The small intestine Flashcards
how long is the small intestine?
what is the diameter?
6m long
3.5 cm in diameter
what are the parts of the small intestine?
how long is each one?
(duodenum=25cm, jejunum=2.5m, Ileum=3.75m
what is the mesentry?
- The small intestine is lined on the inside by fan-shaped mesentery
- increases the surface area of the intestine with folds and supports the blood supply.
- The mesentery then has villi (~1mm tall) covering it
what are the layers of the small intestine?
- external wall has longitudinal and circular muscles
- the internal mucosa are arranged in circular folds
- the mucosa are covered in villi
there are invaginations known as the crypts of lieberkuhn
what are the villi made up of?
what is the function of villi?
The Villus is comprised of simple columnar epithelium:
o Primarily enterocytes (absorptive cells).
o Scattered goblet cells.
o Enteroendocrine cells.
needed to increase the surface area
what are the crypts of lieberkuhn made of?
- Stem cells – to replace the high turnover of enterocytes.
- Paneth cells – base of crypt, secrete acidophilic granules, lysozyme etc
features of enterocytes?
life span?
- most abundant cell types
- Feature a basal nucleus and is specialised for absorption
- Has a brush border of microvilli, each of which is covered in glycocalyx.
SHORT life span
what is the glycocalyx?
– carbohydrate layer that traps a layer of water and mucus to regulate rate of absorption and to protect
surface of microvilli from pH of lumen.
how much is the SA increased by the mesentry folds, microvilli and the villi?
x500 increase
features of the goblet cells?
- 2nd most abundant
- Produces mucus (large glycoproteins
- abundance increases as you get further through the bowel as more lubrication is needed
features of Enteroendocrine Cells?
- columnar epithelial cells
- secrete hormones to influence gut motility
- Found in lower parts of the crypts.
where are paneth cells found?
what do they contain?
the base of the crypts
- they contain large acidophilic granules which have
- lysozymes (antibacterial to protect stem cells)
- glycoproteins ( to protect the enzymes from themselves.)
- zinc (co factor to the enzymes)
Can have phagocytic roles to bacteria and protozoa.
life span of Enterocytes & goblet cells ?
short life span (36 hours)
what type of stem cells are in the villi?
how do the stem cells move?
stem cells are PLURIPOTENT
- The stem cells are continually shunted up the villus to replace old cells lost at the tips.
Dividing stem cells in crypt shunted up → at tip, cells become senescent →
apoptosed cells sloughed into lumen → apoptosed cells digested and reabsorbed by intestine.
whys is a rapid turnover of enterocytes needed?
- enterocytes are the first line of defence so may be directly affected by toxic
substances in the diet →
interference in cell function →
short life-span means any lesions are short lived.
what are the characteristics of the Duodenum?
- Brunner’s Glands present
- Submucosal gland found in crypt base, secreting alkaline fluid into chyme.
what are the functions of the alkaline fluid in the duodenum?
- Neutralise acid to protect small intestine epithelium.
- Optimise pH for pancreatic
enzymes.
what are the features of the jejunum?
- Presence of numerous, large folds in the submucosa called plicae circulares
how do the plicae circulares compare in jejunum and the rest of the small intestine?
- in jejunum they are smaller and thinner
what is the main function of the ileum?
what is a distinct characteristic ?
Major immune defence
and uses defences like Paneth cells bactericidal and rapid cell turnover.
- peyers patches
what does cholera do?
Vibrio cholerae results in prolonged opening on chloride channels = water secretion.
what are the three movements of the small intestine?
- mix
- facilitate contraction
- propelling of the contents
what does segmentation do?
- MIX
- Mixes the contents of the lumen.
- stationary contraction of the circular muscles at regular intervals
- More frequent in the duodenum than ileum
what does peristalsis do?
- PROPEL
- Sequential contraction of adjacent rings of smooth muscle to propel chyme.
what is the migrating motor complex?
- waves of peristalsis.
- most prevalent during fasting
- Prevents migration of colonic bacteria and ‘cleans’ intestine of residual food.
how does digestion in the duodenum happen?
- Small intestine digestion is in an ALKALINE environment.
- digestive enzymes are moved through the Sphincter of Oddi into the duodenum
how are carbohydrates digested?
- Digestion begins in the mouth with
salivary a-amylase - so complex carbs are broken into simple carbs
where is pancreatic a amylase secreted?
- Pancreatic a-amylase is secreted into the duodenum and digests starch and glycogen after a meal.
what does pancreatic a amylase work best in?
- Requires Cl- and a slightly alkaline environment ( brunners glands) - works mainly in the lumen - the digestion of amylase products and simple carbs happens at the membrane
how are glucose and galactose absorbed?
- Absorption of glucose and galactose via secondary active transport
- this is requiring a carrier protein and its going against the electrochemical gradient
- SGLT-1 on apical membrane
how is fructose absorbed?
facilitated diffusion.
- Carrier protein = GLUT-5 on apical membrane
what is the main enzyme for protein absorption?
- Major regulator is TRYPSIN
how are proteins digested?
- digestion begins in the stomach with pepsin but pepsin becomes inactivated
by alkaline duodenum - Pancreas secretes zymogen pepsinogen (the pancrease does not digest itself) (activated by enterokinase)
- then trypsin activates other proteases
how are proteins absorbed?
- Amino acids absorbed by facilitated transport and secondary active transport.
- Di- and tri-peptides are absorbed by distinct carrier proteins
- Cytoplasmic peptidases break down most di- and tri-peptides before excretion
what are the four stages of digestion of lipids?
→ Bile & lipase secretion
→ emulsification (because water and fast don’t mix!)
→ hydrolysis of ester linkages
→ micelles are made to increase SA for digestion
how do bile salt micelles digest lipids?
Bile salts are amphipathic – have a hydrophilic (OH-
, carboxyl) and hydrophobic (methyl) side.
therefore splits up the micelle
how do enzymes break down the lipids?
- lipase ( triglycerides into monoglycerides and FFAs)
- Phospholipase A2 (hydrolyses fatty acids)
- Pancreatic cholesterol esterases ( hydrolyses cholesterol ester)
how are lipids absorbed?
- Micelles are absorbed much quicker than emulsions so are important
- Micelles transport across the ‘unstirred layer’ and present FAs and monoglycerides to the brush border
- The whole micelle is then not absorbed – bile salt absorbed in ileum, lipids absorbed by middle jejunum.
-
how are bile salts recycled?
Bile salts are then recycled in the Liver via the enterohepatic circulation.
what are the two ways of lipids being metabolised?
- Monoglyceride Acylation – MAJOR:
- Phosphatidic Acid Pathway – MINOR:
how does Monoglyceride Acylation work?
Fatty acids bind to apical membrane → FABP (Fatty Acid Binding Protein) moves FA
to smooth ER → FA esterified into di-glycerides and triglycerides.
how does the Phosphatidic Acid Pathway work ?
- Triglycerides synthesised from CoA fatty acid and a -glycerophosphate.
what does the ileocacal sphincter do?
Ileum is separated from colon
so the colon is not involved
