Chapter 25 - 3

The Digestive System

LaPointe Spring ‘12Slide #2The Gall bladder

• Hollow, pear-shaped organ

• Stores, modifies and concentrates bile

• Contraction of the gall bladder and release of bile is controlled by CCK and vagal stimulation

• Empties into the Cystic duct

• Gall stones

LaPointe Spring ‘12Slide #3The Gallbladder

LaPointe Spring ‘12Slide #4Bile

• Made of water, billirubin, ions, cholesterol, other lipids

• Acts as an emulsifier to aid lipid digestion

• 90% is reabsorbed in the ileum - enterohepatic circulation of bile

LaPointe Spring ‘12Slide #5Functions of the large intestine

• Reabsorb water and compact material into feces

• 1500 mL chyme enter the cecum, 90% of volume reabsorbed yielding 80-150 mL of feces

• Absorb vitamins produced by bacteria

• Store fecal matter prior to defecation

LaPointe Spring ‘12Slide #6

General anatomy of the large intestine

• Lies inferior to the stomach

• Frames the small intestine

• Parts of the large intestine

• Cecum

• Colon• Ascending• Transverse• Descending• Sigmoid

• Rectum

• Anal canal

LaPointe Spring ‘12Slide #7

The Large Intestine

LaPointe Spring ‘12Slide #8Cecum and Rectum

• Cecum• opening protected by ileocecal valve• veriform appendix

• Rectum• Last portion of the digestive tract• Terminates at the anal canal

• Internal anal sphincter - involuntary smooth muscle

• External anal sphincter - voluntary muscle

• Hemorrhoids -inflamed veins

LaPointe Spring ‘12Slide #9

Rectum

LaPointe Spring ‘12Slide #10Histology of the large intestine

• Columnar cells except the rectum and anal canal (stratified squamous)

• Muscularis externa

• circular muscle

• only 3 bands longitudinal smooth muscle - taeniae coli

• Absence of villi

• Presence of goblet cells

• Deep intestinal glands

LaPointe Spring ‘12Slide #11Physiology of the large intestine

• Absorption in the large intestine includes:

• Water

• Vitamins – K, biotin, and B5

• Organic wastes – urobilinogens and sterobilinogens

• Bile salts

• Some ammonia and other toxins

• Mass movements of material through colon and rectum

• Defecation reflex triggered by distention of rectal walls

LaPointe Spring ‘12Slide #12Movement in Large Intestine

• Mass movements

• Common after meals

• Integrated by the enteric plexus

• Local reflexes

• instigated by the presence of food in the stomach and duodenum

• Gastrocolic reflex: initiated by stomach

• Duodenocolic reflex: initiated by duodenum

LaPointe Spring ‘12Slide #13

Long reflexes of defecation

LaPointe Spring ‘12Slide #14Coordination secretion and absorption

• Neural and hormonal mechanisms coordinate glands

• GI activity stimulated by parasympathetic innervation

• Inhibited by sympathetic innervation

• Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines

• Several hormones alter motility and secretion

LaPointe Spring ‘12Slide #15Summary Hormone Table

Marieb Table 23.1.1

GIP duodenum fats, carbohydrates stim pancreas insulin secretion, inhibits stomach, stim adipose glucose and FA uptake & lipogenesis

LaPointe Spring ‘12Slide #16Summary Hormone Table (continued)

Marieb Table 23.1.1

LaPointe Spring ‘12Slide #17Activities of Major Digestive Tract Hormones

From Martini Figure 24.22

LaPointe Spring ‘12Slide #18Digestion, Absorption, Transport

• Digestion

• Disassembles organic food into smaller fragments• Mechanical: breaks large food particles to small• Chemical: breaking of covalent bonds by digestive

enzymes • Hydrolyzes carbohydrates, proteins, lipids and

nucleic acids for absorption

• Absorption and transport

• Molecules are moved out of digestive tract and into circulation for distribution throughout body

LaPointe Spring ‘12Slide #19Carbohydrate digestion and absorption

• Begins in the mouth (salivary amylase) completed in small intestine

• Salivary and pancreatic amylase • Convert starches to disaccharides, trisaccharides,

monosaccharides

• Brush border enzymes• Make monosaccharides from disaccharides• maltase (glucose / glucose)• sucrase (glucose / fructose)• lactase (glucose / galactose)

• Absorption of monosaccharides occurs across the intestinal epithelia by facilitaed diffusion or Na+ linked co-transporters

LaPointe Spring ‘12Slide #20Lipid digestion and absorption

• Begins in stomach (minor) completed in small intestine

• Lipid digestion utilizes lingual and pancreatic lipases converts triglycerides to monoglycerides

• Bile salts improve chemical digestion by emulsifying lipid drops

• Lipid-bile salt complexes called micelles are formed

• Micelles diffuse into intestinal epithelia which re-synthesis triglycerides and then release lipids into the circulation as chylomicrons - mixtures of phopholipids and proteins

• carried by lymphatic system

LaPointe Spring ‘12Slide #21Lipid Absorption also (see Saladin fig 25.30)

from Seeley, Stephens and Tate

LaPointe Spring ‘12Slide #22Protein digestion and absorption

• In the stomach, the low pH destroys tertiary and quaternary structure allows pepsin to digest specific peptide bonds

• In the small intestine, pancreatic enzymes and intestinal brush border peptidases work at pH 7-8

• Trypsin - breaks bonds next to arginine or lysine

• Chymotrypsin- breaks bonds next to phenylalanine or tyrosine

• Liberated amino acids, and some dipeptides are absorbed through by several different carrier proteins via by facilitaed diffusion or Na+ linked co-transporters

(See Saladin Fig 25.29)

LaPointe Spring ‘12Slide #23Absorption of other molecules

• Water • Nearly all that is ingested is reabsorbed via osmosis

• Ions • Absorbed via diffusion, cotransport, and active

transport

• Vitamins • Water soluble vitamins (C and B vitamins) are absorbed

by diffusion• Vitamin B12 requires intrinsic factor

• Fat soluble vitamins (A,D, E, K) are absorbed as part of micelles

LaPointe Spring ‘12Slide #24Water Intake, Secretion and Absorption

Diarrhea

Constipation

from Seeley, Stephens and Tate

LaPointe Spring ‘12Slide #25

Marieb Figure 23.33a

Great Summary slide (a)

LaPointe Spring ‘12Slide #26

Marieb Figure 23.33b

Great Summary slide (b)