Lecture 39 - Digestive System Overview - From Oral Secretions to the Stomach Flashcards
What is the function of the digestive system?
To processes what we ingest and absorb the nutrients from food to fuel the body
What is ingestion and how are nutrients from the food absorbed?
Ingestion is the act of taking food (or water) into the body. Most food must be digested in order to be absorbed
Describe the path food takes in the digestive system
- Food is ingested, then digested through physical and chemical means
- It’s propelled along the digestive tract
- Nutrients are absorbed into the bloodstream and lymphatic system
- Non-nutrients (and some wastes) are excreted by defecation
What is the digestive system made up by?
Made up by gastrointestinal tract and accessory organs
The gastrointestinal tract (alimentary canal) is a muscular tube that runs from the mouth to the anus. Movement is (generally) in one direction
What is the alimentary canal and describe its organization
A hollow muscular tube lined by a mucus-secreting epithelium
- Mucosa - epithelium + areolar connective tissue
- Submucosa - dense connective tissue, vessels (and sometimes glands)
- Muscle layer - usually 2 layers of smooth muscle
- Serosa (or adventitia) - epithelium + areolar tissue (or dense connective tissue)
What is the function of mesenteries and peritoneum
They are associated with the alimentary canal
Mesenteries - membranes that anchor abdominal organs to the peritoneum
Peritoneum - a double-layered membrane encasing most of the abdominal cavity
What are the locations of neural plexuses?
Neurons of the ENS and ANS are found throughout the digestive tract and organized into 2 plexuses (networks)
Submucosal plexus - contains visceral sensory fibres and parasympathetic and sympathetic postganglionic neurons (ANS)
Myenteric plexus - contains ENS neurons (interneurons and motor neurons)
In most of the digestive tract, what is the type of muscle found and how are they organized?
You mostly see smooth muscle into 2 layers with different orientations:
- Circular layer - arranged around the circumference of the tube
- Longitudinal layer - arranged along the length of the tube
What are the exceptions to the typical organization of muscle layer in the digestive tract?
- Oral cavity, pharynx, upper esophagus have skeletal muscle in the muscularis
- The stomach has 3 layers of smooth muscle
- The colon has an incomplete outer layer
What is the organization of visceral smooth muscle cells?
They are connected by gap junctions and influenced by rhythmically active pacesetter cells. They are a single-unit organization
The pacesetter cells create regular rhythms of depolarization and repolarization (slow wave potentials) which can spread throughout the muscle layer
NTs released from ANS neurons can enhance or diminish slow wave potentials
Name the 5 different processes that occur in the digestive tract
- Propulsion - forward movement (swallowing, salivation)
- Digestion - food breakdown (chewing, salivation)
- Absorption - nutrients into bloodstream
- Coordination - feedback and feedforward loops (tasting)
- Protection - keeping pathogens out of the rest of the body
What is propulsion? What helps it occur?
Refers to the movement of ingested material through the digestive tract (ie. pushing the bolus from the mouth to the anus)
Peristalsis helps this to occur - it involves spreading waves of contraction in both muscle layers in a proximal-to-distal direction. These waves propel food through the tract
What is digestion and what are the 2 parts? Describe the role segmentation plays
Refers to the breakdown of ingested material into smaller components. It involves segmentation, which are rhythmic cycles of circular muscle contraction that fragment the food bolus, but don’t produce net forward movement
- Physical digestion - the fragmentation of food by purely physical forces (ex. mastication, segmentation)
- Chemical digestion - the breakdown of macromolecules into smaller components by chemicals or enzymes
What enhances digestion and propulsion?
Primary and some accessory digestive organs secrete fluids into the lumen of the digestive tract. These secretions typically contain digestive enzymes or chemicals, but as fluids, they also help to reduce friction of food moving through the tract
Almost all of these secretions are reabsorbed before the wastes are excreted
What is absorption?
The uptake of nutrients, water, and vitamins from the digestive tract lumen into the bloodstream. It refers to the movement of nutrients across the mucosa (epithelial layer) into the body tissue
Only certain parts of the digestive tract are actually capable of absorption