascending colon)
• Remaining components are converted into faeces
• Faeces are semi-solid by time reaches transverse colon
• Components of faeces:
  • Dead epithelial cells
  • Undigested food, e.g. cellulose
  • Bacteria
  • Mucous

Control of the Digestive System

• Both the nervous system and the endocrine system control digestive processes
• The brain controls the responses of hunger and satiety, via the ‘feeding centre’ and ‘satiety centre’ respectively, within the hypothalamus
• The endocrine system controls various glands and the release of hormones and enzymes required for digestion of food in the digestive tract, e.g. gastrin and motilin