Digestive System Flashcards
What are the 4 layers of the gut wall?
Mucosa (innermost)
Submucosa
External muscle layers (muscularis externae)
Serosa (outermost)
What are the 3 layers of the mucosa?
Mucosa lines the lumen
Epithelium (surface)
Lamina propria
Muscularis mucosae
What are Peyer’s patches?
Aggregations of lymphocytes present in the lamina propria
Describe the submucosa
A layer of connective tissue bearing glands, arteries, veins and nerves
Describe the muscularis externa
2 layers of smooth muscle - outer longitudinal layer and inner circular layer
Creates successive peristaltic waves to move luminal contents along the gut
What is the name of the outermost layer of the gut wall? Describe it.
Serosa - a serious membrane
Outermost covering of simple squamous epithelium (mesothelium) + a thin layer of adjacent connective tissue
What are the major functions of the gastrointestinal 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 the food along the tract
- to absorb nutrients from the resultant solution
- to eliminate residual waste material
Briefly describe the purpose of digestion
is the conversion of what we eat, by:
physical disruption
chemical disruption
into a solution:
relatively sterile
neutral in pH
isotonic
from which we can absorb our nutrients: a few sugars a few fatty acids approx. 20 amino acids some minerals some vitamins
Describe saliva
- Starts digestion (amylase and lipase)
- Bacteriostatic (contains Immunoglobulin A antibody (IgA))
- High calcium (protects teeth)
- Alkaline
- Assists swallowing
- Protects the mouth
Describe the disruption that occurs in the mouth
Physical - by action of teeth, tongue, and muscles of mastication
Chemical - by action of salivary amylase and lipase
Describe the function of the oesophagus
• Mouth forms a bolus which enters the oesophagus • Upper end oesophagus – voluntary control (some striated skeletal muscle)
Lower end oesophagus – involuntary control (solely smooth muscle)
• Rapid peristaltic transport, which works even when upside down,
transports bolus to stomach (8-9 seconds)
• Fastest GI transport is on entry (oesophagus) and exit (rectum/anus)
- rest of gut’s motility is comparatively slow
Where is there innervation in the oesophagus? (See slide)
Myenteric plexus (auerbach’s plexus) - in the muscularis externa, in between longitudinal and circular layers Submucosal plexus (maissner’s plexus) - in the submucosa
How does the structure of the oesophagus relate to its function?
- Epithelium – stratified squamous non-keratinized (withstands abrasion).
- Submucosa - subtending layer of connective tissue containing mucus secreting glands
- Muscularis externa – smooth muscle layers (inner – circular; outer – longitudinal) which move food by peristalsis.
Describe the physical and chemical disruption that occurs in the stomach
physical - by churning (3 muscle layers and mucosal rugae)
chemical - by acid (HCl) and enzymes (e.g. pepsin)
Describe the function of the stomach
- acts as a necessary food store (we can eat faster than digest)
- wall relaxes so pressure doesn’t rise (called receptive relaxation)
- contracts rhythmically to mix and disrupt (details in GI Unit)
- secretes acid and proteolytic enzymes to break down tissues and disinfect
- protects its epithelium by secreting mucus
- 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 are rugae?
Folds of gastric mucosa forming longitudinal ridges in the empty stomach, allow for distension and greater surface area for production of acid
Describe the histology of the stomach mucosa (see slides)
Shallow gastric pits lines with mucous cells
Long straight tubular gastric glands extend downwards from the gastric pits towards the muscularis mucosae
Describe the surface mucous cells of the gastric pits
Surface mucous cells are abundant in the gastric pits but infrequent in the tubular gastric glands below. Their secreted mucus is resistant to pepsin (enzymatic) degradation. The mucus is released in response to distension, stomach contents, and acid secretion from the gastric glands. The secreted mucus contains HCO3- ions which neutralise the effect of H+ ions and thereby protect the stomach lining. Aspirin impairs the production of mucus by the mucous cells and can therefore lead to their damage but they are quickly replaced by mitosis in deeper cells in the neck of the gastric pit.
Describe the cells found in a gastric gland
The gastric pit is lined by mucus secreting cells, very like goblet cells.
The isthmus is the region in which stem cells divide to populate the gland by upward or downward migration.
Parietal cells secrete H+ ions into the lumen and HCO3- ions into
nearby capillaries, which move it to surface mucous cells.
Chief cells secrete pepsinogens which are converted into pepsins
which partly hydrolyse proteins.
Enteroendocrine cells include G cells which secrete gastrin. The
wider gastric mucosa responds to this hormone by secreting acid.
Describe the duodenum
20-25 cm long, C-shaped, proximal portion of small intestine
Curves around head of pancreas
Walls contain Brunner’s glands which secrete bicarbonate–rich
mucus, to neutralise acidic chyme.
Connects with liver and pancreas
Describe the function of the duodenum
• Water drawn in from ECF to render hypertonic chyme, isotonic
• Liver releases bile (generally via the gall bladder).
bile contains: - water, alkali, bile salts to emulsify fat
• Pancreas & liver secrete alkali to neutralise acidic chyme
(precisely controlled)
• Pancreas, liver and intestine secrete specific enzymes which act, with bile, to complete digestion of chime (enzymes come to lie in ‘unstirred layer’)
Describe the final steps in digestion
Enzymes from pancreas and intestine • Cleave peptides to amino acids • Cleave polysaccharides to monosaccharides • Break down and re-form lipids • Break down nucleic acids
What is required for absorption?
Active process
• Requires lots of energy Requires large surface area (it’s a slow process)
• Gut is folded/villi/micro-villi
• Adequate contact time (control of gut transit)
Good blood supply/drainage
- drainage is via hepatic portal vein; all nutrients, except lipids, travel via the liver.
- lipids go via lacteals and the lymphatic pathway.
What is the length of the small intestine?
22 feet (7 meters) long (can vary substantially)
1 inch (2.5 centimeters) in diameter.
Expected surface area: 0.6 m2
Actual surface area: 25+ m2 (due to villi)
What are pilcae circulares?
Circular folds of mucosa and. Submucosa projecting into gut lumen
What type of epithelium lines the villus of the small intestine?
Simple columnar
What is the lacteal?
Dilated lymphatic capillary vessel
What are the 3 sections of the small intestine and what does each absorb?
• Duodenum/Jejunum/Ileum
– Most active absorption proximally
- Duodenum absorbs iron
- Jejunum absorbs most of sugars, aa’s and fatty acids
- Ileum absorbs Vit B12, bile acids and remaining nutrients
– By terminal Ileum
• Nutrients absorbed
• Much water, still to be absorbed
What are the sections of the colon?
Caecum Ascending colon Transverse colon Descending colon Sigmoid colon
Whole colon approx 1.2m long and 6-9cm wide
What are crypts of Lieberkühn? What type of epithelium lines them?
Intestinal glands in the mucosa of the colon
Simple columnar
Describe the contents by the end of the large intestine
Continues water recovery over a 20 hr transit
By end of large intestine contents semi-solid
• contents await expulsion in colon (not rectum)
• at certain times rapidly propelled into rectum
- stretches rectum
- critical mass gives urge to defaecate
- if higher centre control overrides reflex – reverse peristalsis
What is the purpose of the colon bacteria?
Colon contains most of the GI tract’s bacteria 99% of these anaerobic and most lost in faeces (1011/g) Involved in:
• synthesis of vitamins K, B12, thiamine and riboflavine
• breakdown of 10 to 20 bile acids
• conversion of bilirubin to non-pigmented metabolites - (all readily absorbed)
How much fluid is added of each kg of food consumed?
13l
What problems can occur with gut fluid balance?
It is a delicate balance (why diarrhoea & constipation so common)
• Gut is dealing with large quantities of fluid
Decreased absorption, or increased secretion can result in:
– life threatening dehydration (depletion of body fluids, not just what we recently drank/ingested)
– life threatening electrolyte imbalance
How is motility and secretion in the gut controlled?
Motility and secretion need precise control
• Achieved by overlapping mechanisms
• Neural
• Paracrine (especially important in the stomach)
• Endocrine
Outline the neural control in the gut
– Somatic (innervating striated muscle)
• Ingestion (mouth and first ⅓ of oesophagus)
• Excretion (last sphincter of anus)
– Autonomic nervous system controls the rest
• Post ganglionic neurones form plexuses
- one between muscle layers of gut wall
- one between submucosa and muscularis externa
• The ‘gut nervous system’ (the ‘gut brain’)
• Range of neurotransmitters
• Parasympathetic nervous control – ‘rest and digest’
Outline the paracrine control in the gut
Substances act locally
• Histamine (controls production of acid in stomach)
• Vasoactive substances (affect blood flow in gut)
Describe the endocrine control in the gut
Range of hormones control:
• secretion of stomach acid
• alkali secretion from liver and pancreas
• enzyme secretion
These hormones are all peptides of 2 structurally related families
• very phylogenetically old
(Gut hormones were 1st hormones to be discovered)
As general rule, gut is controlled by hormones it makes itself, rather than hormones made elsewhere.
