SI (Toosi) Flashcards
What is the function of the SI?
Digestion & absoprtion of fats, carbs and proteins, vitamins and mineral
what are the tissue layers? and their general functions?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
General function:
- protection
- secretion
- digestion
- absorption
- increase SA
What are the layers within the mucosa and what are their functions?
mucous membrane (columnar epithelium)
- protection (tight junctions)
- secretion (exocrine and endocrine)
- digestion (enzymes)
- absorption (transporters)
Lamina propria
- support (bedding fro epithelial cells, blood/lymph vessels and nerve fibers)
- protection (GALT)
Muscularis mucosa
- thing layer of smooth muscle
- adjusts villus length
What is the function of the submucosal layer?
- provides support, regulation and control (houses larger blood and lymph vessels, submucosal nerve plexus)
- elasticity and distensibility
- protection (brunners glands - buffer; peyers patches - T and B lymphocytes
What is the function of the muscularis externa?
- movement (segmental mixing and peristalsis_
- regulation and control (myenteric nervous plexus)
- inner circular and outter longitudinal
What is the function of the serosa?
support and maintain (serous fluid for lubrication attached to mesentary and body wall)
What provides protection in the small intestine?
epithelial cells (tight junctions,
laminal propria (GALT)
submucosa (brunners and peyers patch)
not really muscularis
What are three ways in which SA is increased?
folds 3x
villi & crypts 10x
microvilli 20x (digestion and absorption)
What structures affect the length of the villi?
muscularis mucosa can contract and change length
What 6 cells are found within the crypts?
crypt cells at the base divide and give rise to the following cells
- enterocytes
- majority of cells
- secrete H2O, Na and Cl (to help with absorption) - Goblet cells
- mucous (protection) - enteroendocrine
- monitor pH, osmolarity and composition of ingesta
- secrete hormones into blood to signal other areas of the body - paneth cells
- protect cryt stem cells
- release antibacterial subs into lumen - M-cells
- protection, capturing particles and pass them to immune cells
What 3 cells migrate up and are found in the villus?
- absorptive enterocytes (migrate up and change function)
- goblet cells
- M-cells
Which of the discussed cells play a protective role?
- paneth cells
- M-cells
- goblet cells
Explain countercurrent exchange in the villi and how hypoxia can affect this?
oxygen is transffered from arteriole to venule before it reached the tip, this leaves a lower amount of oxygen reaching the tip
in hypoxic condition the tip would not get sufficient oxygen severely affecting digestions and absorption
what is segmentation? what are its functions?
Oscillating ring like contractions of circular smooth muscle (alternating contracted and relaxed segments
functions:
- mixing contractions (mixing chyme nad mixing with digestive juices)
- exposing chyme to absorptive surfaces
- pushing back and forward
What regulates segmentation?
initiated in both duodenum and ileum
- food in stomach –> gastrin release–> gastrin signals segmentation in ileum
- Chyme in duodenum –> increases SI pacemakers –> BER (basic electrical rhythm) –> travels along circular smooth muscle –> contracts
- parasympathetic increases responsiveness and intensity
If segmentation propels food forward and backwards why does it still travel forward in the GIT?
we have 12 pushes a min forward and only 9 pushes backwards and food is not able to pass back through the cardiac sphincter.
What is the migrating motility complex?
weak, repetitive peristaltic contractions (circular and longitudinal)
- moves bolus long distances
- cleans small intestine of undigested materials
- most active between meals
- reduces bacterial proliferation
How is the MMC regulated?
- coordination by vagal sensory afferent and parasym efferent
- full stomach –> gastrin –> stimulates MMC –> provides room for stomach emptying
- duodenum low pH –> secretin –> slows MMC –> beginning of segmentation
- intestinal distension –> pain –> sympathetic discharge –> epi –> cessation of intestinal motility
What is released from the crypt cells that initiates MMC post digestion and absorption?
motilin
How does the bolus of food stimulate muscle contractions? and what contractions occur and where?
mechanoreceptors
distal to bolus: circ -, long +
proximal to bolus: Circ +, Long -
What would the order of small intestine contractions be following a meal?
- increase in peristaltic contractions
- decrease in peristaltic contractions
- increase in segmentation
- decrease in segmentation
- increase in peristaltic contractions
What is the connecting structure between the ileum and the LI? what 2 characteristic does it have?
ileocecal juncture
- folds of tissue create ileocecal valve
- smooth muscle creates ileocecal sphincter
Whats the point of ileocecal juncture?
prevents contamination of colonic bacteria
What are small intestine secretions called? functions?
succous entericus
Functions:
protection, lubrication, neutralization, electrochemical force, moisture for enzymatic digestion, adjusting, adjusting osmolarity
all enzymes from pancreas
what cells participate in secretions?
crypt entercytes
goblet cells
exocrine glands (brunners)
What is secreted from crypt enterocytes?
Na+: electrochemical gradient fro absorptive cells
Cl-
H2O: adjust osmolarity of digesta and solubalizes ions, sugars and aa’s
What is secreted by goblet cells?
mucus in response to mucosal damage and prostaglandin release
What is secreted by brunner’s glands?
how is it controlled?
Na/K: mostly upper duodenum, alkaline secretion fro neutralization
mucous
H2O: adjust osmolarity for sugars, ions, aa’s
low pH –> enteroendocrine cells –> endocrine secretin –> stimulates gland
If a viral invasion has occurred how would mucus secretion change?
prostaglandins released by damage signals goblet cells
covers and protects damaged areas
What mechanism is involved in enterocyte regulation?
what is the cascade?
vagal and ENS
- Meal
- stretch receptors, osmolarity, presence of aa, preduced pH
- sensed by sensory afferent neuronsin duodenum and jejunum
- signals to medulla
- vagal parasympathetic efferent signals
- AcH release
- activates G protein
- G protein activates phospholipase
- PL produces IP3 which moves to ER and causes Ca into cytosol
- Ca binds to calmodulin
- calmodulin activates Cl- channels
- Na follows into lumen
- water is pulled into the lumen with osmotic gradient
What cascade occurs with secretory diarrhea (bacterial toxin) ?
- bacterial toxin activates receptors on enteroendocrine cells
- cell releases serotonin
- Seritonin binds to receptor (SR)
- SR activated Ca channels and calcium enters
- binds calmodulin which activates Cl channels
- Na follows
- water follows
What cascade occurs with secretory diarrhea (e.coli) ?
- e.coli binds to receptor on enterocyte
- G protein activates AC
- AC convert ATP to cAMP
- cAMP activates Ca channels
- same cascade
What cascade occurs with secretory diarrhea (prostaglandin) ?
PGE2 produced in reponse to epithelial dmamge binds to receptors in enterocyte
- G protein then activates PL-A
- PLA catalyzes production of IP3
- IP3 moves to ER open Ca channels
- same cascade
What are the 3 main carbohydrates?
starch (glycogen, sucrose, lactose
Where are they digested?
mouth (amylase - alpha 1-4) SI lumen (pancreatic amylase) Brush border (maltotriase, maltase, sucrase, lactase)
What is the absorbable unit of carbs?
Glucose, fructose and galactose
How is glucose, fructose and galactose absorbed?
fructose - GLUT5
galactose/glucose - Secondary active transport
- e.g. SGLT-1 (requires sodium co transport)
driven by Na/K ATPase pump to maintain gradient
all diffuse out into blood via GLUT2
what is tertiary active transport?
Na/H+ exchanger
H+/glucose co-transport
Na/K ATPase
What protein digestion occurs before the SI in stomach?
- HCl denatures
- Pepsin (breaks proteins at aromatic side chains)
- rennin
protein fragments of 25-100 aa enter duodenum
What protein digestion occurs in SI?
crypt cells (I-cells) secrete CCK
CCK triggers pancreas –> proteolytic enzymes
CCK trigger enterocytes –> enterpeptidase
What does enteropeptidase activate?
trypsinogen > trypsin
chymotrypsinogen > chymotrypsin
pro-elastase > elastase
procarboxypeptidase A&B > carboxypeptidase A&B
1-12 aa fragments come in contact with brush border
What protein digestion occurs at brush border?
intestinal peptidases project and create 1-3 aa fragments
How are single AAs absorbed?
secondary active transport
How are small peptides (2 or 3 aa’s) absorbed?
tertiary active transport
How are 2 and 3 aa peptides transported into the blood if GLUT2 can only transport single aa?
broken down by cellular peptidase and then diffuse once concentration gradient is built up
What fat digestion occurs before the SI in the stomach?
churning causes emulsion with water
results in fat droplets
What fat digestion occurs in the SI?
Crypt cells (I-cells) secrete CCK CCK stimulates gall bladder to release bile salts which emulsify small droplets increasing SA for digestion CCK stimulates pancreas to release enzymes which is activated by trypsin
end result monoglycerides, fatty acids and cholesterol (absorbable units)
What enzymes does typsin activate?
lipase
Pro- colipase –> colipase
phospholipase
cholesterol esterase
What does lipase and colipase do?
lipase turns fats into cholesterol, fatty acids and monoglycerides.
colipase just helps
this allows bile salts to form micelles so they are soluble and can move to absorbable cells
What occurs at the brush border for lipid digestion?
brush borer lipase get the left overs
How is fat absorbed?
cholesterol, fatty acids and monoglycerides freely enter enterocyte because of concentration gradient when micelle binds to border
Why are fatty acids and monoglycerides resynthesized into triglycerides?
- to reduce osmolality of the cytosol
2. maintain concentration gradient for monoglycerides and fatty acids
what are chylomicrons and what is their purpose?
Fat droplets coated with lipoproteins
move outside of the cell from basolateral surface by exocytosis to enter lacteal
joins with thoracic duct and blood circulation
What is the fate of chylomicrons (express apolipoprotein type B) the circulation and what processes do they go through?
- interact with HDL and exchange apolipoprotein C & E
- bind to peripheral tissues (adipose, mammary, skeletal m, cardiac m) with C and deposits TG and CHOL
- remnants bind to liver with E where CHOL and TG can be transferred to VLDL (chylomicron remnants recycled)
- VLDL can deliver TF to peripheral tissues
- remnant of VLDL is LDL which delivers CHOL to arterioles causing atherosclerosis
- HDL can remove this CHOL and TG from arterioles
