Chapter 43: Fueling Body Activities: Digestion
I.
II.
Evolutionary trends
A. Intracellular digestion (food vacuoles/lysosomes)(protozoans)(hydra-endoderm)
To extra cellular digestion (outside cell, in cavity)(most animals)
B. Two way digestive tract (Hydra) to one way digestive tract (mouth – anus)
Human Alimentary Canal (digestive system)
A. Purpose: break down food into usable form and absorb nutrients into cells
B. Sequence of events
Ingestion  Digestion  Absorption  Egestion
(Intake food) (Break down of food into usable form) (Uptake of soluble nutrients into blood) (Elimination of undigested food)
III.
IV.
C. Involves specialized organs, tissues, enzymes and hormones
Neural and Hormonal Control of Gastrointestinal Tract
Pg 901
A. Sight, smell and contact in mouth of food stimulates salivary glands, gastric juices and
peristalsis in stomach (neural stimulation)
B. Proteins in stomach stimulate gastrin (hormone)
1. Secreted from special cells in stomach wall into blood capillaries
2. Gastrin stimulates parietal cells(stomach wall) to secrete HCL
3. Gastrin stimulates chief cells(stomach wall) to secrete pepsinogen (zymogen
enzyme)
C. HCL converts pepsinogen into active pepsin(protease)
D. HCL lowers pH to 2 and inhibits gastrin production (FEEDBACK)
E. Partially digested food + acidic gastric juices = chime (acidic)
F. Chyme leaves stomach through pyloric phincter (circular muscle) enters duodenum(first
25 cm of small intestine)
G. Acidity of chime stimulates 3 hormones…
1. GIP (gastric inhibitory peptide) from duodenum walls
a. FEEDBACK to stomach via blood to inhibit gastric juices and peristalsis in
stomach
b. Secretin from walls of small intestine stimulates pancreas (acinar cells /
exocrine)
2. Secretin from walls of small intestine stimulates pancreas (acinar cells/exocrine)
a. To secrete bicarbonate ions to increase pH to 8 in small intestine
b. To secrete pancreatic juices (amylases, lipases, proteases, nucleases)
3. CCK (cholecytokinin) from duodenum walls
a. Stimulates pancreas acinar cells
b. Stimulates gall bladder to release bile (emulsifier) into small intestine
H. See diagram in textbook page 900
Absorption of Nutrients in Small Intestine – see page 894
A. Digestion of food is completed in small intestine (4.5 meters)
B. Villi – finger-like projections of wall of small intestine (adaptation, increases surface
area)
V.
C. Microvilli – extensions of the cell membrane of villi epithelial cells “brush border”
D. Villi and microvilli increase surface area for
1. Intestinal “glands”
a. Cells secrete digestive (hydrolytic) enzymes to complete digestion
b. Some enzymes embedded in membranes (lactase)
c. Absorption of digestive products (glucose, aa, f.a. and glycerol)
E. Absorption of amino acids and monosaccharides – see page 896
1. Active co-transport with Na+ into epithelial cells of small intestine (against
concentration gradient)
2. Move into blood capillaries by facilitated diffusion
3. Transported to liver by hepatic portal vein
F. Absorption of fatty acids (monoglycerides) & glycerol – see page 896
1. Absorbed into epithelial cells and resynthesized into triglycerides
2. Protein coat added to form chylomicrons
3. Chylomicrons absorbed into lymphatic capillaries (lacteals)
4. Lymph empties blood into vein near neck (vena cava)
Accessory Glands
A. Liver
1. Stores (glycogen) and releases carbohydrates
2. Regulates cholesterol levels
a. HDL (high density lipoproteins) – “good”
b. LDL (low density lipoproteins) – “bad”
3. Stores fat soluble vitamins A, D, E
4. Produces bile* from cholesterol and bilirubin
a. Bilirubin – yellow pigment from breakdown of RBC in liver
b. Jaundice – bilirubin in bloodstream
c. *Emulsifier of fats in small intestine – increases surface area for
digestion
5. Detoxification of drugs and alcohol
6. Produces plasma proteins and clotting factors
7. Breaks down excess amino acids and forms urea
B. Gall Bladder
1. Stores and releases bile into duodenum through bile duct
C. Pancreas – see page 902
1. Exocrine Gland (acini or acinar cells)
a. Produces pancreatic juice (enzymes), into duodenum by pancreatic duct
2. Endocrine Gland (Islets of Langerhans)
a. Produces hormones insulin (beta cells) and glucagon ( alpha cells)
(antagonistic hormones – opposite effects)
b. Regulates blood sugar levels (Feedback)