small bowel Flashcards
Q. What is the function of the small bowel?
To absorb nutrients, salt & water
what is the duodenum
The duodenum is the first segment of the small intestine. It’s largely responsible for the continuous breaking-down process
whta is the jejunum
The middle part of the small intestine. It is between the duodenum (first part of the small intestine) and the ileum (last part of the small intestine). The jejunum helps to further digest food coming from the stomach.
what is the ileum
The ileum is the final section of the small intestine. The function of the ileum is mainly to absorb vitamin B12, bile salts, and any products of digestion that were not absorbed by the jejunum
what is the transition between them
no sudden transition between them
how do the histological organistaion differ
All have same basic histological organisation
what is the mesentry
The mesentery attaches your intestines to the wall of your abdomen. This keeps your intestines in place, preventing it from collapsing down into your pelvic area.
functions
Suspends small & large bowel from posterior abdominal wall
anchoring them in place
whilst still allowing some movement
Provides a conduitfor blood vessels, nerves & lymphatic vessels.
where can you find villi
only occur in the small intestine
what is the blood and lymph supply like
Have a rich blood supply & lymph drainage for absorption of digested nutrients
what is the nerve innervation like
Have good innervation from the submucosal plexus.
describe the structure of the villi
Have simple epithelium
1 cell thick
dominated by enterocytes (columnar absorptive cells)
motile
what are enterocytes
(columnar absorptive cells)
what are villi lined with
simple columnar epithelium consisting of:
primarily enterocytes (absorptive cells)
scattered goblet cells
enteroendocrine cells
what is in the crypt epithelium
Crypts of Lieberkühn - epithelium includes:
Paneth cells
Stem cells
what is the most abundant cell in the small bowe,
enterocystes
describe their structure
Tall columnar cells with microvilli & a basal nucleus.
function
Specialised for absorption & transport of substances.
lifespan
Short lifespan of 1-6 days.
how is the surface area increased
Folds, villi & microvilli ^ surface area
what does the microvilli make up
brush border
what are the microvilli made up of
glycocalyx
what is a glycocalyx
rich carbohydrate layer on apical membrane
function
serves as protection from digestional lumen
yet allows for absorption.
regulates rate of absorption from intestinal lumen
what is the unstirred layer
the glycocalyx traps a layer of water & mucous known as “unstirred layer”
what do mucous cells contain apical
Mucous containing granules accumulate at apical end
what is mucous
→ large glycoprotein that facilitates passage of material through bowel.
how does the compisition of goblet cell change through the legnth of the bowel
^ abundance of goblet cells along entire length of bowel
what are enterendocrine cells
what is their structure
Columnar epithelial cells
where are they commonly found
most often found in lower part of crypts.
function
Hormone secreting
e.g. to influence gut motility (see Regulation of function lecture)
where are paneth cells found
Found only in the bases of crypts
what do they contain
Contain large, acidophilic granules
what are the contents of the granules
antibacterial enzyme lysozyme (protects stem cells)
Glycoproteins & zinc (essential trace metal for a no. of enzymes)
what is another function
Also engulf some bacteria & protozoa
May have a role in regulating intestinal flora
what are stem cells
Undifferentiated cells which remain capable of cell division to replace cells which die
why does the GI tract have stem cells
Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium
Differentiate into various cell types (pluripotent)
describe their life cycle
Migrate up to tip of villus, replacing older cells that die by apoptosis -> digested and reabsorbed
what is the relative life cycle of enterocytes and goblet cells of the small bowel
short life span (about 36 hrs)
what is their relative lifespan in comparison to other epithelial cells
Rapid turnover contrasts with lifespan of weeks/months for other epithelial cell types (e.g. lung, blood vessels)
why are the lifespans so different
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
how can you distniguish between the duodenum and the jeunum and iluem
Distinguished by presence of Brunner’s glands
how does the duodenum secreate alkaline fluid
Submucosal coiled tubular mucous glands secreting alkaline fluid
why the alkaline secreation
Neutralizes acidic chyme from stomach, protecting proximal small bowel
Help optimise pH for action of pancreatic digestive enzymes.
Functions of 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
what are the 3 phases of motility
Segmentation (mixing)
- Peristalsis (propelling)
- Migrating Motor Complex
what is the segmentation
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
what is peristalisis
. Peristalsis (propelling)
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
- Migrating Motor Complex
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
describe the digestion in the small bowel
Digestion in small bowel occurs in an alkaline environment
how do digestive enzyme enter the duodenum and bile enter
Pancreatic digestive enzymes & bile enter duodenum from MPD & CBD
why doesn’t the duodenal epithelial produce any of its oen digestive enzymes
Duodenal epithelium also produces its own digestive enzymes
where does digestion occur in the dueodnum
Digestion occurs in lumen & in contact with the membrane
what are the 2 carbihydrate strcutures
Simple carbohydrates (monosaccharides - glucose & fructose disaccharides - sucrose & maltose)
Complex carbohydrates
(starch, cellulose, pectins
→ sugars bonded together to form a chain)
what is the first step in digetsion of carbs
and where does most of it take place
Digestion begins in mouth by salivary -amylase (destroyed in stomach (acid pH)
Most of digestion of carbohydrates occurs in small intestine
when is Pancreatic a-amylase released
Secreted into duodenum in response to a meal
what is the optimum envoroment
Needs Cl- for optimum activity & neutral/slightly alkaline pH
where does pancreatic a-amylase act
Acts mainly in lumen (some also adsorbs to brush border)
Digestion of amylase products & simple carbohydrates occurs at the brush border
how is glucose and galactose absorbed
2o
active transport
what carrier protien
= SGLT-1 on apical membrane
how is fructose absorbed
by facilitated diffusion.
how
= GLUT-5 on apical membrane
what is the function of glut 2
facilitates exit at basolateral membrane
where are protiens first digested
Protein digestion begins in lumen of stomach by pepsin
pepsin then inactivated in alkaline duodenum
what then occurs in the small bowel
5x pancreatic proteases secreted as precursors → lumen of small bowel (e.g. trypsinogen)
what activates trypsin
enterokinase
where is enterokinase
an enzyme located on duodenal brush border
what does trypsin do
activates other proteases
hydrolyse proteins → single amino acids (AA) & oligopeptides (AA)n
what is the junction between the duodenam and jejunum
duodenojejunal flexure
what is the outer part of the small bowel called
serosa
what os under that
longitudinal and then circular muscle layer
what is cu=ircular muscle iimportant for
motility
what is brunners gland
submucosal coiled tubular mucous glands secreting alkaline fluid
where do they open
in the cypt
what is the difference between the jejunum and ileum
jejunum is wider and thicker walled
redder
where is the cypt
base of the villi
why is the jejunum more thicker
more plicae circulares
where do peptidases act
Action of luminal, brush-border & cytosolic peptidases
what do enterocytes absorb
Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1
are lipids soluble in water
Lipids are poorly soluble in water
more complicated to digest.
how are they digested
Secretion of bile salts & pancreatic lipases
Emulsification (↑s surface area for digestion)
Enzymatic hydrolysis of ester linkages (Colipase
complexes with lipase – prevents bile salts displacing lipase from fat droplet)
Solubilisation of lipolytic products in bile salt micelles
how are lipids absorbed
Unlike AAs & simple sugars -> lipids transformed as absorbed via enterocytes
describe their absoroption
Fatty acids (FAs) & monoglycerides (MG) leave micelles and enter enterocytes
FAs & MG resynthesized into tri-glycerides (TGS) by 2x pathways:
Monoglyceride acylation (major) Phosphatidic acid pathway (minor)
Chylomicrons - lipoprotein particles synthesised as an emulsion (80-90% TGs, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate) in Golgi apparatus
Chylomicrons secreted across basement membrane by exocytosis
Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel
what is the Ileocaecal valve
Ileum is separated from the colon by the ileocaecal valve
function
Relaxation & contraction controls passage of material into colon
Also prevents back flow of bacteria into ileum
