Phys: Small Bowel & Colon

Question 0

J: This refers to the primary type of gastric motility in the small bowel.

Answer 0

What is segmentation contraction?

Question 1

How long is the small bowel?

Answer 1

~21 feet.

Question 2

What is the rate of segmentation contraction in the small bowel?

Answer 2

9-12 per minute.

Question 3

What are the benefits of segmentation contractions?

Answer 3

(1) mixing of lumenal contents

2) enhanced contact time with the surface epithelium (3-5 hrs

Question 4

How does the autonomic nervous system influence the small bowel?

Answer 4

(1) parasympathetic stimulates

(2) sympathetic depresses

Question 5

How does gastrin influence the small bowel?

Answer 5

It activates post-prandial segmentation of the ileum and terminates the migrating motor complex.

Question 6

What is the migrating motor complex?

Answer 6

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.

Question 7

What initiates the migrating motor complex? What terminates it?

Answer 7

(1) motilin secretion by the ileum

(2) post-prandial gastrin

Question 8

Where is the vomiting center in the brainstem?

Answer 8

The lateral reticular formation in the medulla.

Question 9

What is the step-by-step mechanism of vomiting?

Answer 9

(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

Question 10

What substances are secreted by the small bowel and what is the purpose of each?

Answer 10

(1) mucus: lubrication and cytoprotection

(2) water: enzymatic hydrolysis of nutrients

Question 11

What is the mechanism of water secretion in the small bowel?

Answer 11

Chloride is secreted into epithelial crypts (primarily by CFTR) and water follows.

Question 12

How is absorption in the small bowel regulated?

Answer 12

It isn’t.

Question 13

How does the osmolarity of the lumen of the small bowel compare to that of the epithelial cytoplasm?

Answer 13

They are iso-osmotic.

Question 14

Where is the digestion of carbohydrates initiated?

Answer 14

In the mouth, by salivary amylase.

Question 15

Describe the digestion and absorption of carbohydrates in the small bowel.

Answer 15

(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.

Question 16

Describe the digestion of proteins.

Answer 16

(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.

Question 17

Describe the absorption of peptides.

Answer 17

(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.

Question 18

What enzymes and secretions are involved in the digestion and emulsion of fats in the GI lumen and what are their roles?

Answer 18

(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

Question 19

What is the composition of a stable emulsion?

Answer 19

Inside: triglycerides, diglycerides, cholesterol, fat-soluble vitamins

Outside: phospholipids, monoglycerides, fatty acids, bile salts

Question 20

J: Formed by bile salts and smaller than emulsion droplets, these are transported into enterocytes carrying monoglycerides, fatty acids, cholesterol, and vitamins.

Answer 20

What are micelles?

Question 21

Where are bile salts reabsorbed?

Answer 21

The terminal ileum.

Question 22

Describe the processing of fat within enterocytes.

Answer 22

(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.

Question 23

Where in the GI tract are vitamins digested?

Answer 23

They aren’t.

Question 24

What are the water-soluble vitamins?

Answer 24

Vitamin C and B_.

Question 25

What are the fat-soluble vitamins?

Answer 25

Vitamins K, A, D, and E.

Question 26

Describe the path taken by vitamin B12 from ingestion to absorption.

Answer 26

(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.

Question 27

What is the difference in charge between the blood and the enterocyte?

Answer 27

The blood is slightly positive.

Question 28

What transporters can be found on the apical membrane of enterocytes?

Answer 28

(1) Na/Cl cotransporter (2) Na/K/2Cl cotransporter (3) Na/H and HCO3/Cl linked cotransporters (4) cation channel (5) Na/nutrient cotransporters

Question 29

What are the basic functions of the colon?

Answer 29

(1) salt and water absorption (2) waste storage (3) NO digestion

Question 30

What controls the opening and closing of the ileocecal valve?

Answer 30

(1) open: distention of the ileum | (2) closed: contraction of cecum

Question 31

J: These refer to the pouches along the length of the colon.

Answer 31

What are haustra?

Question 32

Which is more permeable to small ions: the small bowel or the colon?

Answer 32

The small bowel.

Question 33

What are the two primary cells in the colon and what is their absorption/secretion profile?

Answer 33

Surface epithelial cells: absorb Na and Cl, secrete K and HCO3 Crypt cells: secrete Cl

Question 34

Which absorbs more water: small bowel or colon?

Answer 34

The small bowel.

Question 35

What is the predominant motility pattern in the colon?

Answer 35

Segmentation contractions.

Question 36

What is the frequency of segmentation contractions in the colon?

Answer 36

2 per hour.

Question 37

J: This refers to the simultaneous, strong propulsive contractions in the proximal colon, especially after breakfast.

Answer 37

What is the gastrocolic reflex?

Question 38

What influences the gastrocolic reflex?

Answer 38

Gastrin and extrinsic nerves.

Question 39

What are the benefits of colonic bacteria?

Answer 39

(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

Question 40

What gases are released from the GI tract?

Answer 40

N2, O2, CO2, H2, CH4, trace.

Question 41

In defecation, what muscles are under voluntary control and what do they do? Involuntary?

Answer 41

Skeletal: Puborectalis and external anal sphincter relax. Levator ani, rectus, and diaphragm contract. Smooth: Internal anal sphincter relaxes. Rectum contracts.

Question 42

Describe the process of defecation.

Answer 42

(1) stool collects in rectum. (2) hypothalamus recognizes the need for a bowel movement. (3) abdominal muscles and diaphragm contract. (4) colon and rectum contract. (5) internal anal sphincter relaxes. (6) puborectalis relaxes and levator ani contracts. (7) rectoanal angle straightens. (8) external anal sphincter relaxes.