Flashcards in Session 1 - Introduction to the GI system Deck (56):
What is the overall function of the gastrointestinal system?
What are the qualities of the products of digestion?
What solutions does the process of digestion create?
• Small sugars
• Amino acid and small peptides
• Lipids in very small particles
• Specific active or passive uptake of nutrient molecules, water and electrolytes
Give three waste products of the GI tract
• Residue from food
• Gut debris
• Materials secreted from liver
What needs to happen to food for digestion to occur?
• Disrupted physically to release large molecules
• Broken down chemically to release small molecules
Why do ingested foods need to be stored?
• We can eat much faster than we can digest
Outline the overall process of digestion
• Initial physical disruption
• Ingestion & transport to storage
• Initial chemical disruption & creation of suspension – forming chyme
• Controlled release of chyme
• Dilution and neutralisation of chyme
• Completion of chemical breakdown
• Absorption of nutrients and electrolytes
• Final absorption of water and electrolytes,
• Producing faeces for controlled excretion
List the two mechanisms involved in physical disruption of food
Outline the functions of saliva
• Protects mouth
• Lubricates food
• Starts digestion
Give four ways in which saliva protects the mouth
• Wet - maintains mucosae
• Alkaline - protects teeth
• High calcium - protects teeth
What does saliva initially digest?
What is food called after it has been physically disrupted?
Where does storage, initial disruption and disinfection take place?
How is chyme produced in the stomach?
• Action of acid, enzymes and agitation
Where does dilution and neutralisation of chyme take place?
• Duodenum and jejunum
Why does dilution take place?
• To ensure that the chyme is of the same osmotic potential as the small intestine
What do enzymes from pancreas and intestine do?
• Cleave peptides to amino acids
• Cleave polysaccharides to monosaccharaides
• Breakdown and re-form lipids
• Break down nucleic acids
How does absorption of nutrients and electrolytes take place?
• Intestine has large SA due to brush border
• Epithelial cells absorb small molecules - some actively, some passively
• Often coupled to sodium absorption
What are absorbed nutrients taken into?
• Hepatic portal circulation
Where does final absorption of water and electrolytes occur?
• Large intestine
Where does faeces accumulate?
• Descending and sigmoid colon
How does defecation occur?
• Faeces propelled into rectum
• Controlled relaxation of sphincters and expulsion of faeces occurs
What is the function of the stomach?
• Relaxes to accommodate food
• Rhythmical contraction
• Secretes acid and proteolytic enzymes to break down tissues
• Disinfects bolus
Outline the process of receptive relaxation
• Stomach strongly contracted between meals
• Relaxes as bolus enters
• Prevents a rise in pressure of the stomach
What takes place in the duodenum and jejunum?
• Water drawn in from ECF
• Bile added to chyme
• Pancreas, liver and intestinal secretions (enzymes and bile acids)
• Liver and pancreas secrete alkali and neutralise acid
How is duodenum adapted to the highly concentrated chyme it receives from the stomach?
• Wall permeable to water
• Draws water in to dilute contents
Outline the four layers of alimentary canal
• Mucous membrane
• Muscularis externae
What makes up the mucous membrane?
• Lining epithelium
• Connective tissue
• Thin layer of smooth muscle
What makes up the submucosa?
• Fibroelastic tissue with
• Nerves of the submucosal plexus
• Fat cells
What makes up the muscularis externa?
An inner circular layer
Outer longitudinal layer
What lies between the two layers of the muscularis externae?
• Myenteric plexus
What is the serosa?
• A thin outer covering of connective tissue
What is endoscopy?
• A long, thing tube which allows direct visual examination, biopsy sampling and therapeutic treatment of the gastro-intestinal tract
What is nasendoscopy?
• Allows visualisation of the nose, mouth and pharynx
What structure marks the beginning of the oesophagus?
• Cricopharyngeal sphincter
Where does the diaphragm cuff the oesophagus, and why is this clinically relevant?
• At the oesophagogastric mucosal junction
• This relationship may be disrupted by a hiatus hernia, which allows the stomach to herniate into the thorax
Outline the venous drainage of the oesophagus
• Drains into the left portal system via the left gastric vein
How is the oesophagus adapted to rapid transport?
• Streamlined structure which minimises friction
What are the longitudinal ridges of the stomach called?
• Rugae, increase SA
Where is gastric ulceration most common?
• Lesser curve at the angulus
What is the normal maximal fluid contents of the gut?
• 1kg food, 1.5L saliva, 2.5L gastric secretions, 9l of water and alkali
How are the contents of the gut removed?
• Small intestine absorbs 12.5l
• Large intestine absorbs 1.35
• 150g faeces expelled
What happens if the balance between secretion and absorption altered?
• Considerable loss of water and electrolytes, mostly from body fluids
• Rapid dehydration and electrolyte disturbance
Name three control systems of the gut
Why are three overlapping control systems needed in the gut?
• Motility and secretion need precise control
What is the somatic motor used for in the gut?
• Ingestion (chewing) and excretion (defecation)
What is the most significant neural control system of the gut?
• Autonomic (specifically parasympathetic) control
What do post ganglionic neurones form in the gut?
What does the "gut nervous system" control?
• Coordinates secretion and motility
What enteric nervous system of the gut made up of?
• Two nerve plexuses which may act independently of CNS and be modified by both branches of the ANS
What is paracrine secretion?
• Chemical messengers diffuse locally
Name a chemical messenger secreted via the paracrine method
Give three factors the endocrine system controls in the digestive system
• Stomach acid
• Alkali secretion from liver and pancreas
• Enzyme secretion
What type of structure do gut hormones share?
• All peptide derived