Intestinal Transport I Flashcards
1
Q
small intestine
A
- begins just distal to the pyloric sphincter of stomach and extends 21 feet to cecum
- duodenum-1 foot long
- jejunum- proximal 2/5, 8 feet
- ileum- distal 3/5, 12 feet
2
Q
absorptive surface area per cm of gut
A
- jejunum has 3x more SA than ileum, greater absorption occurring in jejunum
- SA increased by foldings
- mucosal folds- 75 vs 24
- villi- 750 vs 240
- microvilli 15000 vs 4800
3
Q
small vs large intestine
A
- SI amplified at three levels: macroscopic folds, microscopic villi and crypts of liberkuhn, and submicroscopic villi
- colon has folds, crypts but not villi, and micro villi
4
Q
total intestinal absorptive area
A
- average length- 21 feet, 700 cm
- area of simple cylinder=7,000 cm2
- increase due to mucosal folds- 35,000 cm (5x)
- due to villi- 350,000cm2 (10x)
- microvilli- 7 million, 20x
- absorptive SA is 700 sq meters- 1000x- 1/3 of a football field
- half of it required, half can be surgically removed
5
Q
celiac disease
A
- gluten is insoluble protein of wheat and other grains
- destroys absorptive cells and decreases number of functional villi
- food can’t be absorbed, diarrhea and malnutrition
- dehydration can be fatal
- restricted diet
- tropical sprune is and infectious disease present in certain areas of tropical countries associated with diarrhea, malabsorption, and nutritional deficiencies
6
Q
two villi and a crypt
A
- villi composed of layer of absorptive enterocytes
- goblet cells stimulated by Ach from PNS fibers to secrete protective mucous
- crypt cells migrate towards top of villi and are extruded 3-8 days later
- quick turnover
- 250g shed each day
- replaced after 1 week
- 5 days to mature and reach tip
- extruded cells digested in the gut
- crypt cells secrete Nacl from the blood to the lumen and water follows
- they stop as they migrate up villus and take on absorptive function
7
Q
radiation sickness
A
- goblet cells and enterocytes have a common stem cell base of the crypts
- high turnover leads to sensitivity to radiation damage
- radiation victims experience intestinal bleeding, diarrhea, and slow death from malabsorption and dehydration
- side effects of certain types of radiation cancer therapy are similar
8
Q
enterocyte
A
- microvilli on brush border
- tight junctions, intercellular spaces, basolateral membrane, basement membrane
- transcellular and paracellular pathway
- trancellular across brush border then through cyto then basolateral
- paracellular is shunt, through tight junction and extracellular space
- low perm to anions
9
Q
brush border
A
- glycoprotein matrix and major membrane for absorption
- contains ectoenzymes that complete digestive process
10
Q
basement membrane
A
- lamina propria
- contains capillaries and lacteals
11
Q
junctional permeability
A
- tight junction wraps around like a collar
- shunt is permeable to water and cations, not to anions
- intestine gets tighter as you go down
- liquid rapidly absorbed in jejunum
12
Q
intestinal fluid and electrolyte balance
A
- input equals output- 9.5L
- 2.5 input from diet, 1 from saliva, 2 from gastric juices, 1 from bile, 2 from pancreatic juice, 1 from intestinal secretions
- output-5 from jejunal absorption, 2.5 from ileal, 1.8 from colonic, 0.2 in feces
- difference between dietary intake and fecal output equals loss through kidney, lungs, and sweat to maintain body fluid in steady state
13
Q
site of intestinal absorption of solutes and nutrients
A
- water and most nutrients to duodenum and upper jejunum
- completely absorbed by end of jejunum
- except B12, need intrinsic factor;and ionized bile salts which undergo enteroheptatic circulation
- ileum absorbs some fluid and electrolytes, along with B12 and takes up ionized bile salts
- if jejunum is removed, ileum will take over
- if distal ileum removed, other parts can’t adapt
- B12 gone-pernicious anemia-macrocytic anemia with large RBCs
- intrinsic factor is secreted by parietal cells, complex is absorbed in distal ileum
14
Q
NaHCO3 Absorption by jejunal absorptive cell
A
- jejunal enterocyte absorbs Na and HCO3, glucose and aa
- panreas secretes NaHCO3 which serves to neutralize acid from the stomach
- the jejunal cell absorbs it to maintain fluid balance
- Na enters by Na/glu and Na/aa cotransport Na/H antiport
- antiport serves to keep internal pH of the enterocyte near neutral and away from electrochemical eq
- Na influx balanced by active efflux across basolateral membrane with Na/K pump
- 1 Na and HCO3 from lumen goes to the blood
- membrane potential 5mV on serosal side and 15mV during absorption after a meal
- HCO3 dissociates, crosses, and re-associates`
15
Q
modes of active Na absorption by intestine
A
- nutrient coupled in villous cells of jejunum and ileum, primary mechanism
- Na/H exchanger-jejunum and duodenum
- parallel Na-H and Cl-HCO3 exchangers in ileium and proximal colon
- epithelial Na channel in distal colon
