Small Bowel Flashcards
(25 cards)
What is the function of the small bowel?
Absorb nutrients, salt and water
What are the 3 sections of the small bowel?
- Duodenum - 25cm
- Jejunum - 2.5m
- Ileum - 3.75m
- total: 6m long
- diameter: 3.5cm
What is the mesentery?
Fold of membrane attached to the bowels
—> Suspends small and large bowel from posterior
abdominal wall (anchors)
—> Conduit for blood and lymphatic vessels
Which 5 blood vessels pass through the mesentery?
- Middle colic artery
- Right colic artery
- Ileocolic artery
- Superior mesenteric artery
- Jejunal and ileal arteries
What are the 4 layers of small bowel tissue?
- Serosa —> outermost
- Muscular - longitudinal
- circular - Submucosa
- Mucosa
How does the small bowel have such a large surface area?
Plicae circulares —> inner lining folds
- thicker in jejunum
Villi on plicae circulares
Microvilli on villi
What are villi?
Projections
- motile
- rich blood supply
rich lymph drainage
- innervated by submucosal plexus
- simple epithelium —> 1 cell thick
- separated by crypts
What are the 5 cell types in the small bowel?
Villi lining:
1. Enterocytes (most) —> absorption
2. Goblet cells —> secrete mucus
3. Enteroendocrine cells
Crypts of Lieberkühn lining:
4. Paneth cells
5. Stem cells (at bottom)
What are enterocytes?
Tall columnar cells for absorption and transport
- most abundant
- lifespan: 1-6 days
- villi and microvilli increase surface area from 0.4m2
to 200m2 (x500)
What are microvilli?
Projection on villi —> brush border
- 0.5-1.5µm tall
- 1000s per villi
- coated in glycocalyx
What is glycocalyx
Carbohydrate-rich coating of microvilli
- protection of microvilli from digestion in lumen
- traps water and mucus —> unstirred layer
- regulates rate of absorption
What are goblet cells?
Mucus-secreting cells —> mucus granules (apical side)
- 2nd most abundant
- mucus = large glycoprotein facilitating passage of
material through bowel
- abundance increases along bowel
What are enteroendocrine cells?
Hormone-secreting columnar epithelial cells
- usually in lower crypt
- hormones —> gut motility
What are paneth cells?
Immune cells at base of crypts
- granules - large + acidophilic
—> lysozyme (antibacterial) - protect stem
cells
—> glycoproteins and zinc - trace metal for
enzymes
- engulf some bacteria and protozoa
- may regulate gut flora
What are small bowel stem cells?
Undifferentiated cells that differentiate to replace cells that die (pluripotent)
- continually divide via mitosis
- cell dies —> digested and reabsorbed —> stem cell
differentiates —> migrates to top of villus
Why do enterocytes have rapid turnover?
Constantly damaged
- first line against pathogens
- toxic substances in diet
- issues with turnover —> severe intestinal
dysfunction
What are Brunner’s glands?
Mucus glands secreting alkaline fluid
- in submucosal layer
- coiled and tubular
- open into base of crypts
- alkaline —> neutralises acidic chyme
- protects small bowel
- optimises pH for pancreatic enzymes
What are the 3 differences between the jejunum and ileum?
- Jejunum —> thicker wall ∵ more + thicker plicae
ciculares - Ileum —> Peyer’s Patches
- Jejunum —> longer arterial arcades
Why does the small bowel need motility? (3)
- Mix food with secretions and enzymes
- Facilitate contact between contents and mucosa
- Move contents along bowel
What are the 3 types of small bowel motility?
- Segmentation —> mixing
- stationary contraction of circular muscles at
intervals (faster in duodenum) - pushes chyme in both directions but net
movement to colon
- stationary contraction of circular muscles at
- Peristalsis —> propelling
- sequential contraction of adjacent rings of muscle
- waves —> around 10cm
- Migrating motor complex
- cycles of contractions from stomach —> small
intestine —> colon (restart at duodenum) - prevents migration of colonic bacteria into ileum
- cycles of contractions from stomach —> small
How does digestion occur in the duodenum?
- alkaline environment
- pancreatic juice in via MPD (main pancreatic duct)
bile in via CBD (common bile duct) - enzymes from duodenal epithelium
How are carbohydrates digested? (3)
- Mouth —> salivary α-amylase
- little digestion ∵ destroyed by stomach acid - Small bowel lumen —> pancreatic α-amylase
- secreted into duodenum
- conditions —> Cl- (optimum)
—> neutral pH - Brush border —> simple carb digestion
—> absorption
- glucose —> SGLT-1 (2 active t)
galactose —> SGLT-1 (2 active t)
fructose —> GLUT-5 (fac diff)
- to blood via GLUT-2
- absorb up to 10kg simple sugars/day
- monosaccharides: glucose, fructose, galactose
disaccharides: sucrose, maltose, dextrose
complex: starch, cellulose, pectin
How are proteins digested? (3)
- Stomach —> pepsin
- inactivated in alkaline duodenum - Small bowel lumen —> 5 proteases
- trypsinogen to trypsin via
enterokinase on duodenal
brush border
- trypsin activates others - Brush border —> PepT1 (H+/oligopeptide co-t)
- absorb small AA chains —> digested
to single AAs via cytoplasmic
peptidases in enterocytes
- proteins hydrolysed to single amino acids and
oligopeptides (AA)n
How are lipids digested? (7)
- Emulsification: fat globule —> droplets
- bile salts
- pancreatic lipases
- Enzymatic hydrolysis
- colipase complexes —> prevent bile salts
displacing lipases
- colipase complexes —> prevent bile salts
- Solubisation: produce micelles
- Enter enterocytes —> fatty acids and monglycerides
released - Triglyceride synthesis - monoglyceride acylation
- phosphatidic acid pathway - Chylomicron formation - in golgi
- 80-90% triglyceride
8-9% phospholipid
2% cholesterol
2% protein
trace carbohydrate - Exocytosis of chylomicron to lacteal
