digestive iii Flashcards
Small intestine gross anatomy
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)
Duodenum
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
Gross anatomy of small intestine
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
Structural modifications
- Circular folds (plicae circularis)
- permanent folds (1 cm deep) that force chyme to slowly spiral through lumen –> more nutrient absorption - Villi
- extensions (1 mm) of mucosa with capillary bed and lacteal for absoption –> increase surface area - Microvilli (brush border)
- contain enzymes for carbohydrate and protein digestion –> increase surface area
Intestinal crypts
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
Mucosa
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
Intestinal juice
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
Digestion in small intestine
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
Requirements for digestion and absorption in small intestine
slow delivery of acidic, hypertonic chyme
Delivery of bile, enzymes, an bicarbonate ions from liver and pancreas
mixing
Motility of small intestine: segmentation
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
Motility of small intestine: peristalsis
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
Motility of small intestines: neurons
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
Motilty of small intestine: Ileocecal junction
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
Large intestine unique feature
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
Large intestine regions
cecum appendix colon rectum anal canal
Cecum and Appendix
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
Colon
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
Rectum, anus, and anal sphincters
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
Large intestine microscopic anatomy
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
Bacterial flora
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)
Intestinal flora
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
Digestive processes in large intestine
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
Motility of Large intestine
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)
Defecation
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
Valsalva’s maneuver in defecation
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
Steps of defecation
- feces move into and distend the rectum, stimulating stretch receptors which transmit signals along afferent fibers to spinal cord neurons
- 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
- if it’s convenient to defecate, voluntary motor neurons are inhibited, allowing external anal sphincter to relax so feces may pass
