D’YOUVILLE COLLEGE
PMD 604 - ANATOMY, PHYSIOLOGY, PATHOLOGY II
Lecture 19: Physiology of intestines, liver & pancreas
G & H chapters 64 & 65
• small intestine
- motility: mixing and propulsive contractions; segmentation ‘chops’
chyme via kneading action (fig. 63 – 3 & ppt. 1); peristalsis moves contents toward
ileocecal valve and spreads chyme uniformly through small intestine
- ileocecal valve & sphincter (fig. 63 – 4 & ppt. 2) – thickening of circular
muscle extending along last several centimeters of ileum, leading to cecum
- persistent state of constriction delays emptying into cecum
- with distension or irritation of cecum valve is closed by cecal pressure,
thus preventing emptying of ileum or reflux of cecal contents into ileum
- after a meal, sphincter relaxes, opened by peristalsis in ileum (gastroileal
reflex) so ileal contents can enter cecum
- secretions of small intestine:
- regulated by local submucous plexus
- enzymes that complete process of chemical digestion and promote absorption
of various foodstuffs, e.g., disaccharidases for completion of carbohydrate digestion;
peptidases for completion of protein conversion to amino acids; lipases for
completion of triglyceride breakdown
- serous (watery) secretions & mucous secretions provide generous
aqueous milieu for action of enzymes secreted by pancreas, e.g., product resembling
interstitial fluid from crypts of Lieberkuhn (fig. 64 - 13), alkaline mucus from Brunner’s
glands of submucosa & mucus from goblet cells
• large intestine (fig. 63 – 5 & ppt. 3)
PMD 604, lec 19
- p. 2 -
- motility: sluggish; mixing movements consist of constrictions of thick circular
smooth muscle with contractions of discontinuous longitudinal muscle (=teniae coli);
this activity creates puckered wall (= haustrations) & kneads fecal material to
maximize contact with mucosa; this facilitates absorption of water & dissolved substances
PMD 604, lec 19
- p. 3 -
propulsive contractions: powerful peristaltic contractions (= mass
movements) are delivered 3 - 4x per day by very thick circular smooth muscle layer to
move contents toward rectum; these are governed by stretching of stomach and
duodenum (gastrocolic & duodenocolic reflexes) via signals transmitted by autonomic
nerves
- rectum & anal canal (ppt. 4) participate in defecation reflexes (fig. 63 - 6 &
ppt. 5)
- filling of rectum with feces causes strong contractions of its circular
smooth muscle while the anal sphincter relaxes simultaneously
- weak intrinsic reflex is promoted by enteric ns that invokes peristalsis in distal colon
as well as rectum
- strong parasympathetic reflex promoted by sacral nerves enhances rectal
emptying
- internal & external anal sphincters: internal consists of smooth muscle
under autonomic (involuntary) control; external consists of skeletal muscle under
somatic (voluntary) control; external sphincter is normally constricted unless conscious
signals cause relaxation to permit defecation
- secretions & absorption in large intestine:
- heavy mucus is secreted to facilitate passage of increasingly solid
contents
- main substances absorbed include sodium & chloride ions; sodium
chloride absorption provide strong osmotic uptake of water
PMD 604, lec 19
5.
- p. 4 -
Hepatic Anatomy & Physiology:
• anatomy of liver & biliary system (fig. 64 - 11 & ppt. 6):
- largest gland in body; occupies most of right upper quadrant (right lobe) &
extends across midline (left lobe)
- biliary tree: right & left hepatic ducts unite to form common hepatic duct
- gall bladder, nestled under right lobe, communicates with common hepatic
duct via cystic duct
- common bile duct extends inferiorly from union of common hepatic and
cystic ducts to duodenal wall, joining with pancreatic duct (hepatopancreatic ampulla)
- liver lobule (ppt. 7): parenchyma is organized into cylindrical columns of
hepatocytes radiating from central vein; laced with leaky capillaries (sinusoids) that
facilitate maximal exchange between hepatocytes & blood
- hepatic circulation: blood supply furnished by hepatic portal vein & common
hepatic artery; branches merge to supply sinusoids that drain into central vein of each
lobule; these drain into hepatic veins & to inferior vena cava (ppt. 8)
• physiology of liver:
- hepatic secretion: bile stored and concentrated in gall bladder
- bile release (gall bladder contractions) stimulated by cholecystokinin + some
stimulation by enteric nervous system (ppt. 9)
- enterohepatic cycle (ppt. 10): hepatocytes release bile (table 64 – 2) into
bile canaliculi (ppt. 7) that drain into biliary tree
- bile salts are an important constituent of bile (facilitate fat digestion and
absorption by emulsifying fat in the intestine) (ppt. 11)
- most bile salts are reabsorbed (ileum) and returned to liver for reuse; 20%
excreted in feces
PMD 604, lec 19
- p. 5 -
- metabolic functions: regulates blood glucose levels by absorbing & storing
it as glycogen (glycogenesis) or secreting it by breaking down glycogen (glycogenolysis)
or synthesizing glucose from amino acids (gluconeogenesis)
PMD 604, lec 19
- p. 6 -
- regulates protein levels (synthesizes many plasma proteins) to maintain
osmotic balance, provide transport molecules, clotting factors; maintains amino acid
pool; excretes ammonia from protein breakdown by forming urea
- maintains lipids in blood (formation of lipoprotein); synthesizes or
excretes cholesterol (in bile) as needed
- processes bilirubin (bile pigment from hemoglobin breakdown) to
excrete with bile
- detoxifies drugs & other chemicals (e.g. alcohol)
- stores vitamins (A, B12 & other B vitamins, D & K), glycogen,
triglycerides, copper and iron
- inactivates several hormones
6.
Pancreas Anatomy & Physiology:
• anatomy of pancreas:
- mixed gland (exocrine & endocrine) posterior and inferior to the greater
curvature of the stomach; head nestles in notch of duodenum, body extends toward
spleen & ends with tail (adjacent to spleen); the latter is the only mobile part (ppt. 12)
- acini: secretory units of exocrine pancreas (ppt. 13); empty into ductules that
lead to large pancreatic duct, which joins the common bile duct (from liver) to form
hepatopancreatic duct (empties into the duodenum via hepatopancreatic ampulla &
sphincter of Oddi)
• pancreatic secretions:
- bicarbonate-rich juice stimulated by secretin (fig. 64 – 10 & ppt. 14) in
response to acidic chyme; secreted by duct cells with copious water
PMD 604, lec 19
- p. 7 -
PMD 604, lec 19
- p. 8 -
- enzyme-rich juice stimulated by cholecystokinin (response to arrival of
protein-rich &/or fat-rich chyme from stomach); secreted by acinar cells and enzymes
largely remain in acinar lumen until 'washed' into pancreatic duct by bicarbonaterich juice
- proteases, synthesized in inactive form, later activated in intestinal
lumen: trypsinogen (activated by enterokinase from intestinal mucosa), chymotrypsinogen
(activated by trypsin) and procarboxypeptidase (activated by trypsin) (ppt. 15)
- pancreatic amylase converts complex carbohydrates to maltose (a
disaccharide)
- lipases, including phospholipases, break down triglycerides and
phosphoglycerides, respectively; cholesterol esterases degrade esters of cholesterol (ppt.
16)
- neural reflex stimulation of pancreatic secretion (acetylcholine from
parasympathetic fibers) involves cephalic, gastric & intestinal phases similar to
reflexes for stimulation of gastric secretion (ppt. 17)