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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
Rate: ileum>jejunum>duodenum


What are functions of colon? (2)

Salt/water absorption (but doesn't concentrate)
Waste storage

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


How does the ileocecal valve work? How is it regulated?

The ileocecal valve opens with distention of ileum and closes with contraction of cecum

Ileum brake works via fat sensors that inhibit gastric motility via Peptide YY