Overview of GI tract Flashcards
(20 cards)
What is bolus?
Small, rounded masses of substance
What is chyme?
Pulpy acidic fluid consisting of gastric fluid and partly digested food
What is the mucosa?
The innermost layer with rugae (ridges) that expand when full
Aids digestion, absorption and secretion
What is the submucosa?
A connective tissue layer with blood vessels, lymphatics and nerves
Supports and protects the mucosa
What is the muscularis externa?
Three muscle layers; the inner oblique, circular and longitudinal
They contract to churn and break down food
What is the serosa?
The outermost layer that secretes fluid to reduce friction allowing smooth organ movement
Parietal cells
Produce hydrochloric acid and an intrinsic factor which helps absorption
Chief cells
Secrete pepsinogen, the inactive proenzyme form of pepsin which helps break down proteins
Enteroendocrine cells
Secrete various hormones including gastrin and ghrelin
Mucous cell
Secrete mucin which has several protective functions
Secretory cells in the stomach can be exocrine or endocrine
Exocrine glands secrete substances into a ductal system to an epithelial surface which includes parietal, chief and mucous cells
Endocrine glands secrete products directly into the blood stream, like enteroendocrine cells
The pyloric sphincter
Controls the release of chyme from the stomach into the small intestine
Releases small amounts of chyme at a time to allow efficient digestion and absorption
Six steps of the digestive system
Ingestion
Propulsion
Mechanical digestion
Chemical digestion
Absorption
Defecation
The oesophagus
Mostly made up of smooth muscle which contracts through peristalsis
Muscles behind the food contract, muscles ahead of the food relax
The lower oesophageal sphincter opens to let food enter the stomach then closes to prevent reflux
The small intestine
Made up of duodenum, jejunum and the ileum
Absorbs nutrients and extracts water
Circular folds, villi and microvilli
Same four layers as the stomach
Chemical digestion ends here
Loss of microvilli can result in diarrhoea
Produces its own digestive juice to further breakdown digestive components
Chyme from the stomach mixes with bile from the liver and pancreatic juices in the duodenum
Movement from the small to large intestine
Chyme enters the large intestine via the sphincter
From ileum to cecum
Movement of chyme is achieved by slow waves of peristalsis over a period of several hours
The large intestines
Made up of cecum, colon, rectum and anus
Completes absorption of nutrients, water and synthesises certain vitamins
Bacteria digest substances in chyme that aren’t digestible by the human digestive system
Simple columnar epithelium lines its mucosa
Goblet cells secrete mucous which aids the movement of faeces
Principal control mechanisms - endocrine regulation
Enteroendocrine cells detect stimuli and release hormones that travel via the bloodstream to distant cells
GI hormones influence digestion, pancreatic function and metabolism in organs like the liver, muscle and brain
Secretin regulates pancreatic secretion
Gastrin is released in the stomach and stimulates gastric acid secretion
Principal control mechanisms - paracrine
Chemical messengers or peptides are released by a sensing cell (often an EEC) and act on nearby target cells via diffusion
Target cells tend to be smooth muscle cells, secretory cells and other EEcs
Histamine is released by enterochromaffin cells in the stomach and stimulates acid production in parietal cells
Principal control mechanisms - neural regulation
Neurotransmitters regulate GI function by acting on target cells
The extrinsic nervous system modulates gut activity through sympathetic and parasympathetic input. The nervous originate outside the gut wall
The intrinsic nervous system is located within the gut wall and can function independently but is influenced by extrinsic nerves
