The Digestive System Flashcards
What are the four layers of the gut wall?
- Mucosa (innermost)
- Submucosa
- Muscularis externae (external muscle layers)
- Serosa (outermost)
What is the submucosa?
A layer of connective tissue bearing glands, arteries, veins and nerves.
What is the muscularis externa?
Two layers of smooth muscle
How does the muscularis externa help move luminal contents along the gut?
By creating peristaltic waves
What is in the serosa?
A simple squamous epithelium (mesothelium) and connective tissue
What are the differences between the inner and outer layer of smooth muscle in the muscularis externa?
Inner layer = circular with nuclei in the centre of the cell.
Outer layer = longer and flattened with cigar shaped nuclei.
What’s are the major functions of the GI tract?
- To provide a port of entry for food into the body
- To mechanically disrupt the food
- To temporarily store the food
- To chemically digest the food
- To kill pathogens in the food
- To move food along the tract
- To absorb nutrients from the resultant solution
- To eliminate residual waste material
Characteristics of Saliva:
- Starts digestion (amylase and lipase)
- Bacteriostatic (contains IgA)
- High calcium
- Alkaline
- Assists swallowing
- Protects the mouth
Upper end Oesophagus
Voluntary control (some striated skeletal muscle)
Lower end Oesophagus
Involuntary control (solely smooth muscle)
How does the Oesophagus work?
Rapid peristaltic transport transports bolus to stomach (8-9s)
(Fastest GI transport is on entry and exit)
Innervation in the Oesophagus
Myenteric plexus
Submucosal plexus
What is in the mucosa?
Epithelium
Lamina propria - loose connective tissue bearing blood is and lymph vessels, some smooth muscle cells and immune cells
Muscularis mucosae - thin layer of smooth muscle cells
Stomach storage:
Acts as a necessary food store (we eat faster than we digest)
Wall relaxes so pressure doesn’t rise (receptive relaxation)
Stomach initial disruption:
Contracts rhythmically to mix and disrupt
Stomach disinfection
Stomach secretes acid and proteolytic enzymes to break down tissues and disinfect
Stomach final action
Produces hypertonic chyme by combined action of acid, enzymes and agitation
Delivers incompletely digested chyme slowly, and in a controlled way to the duodenum
What is the name of the folds of gastric mucosa in the stomach?
Rugae (pronounced roojie)
Describe the gastric glands of the mucosa:
Long, straight tubular gastric glands extending from the gastric pits to the muscularis mucosa
Describe the gastric pits of the mucosa of the stomach
Shallow, lined by mucous cells
How does the mucus secreted by surface mucous cells protect the stomach lining?
The mucus contains HCO3- which neutralises the H+ ions.
It is also resistant to pepsin degradation
Constituents of the gastric gland:
- Gastric pit - lined by mucus secreting cells
- Isthmus - where stem cells divide to populate the gland by up/downward migration
- Parietal cells - secrete H ions into the lumen and HCO3 ions into capillaries
- Chief cells - secrete pepsinogens
- Enteroendocrine cells - include G cells which secrete gastric
Shape of duodenum:
20-25cm long, C shaped, proximal portion of small intestine
Curves around head of pancreas
Steps of dilution in the duodenum:
- Water drawn in from the ECF
- Liver releases bile (water, alkali, bike salts)
- Pancreas and liver secrete alkalis
- Pancreas, liver and intestine secrete enzymes which with bile complete digestion
Completion of digestion with enzymes from pancreas and intestine steps:
- Cleave peptides to amino acids
- Cleave polysaccharides to monosaccharides
- Break down and re-form lipids
- Break down nucleic acids
Requirements for absorption
- Requires a lot of energy
- Requires a large surface area (gut is folded, villi, micro-villi)
- Adequate contact time
- Good blood supply/drainage (liver/hepatic portal vein)
Dimensions of Small Intestine
22 feet/7 meters
2.5 cm in diameter
SA of 25 (meters squared)
What is the name of the folds of the Jejunum?
Plicae circulares
What does the Duodenum absorb?
Iron
What does the Jejunum absorb?
Most of the sugars, AA’s and fatty acids
What does the Ileum absorb?
Vitamin B12, bile acids and remaining nutrients
What are the areas of the Large Intestine?
- Caecum
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
Dimensions of the Large Intestine:
1.2 m long and 6-9cm wide
What does the epithelium of the crypts of Lieberkuhn produce?
Mucus and supplies cells to the surface
Surface epithelial cells absorb water and electrolytes
Bacteria of the Large Intestine are involved in:
- Synthesis of vitamins K, B12, thiamine and riboflavine
- Breakdown of 1’ to 2’ bile acids
- Conversion of bilirubin to non-pigmented metabolites
Decreased absorption or increased secretion of the gut can result in:
Life threatening dehydration
Life threatening electrolyte imbalance
Somatic neural control is involved in:
Ingestion (mouth and first third of Oesophagus)
Excretion
Autonomic nervous system controls what parts of the digestion:
All apart from ingestion and excretion
Paracrine control of the gut
Histamine (production of acid in stomach)
Vasoactive substances (affect blood flow in gut)
Endocrine control of the gut hormones control:
- Secretion of stomach acid
- Alkali secretion from liver and pancreas
- Enzyme secretion
What does Secretin do?
Promotes bicarbonate secretion from duct cells of pancreas
Promotes bike production by the liver
Inhibits secretion of acid by parietal cells of stomach
What does Cholecystokinin (CCK) do?
Made and secreted by enteroendocrine cells of the duodenum
Promotes release of digestive enzymes from the pancreas
Promotes release of bike from the gall bladder
Is a hunger suppressant
What does Gastrin do?
Promotes HCl production (gastric acid) by the parietal cells of the stomach.
Released by G cells of pyloric antrum of stomach, pancreas and duodenum.
