Small Intestine Flashcards
What are the 3 parts of the small intestine?
duodenum
jejunum
ileum
What is the small intestine?
primary site of digestion and absorption in GI tract
Motility in Small Intestine
What are the two predominant motility paradigms in small intestine?
- segmentation
- migrating mobility complex
Motility in Small Intestine
What is segmentation and what does it do?
alternating contractions and relaxations of adjacent sections of small intestine
ensures thorough mixing during processing of chyme, and helps move chyme toward large intestine
Motility in Small Intestine
What does the migrating mobility complex do?
moves luminal contents along small intestine, in the period between meals
Motility in Small Intestine
What is the primary motility during a meal?
segmentation
Motility in Small Intestine
What is segmentation initiated by? (3)
- distension of lumen
- presence of enterogastrone gastrin
- parasympathetic input (sympathetic input decreases segmentation)
Motility in Small Intestine
When does segmentation get replaced by migrating mobility complex?
after absorption of a meal
Motility in Small Intestine
Describe the process of migrating motility complex.
- begins at duodenal-gastric junction
- consists of weak peristaltic contractions that travel for a short distance
- second wave begins slightly more distally than initiation site of first wave and travels slightly further
- ~2 hrs to travel from stomach to large intestine
- cycle repeats itself until ingestion of another meal initiates segmentation again
What are the two structures at the ileocecal junction?
- ileocecal valve
- ileocecal sphincter
What does the ileocecal valve do?
contents of ileum can push through the valve…
cecal contents pushing backward, close the valve ‘flaps’, preventing reverse flow
What controls the ileocecal sphincter?
neuronal and hormonal control
What causes relaxation of the ileocecal sphincter?
distension on ileal side
What causes constriction of the ileocecal sphincter?
pressure on cecal side
What also inhibits the ileocecal valve?
gastrin
What does the mucosal epithelium of small intestine secrete?
~1.5 L of aqueous salt and mucus solution (succus entericus)
What does aqueous salt and mucus solution (succus entericus) do? (3)
- lubricates passage through lumen
- protects mucosa from acid injury
- H2O molecules required for hydrolysis reactions
What is the stimulus for aqueous salt and mucus solution (succus entericus) production and release?
presence of luminal chyme
What are fats digested by?
completely hydrolyzed by pancreatic lipase
What are the absorbable components of fats?
monoglycerides and free fatty acids (FFAs)
How are proteins digested?
reduced to small peptide chains and some unitary amino acids by pancreatic proteolytic enzymes
peptide fragments require further hydrolysis by aminopeptidases in epithelial brush border prior to their absorption
How are carbohydrates digested?
broken into disaccharides and some monosaccharides
further hydrolyzed by disaccharidases in epithelial brush border
What are the adaptations of the small intestine for absorption?
- circular folds or ridges
- villi
- brush border
What are the circular folds or ridges of the small intestine?
on inner surface of small intestine – increase surface area 3x
What are villi?
microscopic projections extending into lumen – increase surface area further 10x
What is the brush border of the small intestine?
visible only by electron microscopy – increase luminal surface area further 20x
each epithelium has between 3000-6000 hair-like microvilli projecting into lumen
What are the 5 main parts of a villus?
- epithelial and mucous cells
- connective tissue core (lamina propria)
- network of capillaries
- lymphatic vessel (central lacteal)
- crypts of Lieberkuhn
Structure of Villus
What are epithelial and mucous cells held together by?
tight junctions
Structure of Villus
What does the brush border of epithelial and mucous cells contain?
- enzymes that hydrolyse peptides/disaccharides
- enterokinase
- transmembrane carrier proteins
Structure of Villus
What is lamina propria?
connective tissue core that gives villus its structure
Structure of Villus
What does the network of capillaries receive and drain?
receives arterial blood and drains into venous outflow
Structure of Villus
What is the central lacteal?
lymphatic vessel that accepts fat digestion products, and is involved in immune responses
Structure of Villus
What are the two cells of the crypts of Lieberkuhn, and what do they do?
- mucous cells are primary locus for succus entericus secretion
- stem cells that constantly produce new epithelial cells
Carbohydrate Absorption
What hydrolyzes disaccharides into monosaccharides?
disaccharidases in brush border
Carbohydrate Absorption
Glucose and galactose are absorbed via what mechanism?
secondary active transport
- cotransported into cell with Na+ ions
- passively diffuses into interstitial fluid through membrane channel
- enter capillary
Carbohydrate Absorption
Fructose is absorbed via what mechanism?
passively diffuses through epithelial cell by means of facilitated diffusion
Protein Absorption
Digested protein products absorbed in small intestine arise from…
ingested and endogenous proteins
Protein Absorption
Where do endogenous proteins arise from? (3)
- digestive enzymes that have been digested themselves
- proteins from sloughed epithelial cells
- plasma proteins that have leaked into GI tract lumen
Protein Absorption
What can endogenous proteins account for?
half of each days protein absorption
Protein Absorption
What are peptide fragments hydrolyzed by?
aminopeptidases in brush border
Protein Absorption
Hydrolyzed peptide fragments are absorbed via what mechanism?
secondary active transport
- with Na+ ions
- enter capillaries in lamina propria of villus
Fat Absorption
How does fat absorption differs from that of proteins and carbohydrates?
complete digestion by pancreatic lipase
Fat Absorption
Describe the process.
- micelles deliver monoglycerides and FFAs to brush border
- digested fat products diffuse into cell
- bile salts, lecithin, and cholesterol are free again to aggregate with luminal fat digestion products
- monoglycerides and FFAs are combined again to form triglycerides
- aggregate with bipolar lipoproteins forming chylomicrons
- exocytosed into interstitial fluid
Fat Absorption
Where are chylomicrons?
- cannot cross the basement membrane of capillaries
- enter central lacteal of the lymphatic system
Where is water primarily absorbed?
small intestine
Na+ and Water Absorption
Describe the process.
- active pumping of Na+ ions into interstitium via Na+/K+ ATPase, along basolateral membrane
- pumping Na+ ions out of basolateral membrane creates concentration gradient
- favours Na+ entry into epithelial cells across brush border via Na+ channels - osmotic pressure of this hypertonic region attracts water from lumen
- hydrostatic pressure moves water into capillary network
When does iron absorption occur?
only when needed
Iron Absorption
How much iron is absorbed?
daily intake ranges from 15-20 mg, but absorb only 0.5-1 mg (male) or 1.0-1.5 g (female)
Iron Absorption
Is ferrous iron (Fe2+) or ferric ion (Fe3+) more easily absorbed?
ferrous iron (Fe2+)
Iron Absorption
What can prevent iron absorption?
phosphate molecules binding to iron
Iron Absorption
What can enhance iron absorption?
vitamin C can enhance absorption by increasing Fe2+ pool
Iron Absorption
Where is iron transported?
- actively transported into epithelial cells (a regulation site influenced by body iron need)
- iron needed for RBC production immediately enters blood while the rest remains bound to ferritin in epithelial cell
Iron Absorption
DO NOTES ON DIAGRAM
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When does calcium absorption occur?
only when needed
Calcium Absorption
Where is calcium absorption regulated?
at site of active transport of Ca2+ into epithelium
Calcium Absorption
What does vitamin D enhance?
brush border Ca2+ active transport
Calcium Absorption
How much calcium is absorbed?
normally, about ⅔ of ingested 1000 mg Ca2+ is absorbed each day
Biochemical Homeostasis
What does the GI system do?
maintains acid-base balance of body
Biochemical Homeostasis
What does intestinal absorption ensure?
that ions are recycled
Biochemical Homeostasis
What does a loss of electrolytes cause?
loss in large quantities produces acid-base imbalance – can occur during vomiting and diarrhea
Biochemical Homeostasis
What does excessive vomiting result in?
loss of stomach acid, and can result in metabolic alkalosis
Biochemical Homeostasis
What does diarrhea result in?
loss of HCO3- in feces, and results in metabolic acidosis
What does diarrhea influence?
nutrient and water absorption in small intestine
What happens in the small intestine during diarrhea?
- small intestine rapidly empties its contents
- rapid exit of chyme decreases absorption time
What does prolonged diarrhea lead to?
- malnutrition
- dehydration
- acid-base disturbance
What can diarrhea be caused by?
- excessive intestinal motility:
- hyper-osmotic chyme, resulting in luminal fluid accumulation
- toxins from bacteria (ie. Vibrio cholera)
How does excessive intestinal motility cause diarrhea?
- irritation of GI tract wall
- bacterial/viral infection
- emotional stress
How do toxins from bacteria (ie. Vibrio cholera) cause diarrhea?
- inhibit Na+ absorption or stimulate Cl- excretion
- osmotic fluid movement into instead of from lumen
