Small Bowel Flashcards
What is the function of the small bowel?
Absorb nurtrients salt and water
Describe the structure of the small bowel
du 25cm
je 2.5 m
ile3.75
What functions does the Mesentary have?
Suspends small and larger bowel from posterior abdominal wall: anchoring them in place
provides a conduit for blood vessels, nerves and lymphatics
What does the superior mesenteric artery supply?
COMES OUT FROM THE inferior border of the pancreas
supplies duodenum and jejunum and ilieum
What is the serosa?
Outer covering of digestive epithelium.
What are plicae circulares
Also known as the valves of krekhering …
circular folds, termed the plicae circulares, which amplify the organ’s surface area, promoting efficient nutrient absorption.
Describe the structure of the villus?
Found on the pilcae circulares
Have endocrine, goblet, paneth, enterocyte cells.
Have a brush border - microvilli
What are the adaptations of the villi?
Only in small bowel
Motile
rich blood supply and lymph drainage
simple epothelial
1 cell thick
sominated by enterocytes
What provides innervation to the villi?
good innervation from submucosal plexus
What cells are the villi lined with?
Simple columnar epithelial consisting of:
enterocytes (most)
scattered goblet cells
enteroendocrine cells
What are the crypts of lieberkuhn? What cells make it up?
Epithelium in the cell bowel that contain:
Paneth cells
Stem cells
What are the characteristics of enterocytes?
Abundant in s.bowel
Columnar with microvili and basal nuclei
Absorption and transport
measurements of microvlli?
Surface area
Cylindrical internal surface area of small bowel is 0.4m2.
Folds, villi & microvilli ↑ surface area to ~200m2 (size of a tennis court)
At least a 500 fold ↑
Why do enterocytes and goblet cells have a short life span ( 36 hrs )
Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet.
Effects of agents which interfere with cell function, metabolic rate etc will be diminished.
Any lesions will be short-lived.
If escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur
What covers the surface of the microvilli?
Glycocalyx :
rich carb layer on apical membrane
protection from digestional lumen
Traps water and mucous known as “unstirred layer” which :
regulates rate of absorption from intestinal lumen
What are the characteristics of Goblet cells?
2nd most abundant cell type
mucous: larger glycoprotein
What gives goblet cells their goblet shape?
mucous containing granules accumulation at apical end of cell - goblet shape
How do goblet cells change down the bowel ( arrangement )
Increased abundance of goblet cells along entire length of bowel
↓ in duodenum
↑ in colon
What are enteroendocrine cells?
Columnar epithelial cells
scattered among enterocytes
most often found in lower part of crypts.
Hormone secreting
e.g. to influence gut motility (see Regulation of function lecture)
*referred to as chromaffin cells (affinity for chromium/silver salts).
Paneth cells contain large acidophilic granules, what are in these?
Granules contain:
antibacterial enzyme lysozyme (protects stem cells)
Glycoproteins & zinc (essential trace metal for a no. of enzymes)
What roles can paneth cells play?
Also engulf some bacteria & protozoa
May have a role in regulating intestinal flora
Where do stem cells in the Gi tract migrate?
To the tip of villus as they divide mitotically
Replace dead cells ( apoptosis )
How is the duodenum distinguised?
Distinguished by presence of Brunner’s glands
What are Brunner’s glands?
Submucosal coiled tubular mucous glands secreting alkaline fluid
Open into the base of the crypts
Alkaline secretions of Brunner’s glands
Neutralizes acidic chyme from stomach, protecting proximal small bowel
Help optimise pH for action of pancreatic digestive enzymes.
What are the functions of the small intestine motility?
To mix ingested food with digestive secretions & enzymes
To facilitate contact between contents of intestine & the intestinal mucosa
To propel intestinal contents along alimentary tract
How does segmentation occur ( mixing ) in the small bowel?
Mixes contents of lumen
Occurs by stationary contraction of circular muscles at intervals.
More frequent contractions in duodenum cf. ileum
allow pancreatic enzymes & bile to mix with chyme
Although chyme moves in both directions, net effect is movement → colon
How does Peristalsis occur ( propelling) in the small bowel?
Involves sequential contraction of adjacent rings of smooth muscle
Propels chyme towards colon
Most waves of peristalsis only travel about 10cm
Segmentation & peristalsis result in chyme being segmented, mixed & propelled → colon
How does Migrating motor complex occur in the small bowel?
Cycles of smooth muscle contractions sweeping through gut
Begin in stomach → small intestine → colon → next wave starts in duodenum
Prevents migration of colonic bacteria into ileum
What are the anatomical differences between the jejnum and the ileum?
Jejunum is wider, thicker walled and redder than the ileum
thicker because plicae circulares are larger, more numerous and closely set
- jejunal mesentray is attached to the above and left of aorta whereas ileal mesenatary which is attached below and to the right of the aorta
jejunal mesenteric branches have 1 arterial arcades and long arterial vessels whereas ileum has multiple arcades and shorter vessels.
What envinronment does digestion in the duodenum take place?
Digestion in small bowel occurs in an alkaline environment
Pancreatic digestive enzymes & bile enter duodenum from MPD & CBD
Duodenal epithelium also produces its own digestive enzymes
Digestion occurs in lumen & in contact with the membrane
Name some simple carbohydrates?
monosaccharides - glucose & fructose
disaccharides - sucrose & maltose
Describe some complex carbohydrates
starch, cellulose, pectins
→ sugars bonded together to form a chain
How does digestion of carbohydrates start in the mouth?
salivary a-amylase (destroyed in stomach (acid pH)
*Most of digestion of carbohydrates occurs in small intestine
When is pancreatic a-amylase secreted into the duodenum
In response to a meal
Digestion of starch and glycogen in S.B
Acts in the lumen
Digestion of amylase products & simple carbohydrates occurs at the brush border
WHat does pancreatic a-amylase need for optimum activity?
Cl- ions
Slightly alkaline pH
What carrier protein absorbs glucose and galactose?
SGLT-1 on apical membrane
Secondary active transport
- GLUT 2 facilitates exit at basolateral membrane
How is fructose absorbed?
Diffusion through GLUT-5 on apical membrane
- GLUT 2 facilitates exit at basolateral membrane
Where does the digestion of protein begin?
in the lumen of stomach by PEPSIN
it is then inactivated in alkaline duodenum
What protein digestion occurs in the small bowel?
5x pancreatic proteases secreted as precursors → lumen of small bowel (e.g. trypsinogen)
Trypsin activated by enterokinase
an enzyme located on duodenal brush border
Trypsin → activates other proteases
hydrolyse proteins → single amino acids (AA) & oligopeptides (AA)n
Where do multi amino acids get broken down into monoaminoacids ?
Variety of peptidases at brush borders of enterocytes progressively hydrolyse (AA)n → AAs
Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1
These small peptides are digested to AAs by peptidases in cytoplasm of enterocytes
What are the four stages of digestion of lipids?
- Secretion of bile salts & pancreatic lipases
- Emulsification (↑s surface area for digestion)
- Enzymatic hydrolysis of ester linkages
- Solubilisation of lipolytic products in bile salt micelles
What is enzymatic hydrolysis of ester linkages?
Colipase complexes with lipase – prevents bile salts displacing lipase from fat droplet
How is the ileum separated from the colon?
Ileocaecal valve
What does the relaxation and contraction of the ileocaecal valve do?
Relaxation & contraction controls passage of material into colon
Also prevents back flow of bacteria into ileum
