D’YOUVILLE COLLEGE
BIOLOGY 108/508 - HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 7
DIGESTIVE SYSTEM III
INTESTINES
4.
Abdominal Gastrointestinal Tract (continued):
Small Intestine:
• approximately 20 feet: duodenum (10”), jejunum (8’), and ileum (12’)
• Wall Features: strategies to increase surface area: (figs. 23 – 22 & 23 – 23)
• Plicae Circulares: spiral folding of mucosa and submucosa
• Villi: finger-like folds of mucosa with core of connective tissue (including
microcirculation and lymphatics) and surface layer of absorbing cells and goblet
cells (mucus production)
• Microvilli: microscopic folding of apical membrane of absorbing cells
• Glands:
• Submucous (Brunner’s) glands: (in submucosa) secrete alkaline mucus
(neutralizing acidity of chyme)
• Intestinal Glands (Crypts of Lieberkuhn): watery product (like interstitial
fluid)
• Apical Borders of Absorbing Cells: enzymes (disaccharidases,
dipeptidases, lipases) facilitate digestion and absorption
Physiology of Small Intestine:
a. Digestion and Absorption (fig. 23 – 32):
• Carbohydrates: disaccharides split to monosaccharides (glucose, fructose,
galactose) that are absorbed into blood capillaries
• Proteins: dipeptides split to constituent amino acids that are absorbed by
active transport into blood capillaries (fig. 23 – 33)
• Fats: triglycerides + bile salts (micelles) split to fatty acids and glycerol,
absorbed into mucosal cells; triglycerides reconstituted and combined with protein
to form chylomicrons (extremely low density lipoproteins) which must be absorbed
by lacteals (fig. 23 – 34); bile salts sent back to liver (enterohepatic cycle)
• Electrolytes: actively transported to blood capillaries
• Note: most absorption (via blood capillaries) is carried directly to the liver
by the hepatic portal vein
• chylomicrons (too large to enter blood capillaries) are carried by
lacteals to larger lymphatics to great veins of thorax
b. Motility (fig. 23 – 3):
• mixing via segmentation contractions and pendular movements
• propulsion via peristalsis
• emptying within 3 - 10 hours; relaxation of ileocecal valve
Bio 108/508
lec. 7 - p. 2
Large Intestine:
a. Parts (fig. 23 – 29):
• Cecum: blind-ended pouch inferior to ileocecal junction; site for
vermiform appendix (lymphoid organ)
• Ascending Colon: from cecum to hepatic flexure (right side)
• Transverse Colon: from hepatic flexure below greater curvature of
stomach to splenic flexure; supported by transverse mesocolon
• Descending Colon: from splenic flexure to sigmoid colon (left side)
• Sigmoid Colon: turns medially across brim of pelvis to join rectum at
back of pelvic cavity; supported by sigmoid mesocolon
• Rectum: straight, terminal passageway of large bowel; ends at anal canal
(short chamber between involuntary internal anal sphincter and voluntary external
anal sphincter)
b. Wall Features:
• Mucosa: no villi; abundance of goblet cells and mucous glands
• Muscularis: circular muscle well developed, powerful; longitudinal
muscle discontinuous, disposed as three strips (taeniae coli); features puckering
(haustrations)
c. Functions:
• Absorption: residual water absorption; water removal produces solid
stool (feces) of indigestible material held together by thick mucus
• Bacterial Flora: metabolic conversions of carbohydrate residues
(fermentation), protein residues (putrefaction)
• bacteria produce gas, stool color (bile pigment conversion), stool odor
(skatole, indole, hydrogen sulfide, ammonia)
• Elimination of Waste: 3 or 4 times daily, powerful peristaltic contractions
(mass movements) of circular muscle propel contents to rectum
• arrival of feces in rectum initiates rectal reflex, powerful constriction of
rectum resulting in expulsion of feces (defecation) (fig. 23 – 31)
• reflex may also be voluntarily initiated (Valsalva’s maneuver);
elevation of anal canal (levator ani) and relaxation of anal sphincters facilitates
excretion of stool