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