Flashcards in Unit 6 - Intestinal Transport I: Fluid and Electrolytes Deck (47)
what are the components and lengths of the SI?
starts distal to pyloric sphincter, extends 21 feet to cecum
-duodenum: 1 foot long (demarcated from jejunum by ligament of Treitz)
-jejunum: 8 feet long (proximal 2/5 of SI)
-ileum: 12 feet long (distal 3/5 of SI)
compare absorptive SA of jejunum and ileum:
-mucosal folds of Kerkring
-increase 20x (so total increase 1500x; 3x more SA than ileum)
-increase 20x (so total increase 480x)
how is SA of SI amplified?
1. macroscopic folds of Kerckring
2. microscopic villi and crypts of Lieberkuhn
3. submicroscopic microvilli
how is the SA of LI amplified?
1. macroscopic semilunar folds
2. crypts (but not villi)
how much of the total intestinal absorptive area is needed for absorption?
there are 7 million cm2, and only half is needed for absorption
-half can be removed surgically w/o compromising absorptive function
what is celiac disease? (also non-tropical sprue)
malabsorptive syndrome where number and size of microvilli are reduced
-gluten destroys absorptive cells and decreases number of functional villi
-diarrhea and malnutrition result, causing fatal dehydration
what is tropical sprue?
same as celiac disease, but caused by infectious agent
-present in tropical countries associated with diarrhea, malabsorption, and nutritional deficiencies
what are villi and crypts of SI?
villi made of layer of absorptive enterocytes (columnar epithelial cells), and there are 2 villi next to each other with crypt between
what is turnover of SI cells?
crypt cells migrate toward tip of villi, and are extruded 3-8 days later (250 g of cells lost daily, excreted or digested)
-need 1 week to replace intestinal mucosa
-rate of cell renewal is reduced due to radiation, malnutrition, and sprue
what do undifferentiated cells of crypt do?
secrete NaCl from blood into lumen, and water follows osmotically
-crypt cells stop secreting as they migrate up the villus, and take on absorptive function, absorbing either NaHCO3 or NaCl
-mitotic figures can be seen at the base of the villi
--these become goblet cells and enterocytes
what do goblet cells do?
secrete mucous in response to ACh released from parasympathetic cholinergic nerve fibers
-mucous lubricates tissue during peristalsis, and forms protective barrier
what is radiation sickness?
radiation targets dividing cells, and since there is high turnover of intestinal mucosa and goblet cells and enterocytes have common stem cell in base of grypts, radiation victims have intestinal bleeding, diarrhea, and malabsorption/dehydration
what are the 2 parallel pathways for transfer of materials?
1. transcellular pathway: across brush border, thru cytoplasm, then across basolateral membrane
2. paracellular pathway (across shunt pathway): thru tight junction and extracellular space
-very permeable to H2O and cations (Na, K), but not to anions
-tight junctions are low resistance (high conductance) pathway
what is the major membrane for absorption of nutrients? how does it do this?
brush border (AKA apical or lumenal membrane)
-has a glycoPRO matrix
-contains ectoenzymes (enterokinase, disaccharidases, peptidases) to complete digestive process
what does the basement membrane contain?
capillaries and lacteals (also known as lamina propria)
how does the paracellular pathway differ from jejunum to ileum to colon? what does this mean?
leaky in jejunum to tight in colon
-since shunt is permeable to H2O and cations, but not anions, when you drink water, the liquid is rapidly absorbed in jejunum by osmosis
what is succus entericus?
what happens if the jejunum is removed surgically?
ileum adapts to take over its function
what are 2 exceptions that are absorbed only in ileum, instead of mainly in jejunum?
vitamin B12 (requires intrinsic factor from parietal cells) and ionized bile salts
-nonionized bile salts can be absorbed the entire length of SI
what happens if the distal ileum is removed?
other parts of GIT cannot adapt, causing pernicious (macrocytic) anemia
how is Na+ absorbed by jejunal absorptive cells?
Na has 2 mechanisms
1. Na+/glucose and Na+/AA cotransport from lumen into cell
2. Na+/H+ antiport on lumenal side to keep internal pH of enterocyte close to neutral
3. Na+/K+ pump on basolateral membrane
how is HCO3- absorbed by jejunal absorptive cells?
HCO3- has "disappearing ball" mechanism
1. in cell, H2O breaks into H+ and OH
2. H+ from Na+/H+ antiport exits cell, and combines with HCO3- to make H2CO3
3. carbonic anhydrase breaks into H2O and CO2
4. CO2 crosses lumenal membrane, then combines with OH (from (1)) to make HCO3
5. HCO3 then somehow gets past basolateral membrane
what is the membrane potential at rest and during absorption?
+5 mV at rest on serosal side, and rises to +15 mV after meal b/c electrogenicity of Na+/K+ pump
where is there Na+/glucose or Na+/AA cotransport with Na+/K+ pumps?
jejunum (major) and ileum (minor)
where are there Na+/H+ exchangers on both basolateral and luminal sides, plus an Na+/K+ pump on the basolateral side?
duodenum and jejunum (major)
where are there parallel Na+/H+ and Cl-/HCO3- exchangers on the luminal side, with Na+/K+ pumps, water, and CO2 diffusion on the basolateral side?
ileum and proximal colon
where are there Na+ channels on the luminal side, and Na+/K+ pumps on the basolateral side?
only in the colon
what does amiloride block?
Na+/H+ exchangers and Na channels
how is NaCl reabsorbed by ileal absorptive cells?
both enter via Na/H and Cl/HCO3 exchangers on lumenal side, then Na/K pump and Cl diffusion on basolateral side