Session 1 - Introduction to the GI system Flashcards
What is the overall function of the gastrointestinal system?
- Secretion
- Digestion
- Motility
- Absorption
What are the qualities of the products of digestion?
- Sterile
- Neutral
- Isotonic
What solutions does the process of digestion create?
- Small sugars
- Amino acid and small peptides
- Lipids in very small particles
Define absorption
• 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
- Disinfection
- 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
Mastication
Saliva
Outline the functions of saliva
- Protects mouth
- Lubricates food
- Starts digestion
Give four ways in which saliva protects the mouth
- Wet - maintains mucosae
- Bacteriostatic
- Alkaline - protects teeth
- High calcium - protects teeth
What does saliva initially digest?
• Sugars
What is food called after it has been physically disrupted?
• Bolus
Where does storage, initial disruption and disinfection take place?
Stomach
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
- Submucosa
- Muscularis externae
- Serosa
What makes up the mucous membrane?
- Lining epithelium
- Connective tissue
- Thin layer of smooth muscle
What makes up the submucosa?
- Fibroelastic tissue with
- Vessels
- Nerves of the submucosal plexus
- Leucocytes
- 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?
- 14L
* 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
- Neural
- Paracrine
- Endocrine
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?
• Plexuses
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
Histamine
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
What are the two groups of gut hormones?
- Gastrin group
* Secretin group
