1. Chapter 23 Digestive System Lecture 11 Part 2: Small Intestine
Marieb’s Human
Anatomy and Physiology
Ninth Edition
Marieb w Hoehn
Chapter 23
Digestive System
Lecture 11
Part 2: Small Intestine
Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1]
Slides 16 – 38; 50 min [Lecture 2]
118 min (38 slides plus review of course Web sites and syllabus)

2. Three Parts of Small Intestine
“Mixing bowl”; acid neutralization
Bulk of chemical digestion and nutrient absorption occurs here
The ‘bowel’ consists of the small and large intestines.
Vitamin B12 absorption
Main functions of small intestine: 1) chemical digestion 2) absorption of nutrients (90%) from chyme

3. Blood Supply and Drainage of Small Intestine
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

4. Wall of Small Intestine
Plicae circulares – permanent circular folds of mucosa that further increase surface area for absorption – do not flatten out with distention like rugae of stomach.
Especially prominent in lower duodenum and upper jejunum
Submucosa of duodenum contains mucus-secreting glands (Brunner’s glands) that protect the small intestine

5. Intestinal Villi & Glands
Enterocyte = Intestinal Cell
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
Intestinal glands secrete an abundant watery fluid that helps absorb products of digestion. They also contain enteroendocrine cells (enterokinase, gastrin, secretin, CCK)

6. Intestinal Epithelium
Microvilli further increase the surface area available for absorption in the small intestine
Form a ‘brush border’ on the intestine
Digestive enzymes are embedded in the membrane of microvilli
Main function of plicae, villi, and microvilli is to increase the surface area for absorption (from about 3.6 ft2 to about 2200 ft2!)

7. Secretions of Small Intestine
peptidase – breaks down peptides into amino acids
sucrase, maltase, lactase – break down disaccharides into monosaccharides
intestinal lipase – breaks down fats into fatty acids and glycerol
enterokinase – converts trypsinogen to trypsin
gastrin/somatostatin – hormones that stimulate/inhibit acid secretion by stomach
cholecystokinin (CCK) – hormone that inhibits gastric glands, stimulates pancreas to release enzymes in pancreatic juice, stimulates gallbladder to release bile, and relaxes hepatopancreatic sphincter (of Oddi)
secretin – stimulates pancreas to release bicarbonate ions in pancreatic juice; stimulates gall bladder to release bicarbonate-rich bile
Brush border
See Table in Marieb for summary of digestive enzymes – great to use for XC!!

8. Control of Intestinal Secretion and Absorption
mucus secretion (protective) stimulated by presence of chyme in small intestine
distension of intestinal wall activates nerve plexuses in wall of small intestine
motility/secretion stimulated by gastroenteric reflex
parasympathetics trigger release of intestinal enzymes
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

9. Movements of the Small Intestine
Movements in local segments can occur without stimulation by parasympathetic NS. However, nervous stimulation accelerates segmentation and peristalsis.
peristalsis – pushing movements
segmentation – ringlike contractions that aid in mixing and slowing peristalsis
overdistended or irritated wall triggers “peristaltic rush” resulting in diarrhea
“Long distance” movements are triggered by stomach filling: - gastroenteric reflex (↑ motility and secretion along length of small intestine) - gastroileal reflex (relaxation of ileocecal sphincter)

10. Absorption in the Small Intestine
monosaccharides and amino acids
through facilitated diffusion and active transport
absorbed into blood
electrolytes and water
through diffusion, osmosis, and active transport
absorbed into blood
vitamins
fat-soluble dissolve in dietary fats (vit A,D,E,K)
Water-soluble through diffusion, except B12 (active transport)
Vitamin K (large intestine) – with other lipids
absorbed into blood

11. Absorption of Fats in the Small Intestine
fatty acids and glycerol
several steps
absorbed into lymph into blood
Chylomicrons contain TG, cholesterol, and phospholipids