Digestive System

• The digestive system processes what we ingest and absorbs nutrients (including water) that support the entire body

  
• Ingestion


  The act of taking food (or water) into the body

  
• Digestion


  Food must then be digested in order to be absorbed

  
• Parts of the digestive system

  • Oral cavity
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anus

  
• The main function of the digestive system is to extract nutrients from food to fuel the body
• Digestive system function


  1. Food is ingested

  2. Food is digested (broken down by physical and chemical means)

  3. Nutrients are absorbed into the bloodstream and lymphatic system

  4. Non-nutrients (and some wastes) are excreted by defecation

  
• Everything you eat or drink passes through your digestive tract

  
• The digestive system is made up by the gastrointestinal tract and accessory organs

  
• Gastrointestinal tract (alimentary canal)


  A muscular tube that runs from the mouth to the anus, ~9m long in an adult, with movement generally in one direction

  
• Digestive tract wall


  • Mucosa: epithelium + areolar connective tissue
  • Submucosa: dense connective tissue, vessels (and sometimes glands)
  • Muscle layer: usually two layers of smooth muscle
  • Serosa (or adventitia): epithelium + areolar tissue (or dense connective tissue)

  
• Nervous system of digestive tract

  • Submucosal plexus: contains visceral sensory fibres, and parasympathetic and sympathetic postganglionic neurons (ANS)
  • Myenteric plexus: contains ENS neurons (interneurons and motor neurons)
• The digestive system is innervated by the autonomic nervous system (ANS) but also has its own nervous system - the enteric nervous system (ENS)

  
• Muscle layers in digestive tract

  • Longitudinal layer: arranged along the length of the tube
  • Circular layer: arranged around the circumference of the tube

  
• Exceptions to standard muscle layer organization

  • Oral cavity, pharynx, upper esophagus have skeletal muscle
  • Stomach has three layers of smooth muscle
  • Colon has an incomplete outer layer

  
• Visceral smooth muscle


  Single-unit organization, cells connected by gap junctions, influenced by rhythmically active pacesetter cells
• Neurotransmitters released from ANS neurons


  Can enhance or diminish slow wave potentials in visceral smooth muscle
• Processes occurring in digestive tract

  • Digestion (food breakdown)
  • Propulsion (forward movement)
  • Absorption (nutrients into bloodstream)
  • Protection (keeping pathogens out)
  • Coordination (feedback and feedforward loops)

  
• Propulsion


  The movement of ingested material through the digestive tract, also called 'motility'

  
• Peristalsis


  Involves spreading waves of contraction in both muscle layers in a proximal-to-distal direction to propel food
• Segmentation


  Rhythmic cycles of circular muscle contraction that fragment the food bolus but do not produce net forward movement
• Secretions of the digestive tract and accessory organs enhance both digestion and propulsion

  
• Absorption


  The uptake of nutrients, water, and vitamins from the digestive tract lumen into the bloodstream

  
• Digestion refers to the breakdown of food into smaller fragments (or chemical components)

  
• Absorption specifically refers to the movement of nutrients across the mucosa (epithelial layer) into body tissues

  
• Processes occurring in oral cavity

  1. Chewing (physical digestion)

  2. Tasting (coordination)

  3. Salivation (propulsion, chemical digestion)

  4. Swallowing (propulsion)
• Mastication (chewing)


  Contributes to physical digestion by bringing two rows of teeth together
• Gastroscopy allows specialists to look inside a living, churning stomach. It involves the insertion of a camera (and a light) into the stomach on the end of a long, flexible tube.

  
• Adult humans typically have 32 (permanent) teeth, which are our second set of teeth
• Digestion (physical and/or chemical)

  1. Chewing (Physical Digestion)

  2. Tasting (Coordination)

  3. Salivation (Propulsion, Chemical Digestion)

  4. Swallowing (Propulsion)

  
• Gastroscopy has to be done while fasting, and often also involves sedation.
• A swallowable 'pill-cam' can also be used without sedation for a quick view of the alimentary canal.
• Processes occurring in the oral cavity

  • Chewing (Physical Digestion)
  • Tasting (Coordination)
  • Salivation (Propulsion, Chemical Digestion)
  • Swallowing (Propulsion)
• Mastication (chewing)


  • Contributes to physical digestion by bringing two rows of teeth together
  • Coordinated rhythmic motor behaviour involving movements of the mandible, and puts food in contact with moving, grinding teeth
• Esophagus


  • Pyloric Sphincter
  • Stomach (Gastric) Rugae

  
• Adult humans typically have 32 (permanent) teeth, which are our second set of teeth. The first set (deciduous teeth) appears during the first two years of life and then are replaced.
• The stomach is a muscular pouch with four distinct subregions and three muscle layers.
• The extra muscle layer (oblique) enhances the strength and range of contractions of the stomach wall.
• Teeth form within the mandible and maxilla, then gradually 'erupt' into the oral cavity.
• Tongue


  • Muscular organ which manipulates food and is also involved in gustation (taste)
  • Highly mobile muscular organ containing skeletal muscle tissue. Its mobility is increased by extrinsic muscles
  • Contributes to physical digestion and propulsion
  • Secretes lingual lipase which contributes to chemical digestion of fats
  • Principal organ for our sense of 'taste'
• The stomach is found in the upper mid-left abdominal cavity.