Flashcards in Phys: Small Bowel & Colon Deck (43)
How long is the small bowel?
J: This refers to the primary type of gastric motility in the small bowel.
What is segmentation contraction?
What is the rate of segmentation contraction in the small bowel?
9-12 per minute.
What are the benefits of segmentation contractions?
(1) mixing of lumenal contents
(2) enhanced contact time with the surface epithelium (3-5 hrs)
How does the autonomic nervous system influence the small bowel?
(1) parasympathetic stimulates
(2) sympathetic depresses
How does gastrin influence the small bowel?
It activates post-prandial segmentation of the ileum and terminates the migrating motor complex.
What is the migrating motor complex?
Short, organized peristaltic contractions starting in the stomach and moving caudally. It is accompanied by relaxation of the pyloric sphincter and takes about 2 hrs to reach the end.
What initiates the migrating motor complex? What terminates it?
(1) motilin secretion by the ileum
(2) post-prandial gastrin
Where is the vomiting center in the brainstem?
The lateral reticular formation in the medulla.
What is the step-by-step mechanism of vomiting?
(1) increased saliva
(2) contraction of upper intestines
(3) contraction of pylorus
(4) contraction of abdominal muscles and diaphragm
(5) dilation of LES and esophagus
(6) glottis closure
What substances are secreted by the small bowel and what is the purpose of each?
(1) mucus: lubrication and cytoprotection
(2) water: enzymatic hydrolysis of nutrients
What is the mechanism of water secretion in the small bowel?
Chloride is secreted into epithelial crypts (primarily by CFTR) and water follows.
How is absorption in the small bowel regulated?
How does the osmolarity of the lumen of the small bowel compare to that of the epithelial cytoplasm?
They are iso-osmotic.
Where is the digestion of carbohydrates initiated?
In the mouth, by salivary amylase.
Describe the digestion and absorption of carbohydrates in the small bowel.
(1) Pancreatic amylases are secreted into the lumen.
(2) These enzymes digest the carbohydrates to disaccharides.
(3) Brush border disaccharidases reduce these to monosaccharides.
(4) These are then absorbed (primarily in the jejunum) by Na cotransporters.
Describe the digestion of proteins.
(1) Digestion is initiated in the stomach by pepsin and acid.
(2) Enterokinases in the small bowel convert the pancreatic trypsinogen to trypsin.
(3) Trypsin activates other pancreatic proteases.
(4) Proteins are degraded to smaller peptides and amino acids.
(5) Final processing is performed by aminopeptidases on the brush border.
Describe the absorption of peptides.
(1) Amino acids are cotransported with Na+ into enterocytes (there are carriers for neutral, acidic, and basic amino acids).
(2) Dipeptides and tripeptides are cotransported with H+ into enterocytes and are further degraded.
What enzymes and secretions are involved in the digestion and emulsion of fats in the GI lumen and what are their roles?
(1) gastric lipase: degrades triglycerides to fatty acids and diglycerides
(2) pancreatic lipase: degrades triglycerides to fatty acids and monoglycerides
(3) bile salt: stabilizes the emulsion
(4) colipase: brings pancreatic lipase together with triglycerides
(5) phospholipase A2: digests phospholipids on the emulsion surface and helps anchor lipase to the emulsion
What is the composition of a stable emulsion?
Inside: triglycerides, diglycerides, cholesterol, fat-soluble vitamins
Outside: phospholipids, monoglycerides, fatty acids, bile salts
J: Formed by bile salts and smaller than emulsion droplets, these are transported into enterocytes carrying monoglycerides, fatty acids, cholesterol, and vitamins.
What are micelles?
Where are bile salts reabsorbed?
The terminal ileum.
Describe the processing of fat within enterocytes.
(1) triglycerides are reformed from monoglycerides and fatty acids.
(2) triglycerides, cholesterol, and phospholipids are packaged into chylomicrons.
(3) the chylomicrons are coated with ApoA and ApoB and released via exocytosis.
Where in the GI tract are vitamins digested?
What are the water-soluble vitamins?
Vitamin C and B_.
What are the fat-soluble vitamins?
Vitamins K, A, D, and E.
Describe the path taken by vitamin B12 from ingestion to absorption.
(1) B12 is separated from food particles by gastric acid.
(2) B12 binds R protein (a carrier derived from saliva, stomach and bile).
(3) pancreatic enzymes free B12 from R protein.
(4) B12 binds intrinsic factor in the proximal small bowel.
(5) the B12-IF complex is absorbed by a specific transporter in the terminal ileum.
What is the difference in charge between the blood and the enterocyte?
The blood is slightly positive.
What transporters can be found on the apical membrane of enterocytes?
(1) Na/Cl cotransporter
(2) Na/K/2Cl cotransporter
(3) Na/H and HCO3/Cl linked cotransporters
(4) cation channel
(5) Na/nutrient cotransporters
What are the basic functions of the colon?
(1) salt and water absorption
(2) waste storage
(3) NO digestion
What controls the opening and closing of the ileocecal valve?
(1) open: distention of the ileum
(2) closed: contraction of cecum
J: These refer to the pouches along the length of the colon.
What are haustra?
Which is more permeable to small ions: the small bowel or the colon?
The small bowel.
What are the two primary cells in the colon and what is their absorption/secretion profile?
Surface epithelial cells: absorb Na and Cl, secrete K and HCO3
Crypt cells: secrete Cl
Which absorbs more water: small bowel or colon?
The small bowel.
What is the predominant motility pattern in the colon?
What is the frequency of segmentation contractions in the colon?
2 per hour.
J: This refers to the simultaneous, strong propulsive contractions in the proximal colon, especially after breakfast.
What is the gastrocolic reflex?
What influences the gastrocolic reflex?
Gastrin and extrinsic nerves.
What are the benefits of colonic bacteria?
(1) produce short chain fatty acids from undigested carbohydrates, nourishing the colon
(2) produce vitamin K
(3) stimulate immune development and IgA secretion
(4) compete with pathogenic bacteria
What gases are released from the GI tract?
N2, O2, CO2, H2, CH4, trace.
In defecation, what muscles are under voluntary control and what do they do? Involuntary?
Skeletal: Puborectalis and external anal sphincter relax. Levator ani, rectus, and diaphragm contract.
Smooth: Internal anal sphincter relaxes. Rectum contracts.