digestive iii

Question 1

Small intestine gross anatomy

Answer 1

Major organ of digestion and absorption
-2-4 m long; from pyloric sphincter to ileocecal valve

Subdivisions:

• duodenum (retroperitoneal)
• jejunum (attached posteriorly by mesentery)
• ileum (attached posteriorly by mesentery)

Question 2

Duodenum

Answer 2

Curves around head of pancreas; shortest part (25 cm)

Bile duct from liver and main pancreatic duct from pancreas

• join at hepatopancreatic ampulla
• enter duodenum at major duodenal papilla
• entry controlled by hepatopancreativ sphincter

Question 3

Gross anatomy of small intestine

Answer 3

Vagus nerve and sympathetics from thoracic splanchnic nerves serve small intestine

Superior mesenteric artery brings blood supply

Veins drain into superior mesenteric veins –> hepatic portal vein –> liver

Question 4

Structural modifications

Answer 4

1. Circular folds (plicae circularis)
   - permanent folds (1 cm deep) that force chyme to slowly spiral through lumen –> more nutrient absorption
2. Villi
   - extensions (1 mm) of mucosa with capillary bed and lacteal for absoption –> increase surface area
3. Microvilli (brush border)
   - contain enzymes for carbohydrate and protein digestion –> increase surface area

Question 5

Intestinal crypts

Answer 5

Intestinal crypt epithelium renewed every 2-4 days

• most = secretory cells that make intestinal juice
• enteroendocrine cells –> enterogastrones
• intraepithelial lymphocytes (IELs) release cytokines that kill infected cells
• Paneth cells secrete antimicrobial agents (Defensins and lysozymes)
• stem cells divide to make crypt cells

Question 6

Mucosa

Answer 6

Peyer’s patches protect especially distal part against bacteria (may protrude into submucosa)

B lymphocytes leave intestine, enter blood, protect intestine lamina propria with their IgA

Duodenal (Brunner’s) glands of duodenum secrete alkaline mucus to neutralize acidic chyme

Question 7

Intestinal juice

Answer 7

1-2 L per day in response to distension or irritation of mucosa

slightly alkaline; isotonic with blood plasma

Largely water; enzyme poor (enzymes of small intestine only in brush border); contains mucus

Facilitates transport and absoption of nutrients

Question 8

Digestion in small intestine

Answer 8

Chyme from stomach containes

• partially digested carbs and prots
• undigested fats

3-6 hrs in small intestine

• most water absorbed
• pretty much all nutrients absorbed

small intestine, like stomach, has no role in ingestion or defecation

Question 9

Requirements for digestion and absorption in small intestine

Answer 9

slow delivery of acidic, hypertonic chyme

Delivery of bile, enzymes, an bicarbonate ions from liver and pancreas

mixing

Question 10

Motility of small intestine: segmentation

Answer 10

Segmentation

• most common motion of small intestine
• initiated by intrinsic pacemaker cells
• mixes and moves contents toward ileocecal valve
• intensity altered by long and short reflexes and hormones (remember parasymp increases)
• wanes in late intestinal (fasting) phase

Question 11

Motility of small intestine: peristalsis

Answer 11

Initiated by rise in hormone, motilin, in late intestinal phase (every 90-120 mins)

Each wave starts distal to previous
-migrating motor complex

Meal remnants, bacteria, and debris moved to large intestine

From duodenum to ileum takes about 2 hrs

Carb meals move faster, fatty ones move slower

Question 12

Motility of small intestines: neurons

Answer 12

Local enteric neurons coordinate intestinal motility

Cholinergic sensory neurons may activate myenteric plexus

• causes contraction of circular muscle proximally and of longitudinal muscle distally
• forces chyme along tract

Question 13

Motilty of small intestine: Ileocecal junction

Answer 13

Ileocecal sphincter relaxes, admits chyme into large intestine when

• gastroileal reflex enhances force of segmentation in ileum
• gastrin increases motility of ileum

Ileocecal valve flaps close when chyme exerts backward pressure
-prevents regurgitation into ileum

Question 14

Large intestine unique feature

Answer 14

Teniae coli
-three bands of longitudinal smooth muscle in muscularis

Haustra
-pocketlike sacs caused by tone of teniae coli

Epiploic appendages
-fat-filled pouches of visceral peritoneum

Question 15

Large intestine regions

Answer 15

cecum
appendix
colon
rectum
anal canal

Question 16

Cecum and Appendix

Answer 16

Cecum = first part of large intestine

Appendix = masses of lymphoid tissue

• part of MALT of immune system
• bacterial storehouse –> recolonizes gut when necessary
• Twisted –> enteric bacteria accumulate and multiply –> can lead to appendicitis

Question 17

Colon

Answer 17

Retroperitoneal except for transverse and sigmoid regions

Ascending colon (right side- to level of right kidney) –> right colic/hepatic flexure

Transverse colon –> left colic/splenic flexure

Descending colon (left side)

Sigmoid colon in pelvis –> leads to rectum

Question 18

Rectum, anus, and anal sphincters

Answer 18

Rectum

• three rectal valves stop feces from being passed with gas
• hemorrhoidal veins

Anal canal

• last segment of large intestines
• opens to body exterior at anus

Sphincters

• internal anal sphincter = smooth muscle
• external anal sphincter = skeletal muscle

Question 19

Large intestine microscopic anatomy

Answer 19

Thicker mucosa of simple columnar epithelium except in anal canal (stratified squamous)

No circular folds, villi, or digestive secretions

Abundant deep crypts with goblet cells

superficial venous plexuses of anal canal form hemorrhoids if inflamed

Question 20

Bacterial flora

Answer 20

Enter from small intestine or anus

• colonize colon
• synthesize B complex vitamins and vitamin K
• metabolize some host-derived molecules (mucin, heparin, hyaluronic acid)
• ferment indigestible carbs
• release irritating acids and gagsses (500 ml per day)

Question 21

Intestinal flora

Answer 21

viruses and protozoans

Bacteria prevented from breaching mucosal barrier
-epithelial cells recruit dendritic cells to mucosa –> sample microbial antigens –> present to T cells of MALT –> B cell produce IgA antibody-mediated response –> restricts microbes

Question 22

Digestive processes in large intestine

Answer 22

Residue remains in large intestines for 12-24 hrs

No food breakdown except by enteric bacteria

Vitamins (made by bacterial flora), water, and electrolytes (esp Na and Cl) reclaimed

Major funcs = propulsion of feces to anus and defecation

colon isn’t essential for life

Question 23

Motility of Large intestine

Answer 23

Most contraction of colon are haustral contractions

• slow segmenting movements
• haustra sequentially contract in response to distension

Also have gastrocolic reflex

• initiated by presence of food in stomach
• activates three to four slow powerful peristaltic waves per day in colon (mass movements)

Question 24

Defecation

Answer 24

Mass movements force feces toward rectum

Distension initiates spinal defecation reflex

Parasympathetic signals

• stimulate contraction of sigmoid colon and rectum
• relax internal anal sphincter

Conscious control allows relaxation of external anal sphincter

Muscles of rectum conract to expel feces

Question 25

Valsalva's maneuver in defecation

Answer 25

closing of glottis, contraction of diaphragm and abdominal wall muscles --> increased intra-abdominal pressure Levator ani muscle contracts --> anal canal lifted superiorly --> feces leave body

Question 26

Steps of defecation

Answer 26

1. feces move into and distend the rectum, stimulating stretch receptors which transmit signals along afferent fibers to spinal cord neurons 2. a spinal reflex is initiated in which parasym motor fibers stimulate contraction of rectum and sigmoid colon as well as relaxation of internal anal sphincter 3. if it's convenient to defecate, voluntary motor neurons are inhibited, allowing external anal sphincter to relax so feces may pass