The Digestive System

Flashcards about the key components, functions, and processes of the digestive system, including the gastrointestinal tract (GIT) and accessory organs.

Components of the Digestive System (Major (hollow) organs)

. Oral cavity & Pharynx
• Oesophagus
• Stomach
• Small Intestine
• Large Intestine, inc. rectum & anus

Components of the Digestive System (Accessory Organs)

Teeth & Tongue
• Salivary Glands
• Liver
• Gallbladder
• Pancreas

Functions of the Digestive System

The digestive system converts food into its simplest
components to be absorbed into the bloodstream or excreted
as waste It does this via: Ingestion, Mechanical digestion, Chemical digestion, Absorption, Elimination

Ingestion

Entry of food/liquids into the digestive tract via the oral cavity

Mechanical digestion

Crushing/shearing of ingested food

Chemical digestion (extraction of nutrients)

Enzymatic breakdown of food into substances that can be absorbed

Absorption

Movement of nutrients into the bloodstream

Elimination

Indigestible food is compacted into faeces and is excreted

Components of 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, inc. rectum & anus

Functions of the Accessory Digestive Organs: Teeth & Tongue function

Crushing/shearing of food and shaping of food into bolus

Functions of the Accessory Digestive Organs: Salivary glands function

Secretion of enzyme (amylase) to begin breaking down carbohydrates

Functions of the Accessory Digestive Organs: Liver function

Secretion of bile to breakdown fats, detoxification of the blood, storage of iron, glucose and fat-soluble vitamins

Functions of the Accessory Digestive Organs: Gallbladder function

Storage of bile

Functions of the Accessory Digestive Organs: Pancreas function

Secretion of pancreatic enzymes to breakdown protein, fat and carbohydrates

Oral cavity digestion

Mechanical digestion (mastication) and mixing of food with saliva; enzymes in saliva begin the process of chemical digestion

Oesophagus digestion

A fibromuscular tube that transfers food (bolus) into the stomach

Stomach digestion

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) digestion

Chemical digestion: Aided by digestive juices from the pancreas and gallbladder

Small intestine (jejunum and ileum) digestion

Where the bulk of chemical digestion and nutrient absorption occurs

Large intestine function

Cecum, ascending colon, transverse colon,
descending colon Reabsorption of water & some nutrients; compacts waste (faeces) for elimination

Rectum & anus function

Stores and eliminate waste (faeces)

General Structure of the GIT Wall Layers

From inside to out: Mucosa, Submucosa, Muscularis externa, Serosa/Adventitia

Mucosa

Innermost structure of the GIT wall consisting of three sublayers: epithelium, lamina propria, and muscularis mucosa

The Mucosa: 1. Epithelium

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: 2. Lamina propria

Loose connective tissue that is a route by which nutrients are absorbed and 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

The Mucosa: 3. Muscularis mucosa

Thin layer of smooth muscle that causes folds to form in the mucosal layer, increasing local movements and absorption of nutrients

Submucosa

Loose connective tissue that provides physical support to the mucosa and connects it 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

Submucosal (Meissner’s) plexus

Network of sensory neurons and autonomic nerve fibres that innervate the glands and stimulates secretion from glands

Muscularis Externa

Multiple muscle layers organized in circular and longitudinal layers that are essential for mechanical processing and moving materials along the GIT. Composition depends on location within the GIT:

The Muscularis Externa layer: Oesophagus

Contains inner circular and outer longitudinal layers
of muscle. Upper third is skeletal muscle (voluntary), while the rest
is smooth muscle (involuntary)

The Muscularis Externa layer : Stomach

Contains inner oblique, middle circular and outer
longitudinal layers of smooth muscle

The Muscularis Externa layer: Small and large intestines & rectum

Contains inner circular and
outer longitudinal layers of smooth muscle

The Muscularis Externa layer: Anus

Contains inner circular and outer longitudinal layers of
skeletal muscle (voluntary)

The Muscularis Externa also contains Myenteric plexus (Auerbach plexus)

Network of sensory neurons + autonomic nerve fibres located between the circular and longitudinal muscle layers that coordinates digestive muscle activity

Serosa

Outermost layer of GIT organs within the abdominopelvic cavity, continuous with the peritoneum, i.e. distal oesophagus, stomach and intestines. Composed of 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. Composed of Dense (fibrous) connective tissue that anchors the organ in place

structure of GIT links 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 oesophageal sphincter

At junction of pharynx and oesophagus; prevents airflow into oesophagus and also reflux of food into airway

Lower oesophageal sphincter

At gastro-oesophageal junction; controls entry of bolus into stomach and prevents backflow of gastric juice

Layers found in the Oesophagus

1. Mucosa
• Stratified squamous epithelium for protection
• Contains large folds that keep the lumen closed unless
swallowing is occurring, and allow for expansion when swallowing
2. Submucosa
• Large mucus glands for lubrication and to facilitate transport
3. 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
• 4. Adventitia*
• Connective tissue layer anchoring majority of the oesophagus to neighbouring structures
• *except distal portion, which is serosa

Stomach

Expandable muscular organ that stores food and mechanically and chemically breaks down food. Four anatomical regions: Cardia, Fundus, Body, Pylorus

Stomach contains: Rugae

Folds of the mucosa & submucosa that allow expansion of the stomach

Stomach Contains two sphincters: Pyloric sphincter

Distal end of the stomach that prevents early discharge of stomach contents

Stomach Contains two sphincters: Lower gastro-oesophageal sphincter

Controls entry of bolus into stomach and prevents backflow of gastric juice

Stomach layers

1. 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
   2. Submucosa
   3. 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)
   4. 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 in the stomach’s gastric glands: Mucous Cells

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

Specialised Cells in the stomach’s gastric glands: 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 in the stomach’s gastric glands: 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 in the stomach’s gastric glands: Enteroendocrine Cells

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

what the stomach absorbs

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) and is the site of most enzymatic digestion & absorption Three anatomical regions:
• Duodenum: Approx. 5%
• Jejunum: Approx. 40%
• Ileum: Approx. 60%

Small Intestine layers

1. 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
   2. Submucosa
   3. Muscularis Externa
   • Inner circular layer
   • Outer longitudinal layer
   4. Serosa

Small Intestine: Types of Movement: 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

Small Intestine: Types of Movement: Peristalsis

Involuntary contraction/relaxation of both muscularis externa layers that propels chyme onwards through the tract

Large surface area of small intestines is achieved through: Plica circularis

Permanent folds within the mucosa. Present only until the mid-section of the ileum and submucosa that can’t stretch out like rugae (stomach)

Large surface area of small intestines is achieved through: Villi

The surface of the plicae circulares (pl.) contains small vascularised projections of mucosa

Large surface area of small intestines is achieved through: Microvilli

Cell surface feature sometimes referred to as a ‘brush border’

Specialised Cells of Small intestines: Enterocytes

Tall columnar cells at the top it contain brush border of microvilli and Absorb nutrients Secrete enzymes to aid in breakdown
of macronutrients for easier absorption
• Found across the entire surface of a villus

Specialised Cells of Small intestines: Goblet Cells

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

Specialised Cells of Small intestines: Enteroendocrine Cells

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

Specialised Cells of Small intestines: Paneth Cells

Contain the enzyme lysozyme with 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

Large Intestine layers

1. 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
   2. Submucosa
   3. 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
   4.Serosa*
   • *except anus, which is adventitia

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