Lecture 8: Small intestine Flashcards
What is the intestinal phase of digestion?
- Initiated by entry of chyme from the stomach into the duodenum.
- Influenced by size of bolus, particle size, and macronutrient composition. 🌟
Reflex response to 4 things: 🌟
distention, low pH, osmolarity, and digestive products.
What are the responses of the intestinal phase of digestion?
How does a meal digest in the stomach? (think entry and departure)
- Bolus: small particles, emulsion
- Triglycerides: trigs, monoglycerides, FFA
- Proteins: protein, peptides, some amino
- Starch: Starch, oligosaccharides
What increases gastric emptying?
Gastric distention->empty rate is proportional to size of meal
What is gastric emptying inhibited/slowed by ⭐️
- Entry of chyme into duodenum (duodenal
distention) - Fat, protein, cations (electrolytes)
- Acidity (pH < 3.5) in duodenum
- Hypo/hyper-osmotic chyme in duodenum
- Solid food more inhibitory than liquids.
What are some hormones/enterogastrones that might inhibit or slow gastric emptying ⭐️
cholecystokinin (CCK), vasoactive intestinal polypeptide (VIP), Gastric Inhibitory Peptide (GIP), somatostatin, secretin
Inhibits the caudal (or “forward) motion of the contents of chyme
How does gastric emptying from the stomach to the duodenum trigger feedback?
CCK, Secretin and HCO3-
The gastric emptying rate depends on what?
macronutrient composition, osmolality, and caloric density
What are some implications of gastric emptying?
considerations for diabetics, foods effects on drug absorption, metabolism, restoring hydration status (with isotonic solution)
What is the osmolality of meal with hypertonic meal and hypo? where does this occur mostly?
Intestinal motility is coordinated by what? What is is modified by?
- Intestinal motility is coordinated by the enteric nervous system (ENS)
- modified by long and short reflexes, neurotransmitters and hormones
What happens during and shortly after a meal with intestinal motility? ⭐️
Chyme is mixed by segmenting movements of intestinal wall (both directions; circular muscles)
After food is digested and absorbed, segmentation is replaced by what? ⭐️
by peristalsis moving undigested material from small intestine into large intestine (one way; longitudinal muscles).
Circular and longitudinal contractions
Regulated by what?
Myenteric plexus (part of ENS) (aka Auerbach plexus)
Peristalsis vs segmentation
Circular sm vs longitudinal sm
slide 13
Interstitial Cells of Cajal (ICC) generate what?
generate GI slow waves (aka: BER; basal electrical rhythm)
Pacemaker ICC links what?
nervous system to smooth muscle
What is happening in a fed state with the small intestine?
Segmental Contractions
- Slows food propulsion, mixes to expose chyme to enzymes and GI surface
Peristaltic activity (propels food) replaces
segmentation when? ⭐️
completion of absorption
What is the motilin candidate hormone?
- secreted from M cells
- MMCs appear when in a fasted state.
- Motilin receptor agonists stimulate intestinal peristalsis
_ alter amplitude and duration of MMCs
Cathartics (laxatives) alter amplitude and duration of MMCs
Motilin is associated with _
- MMCs
- Plasma motilin peaks in fasting (interdigestive period). Note motilin is released cyclically every 80–120 min to induce contraction of the MMCs from the stomach to the ileum
vili and microvili?
- By projecting into the lumen, the Villi increases the surface area for absorption of nutrients.
- Microvilli (aka brush Border, 100nm) fringe the villi to further increase surface area
What is in the villus that is part of our lympathic system and imp for fats?
lacteal
What are the barriers within the paracellular pathway between intestinal epithelial cells
tight jxns
What happens when barrier function is messed up?
Disruption -> hyper-permeability (“leaky gut”) and potentially autoimmune or neurological disorders
What are the different anatomical features that are increases SA in small intestine
What are the 3 sub phases of intestinal digestion? + what they are doing in each
How is proteins digested in gut? ⭐️
All 3 phases, luminal, brush border and cytosolic digestion may be involved
How are carbs digested in gut ⭐️
Only luminal and brush border digestion – no intracellular digestion by the enterocyte
How are lipids digested in the gut?
All digestion is luminal (micelle);triglyceride is re-formed in the enterocyte (chylomicron)and transferred to lacteal (not capillary).
How does proteins get absorped?
Active transport of amino acids and small peptides (< 5 amino acids).
How does carbs get absorped in gut
- Uptake of monomers only
- Active transport of Glucose; facilitated diffusion for Fructose
How does lipids get absorped in gut?
- Uptake of free fatty acids and glycerol.
- Mechanism of uptake by the enterocytes is probably diffusion.
Starches digested in duodenal lumen by what? What does that produce?
Starches digested in duodenal lumen by amylase to produce oligosaccharides and disaccharides
What forms monosaccharides? +enzymes
Brush border digestion of polymers by specific amylases and disaccharidases forms monosaccharides. Note: Maltase, lactase and sucrase are at brush border
Simple sugars Glucose (and Galactose) taken up by what?
active transport processes into enterocytes.
Generally NO uptake of what?
disaccharides (trehalose) or oligosaccharides (fiber)
What are the transporters on the brush border membrane for monosaccharide absorption?
Note: SGLT2 is in Kidney (glucose reabsorption function)
meal glucose is completely absorbed where?
duodenum
Meal lactose absorption is where? variations?
For protein digestion, proteases stored how and where?
in inactive form in pancreas & secreted in response to neurohormonal stimulation
Pancreatic trypsinogen converted to what?
to active form (trypsin) by duodenal brush-border enterokinase
What does trysin do? 🌟
Trypsin activates all other luminal peptidases
Digestion of oligopeptides in lumen and small peptides happens where?
brush border
_ _ of free amino acids,di-andtri- peptides.
active transport
protein
_ degradation of di- and tri-peptides.
Cytosolic degradation of di- and tri-peptides.
protein
What is the activation of proteases in the small intestine?
What are the three sites of protein digestion?
lumen, brush border and cytoplasm
Short peptide uptake coupled to what?
to proton transport followed by cytosolic digestion
Removal of protein by digestion and absorption occurs where?
Lipid digestion in _ phase only.
luminal
Digestion of lipids require what?
bile salts, pancreatic lipase, co-lipase and phospholipids to form micelles
Lipids emulsify by what?
bile salts and phospholipids
Triglyceride digested to what?
to form free fatty acids and a monoglyceride
Triglyceride digestion productions are taken up by diffusion and packaged into what?
chylomicrons
Bile salts and phospholipids convert large fat globules into what?
smaller pieces with polar surfaces that inhibit reaggregation
Absorbed products of lipolysis are synthesized into triglycerides in the _ and exit as _
enterocytes
chylomicrons
Lipid digestion process
absorption of lipolytic produces occur where?
Sodium absorption generally coupled to what?
to nutrient absorption (e.g. SGLT1, PEPT1).
Electrogenic:
forces anions (mainly chloride) to passively follow the sodium transport by paracellular route
Water transported _ , following _ _ (favoring absorption).
Water transported passively, following osmotic gradients (favoring absorption).
What is the major mediator of mvt of water?
cl-
What is the driving force for chorlide efflux
potassium efflux
Cystic fibrosis transmembrane conductance regulator?
Mutations of the CFTR gene affecting chloride ion channel function lead to dysregulation of epithelial fluid transport in the lung resulting in cystic fibrosis.
Sodium, Potassium, and Chloride co-transporter is driven by what
Na+ gradient
What happens with cholerae?
What is IF and what is it required for?
is a glycoprotein and required for vitamin B12 (cobalamin) uptake
Where is absorption of IF? What protects it?
- Absorption is in terminal ileum.
- “R” Protein (haptocorrin) protects acid-sensitive B12 from degrading
B12 is essential for what?
B12 essential for metabolism, DNA synthesis and RBC production
What are causes of ulcers
Cause (~90%) is mucosal breakdown from H. pylori.
- NSAIDs, gastrinoma
Gastric (peptic) ulcer vs. Duodenal (peptic) ulcer
- Gastric (Peptic) Ulcer - mucosa damage due to leakage of acid from stomach.
- Duodenal (Peptic) Ulcer - More acid present in duodenum than normal (not clear, though) - increased parietal cell mass. Increased HCl + pepsinogen secretion.
