Flashcards in Small Bowel & Colon Deck (23):
What is the main method of motility in the small bowel during the fed state?
Segmentation during the fed state
Consists of variable patterns of peristaltic contractions
How is segmentation regulated?
Distention-- In duodenum, mechanoreceptors activate reflex arc with entry of chyme/fluids
Extrinsic nerves-- stimulated by PS suppressed by sympathetics
Gastrin: Activates segmentation in ileum
What is the method of motility during the fasted state?
Migrating motor complex-- consists of sequential organized short peristaltic waves that are initiated by ileal motion secretion
How does vomiting work?
"Peristalsis in reverse": Irritant receptors in gut to medulla "vomiting center" then efferent outputs via vagal, phrenic, spinal, sympathetic, parasympathetic nerves
Saliva-->Upper intestines contract-->Pylorus contracts-->Abd muscles/diaphragm contract-->LES/esophagus dilates-->Glottis closes
What does small bowel secrete? What are their functions?
Smaller bowel secretes water and mucous.
Mucous provides lubrication, cell protection
H20 provides enzymatic hydrolysis of nutrients
What is the mechanism of water and mucous secretion
Driven by CFTR channel on apical membrane (Water follows Cl osmotically)
Regulated by cAMP
Is absorption in the small bowel regulated?
No-- the small bowel is designed to absorb all nutrients presented to it, and does not concentrate
What are the 2 units of the surface epithelium in the small bowel?
Villi and crypts
What are functions of crypts?
Secretion: CFTR channels
Stem cells to repopulate shredded gut cells
How does digestion of carbohydrates work?
Most occurs in small bowel via pancreatic amylase secretion. Complex carbs are degraded to disaccharides. Brush border disachharidases.
Absorption in jejunum-- cotransport of monosaccharides with Na
Describe the process of protein digestion
Pepsin in stomach
Pancreatic proenzymes activated in lumen (enterokinase activates trypsin, which activates all other pancreatic proteases)-- they degrade to small peptides/AAs
Brush border aminopeptidases
Cotransport of AAs with Na or cotransport of di/tripeptides with H
Describe the first step of fat digestion in the stomach
Gastric lipase converts TG to fatty acid and DG. DG+phospholipids stabilize emulsion
Describe the first step of fat digestion in the duodenum
Bile salts stabilize emulsion
Pancreatic lipase convert TG to FA+MG
What are the functions of colipase and phospholipase A2
Colipase is secreted by the pancreas and brings lipase together with TG
Phospholipase A2 is secreted by pancreas to digest phospholipids on surface of emulsion and anchor lipase to emulsion. It's activated by bile salts
Describe the final product of emulsification step
Inner hydrophobic layer of TG, DG, cholesterol fat-soluble vitamins
Outer hydrophilic layer of phospholipids MG, FA, bile salts
What is second step of fat digestion?
Transfer into cells.
Bile salts form micelles to transport MG, FA, cholesterol and fat soluble vitamins
What are the third/fourth steps of digestion?
Intracellular processing and export.
MG+FA==>TG. Packaging of TG, cholesterol and phospholipids into chylomicrons (coated with apolipoproteins) for export into lymphatics.
Describe absorption for vitamins
Absorbed whole, not digested
Simple diffusion or carrier mediated transport for water-soluble vitamins
Fat soluble vitamins form micelles
How is B12 processed?
Initially bound to food then binds carrier R protein then it is freed by pancreatic enzymes and binds intrinsic factor in proximal small bowel
How is salt/water absorbed?
Active process driven by Na/K/ATPase on basolateral membrane
NaCl absorption drives water absorption
What are functions of colon? (2)
Salt/water absorption (but doesn't concentrate)
No digestive function
What is the ileocecal valve and what is its function?
Ileocecal valve is a one way valve that regulates entry of material into colon