DIGESTIVE PROCESSES
1. Ingestion
2. Digestion (mechanical and chemical)
3. Motility (mixing and propulsion)
4. Secretion
5. Absorption
6. Defecation
GI TRACT AND ACCESSORY DIGESTIVE STRUCTURES
GI TRACT INNERVATION
1
, • Autonomic NS
o Parasympathetic NV (enhances digestion)
o Sympathetic NV (inhibits or slows down digestion)
• Enteric Nervous System
o Submucosal plexus
o Myenteric plexus
ENZYMES originating in the oral cavity
Salivary glands
Parotid glands (under the cheekbones)
Submandibular glands (under the mandible)
Sublingual glands (under the tongue)
• All secrete SALIVA, which contains:
o Mostly water
o Ions
o Buffers
o Lysosyme
o Salivary amylase
Salivary Amylase (made by the salivary glands)
- Break down carbohydrates
- Converts Polysaccharides to monosaccharides, maltose, maltotriose and alpha-dextrins
- Deactivated by stomach acid
Lingual Lipase (made by the lingual glands of the tongue)
- From lingual glands of the tongue
- Break down lipids
- Activated by HCl of stomach
- Converts triglycerides to fatty acids and monoglycerides
THE SIGNIFICANT CELLS OF THE STOMACH AND THEIR SECRETIONS (GASTRIC JUICE)
Parietal Cells
- Secrete HCl
- makes the chyme acidic
- kills bacteria
- denaturing (unfolding) proteins
2
, - activates pepsin along with lingual lipase.
- Secrete intrinsic factor, which allows for absorption of vitamin B12
Chief Cells
- The major stomach cells
- Secrete pepsinogen, which is activated (by stomach acid) and becomes pepsin, a peptidase
- Pepsin breaks down proteins
- Secrete gastric lipase
- breaks down triglycerides into monoglycerides and fatty acids
G Cells
- Secrete Gastrin (hormone): contracts cardiac sphincter; loosens pyloric sphincter; promotes stomach
motility; promotes secretion of pepsinogen from chief cells and HCl from parietal cells
Mucous Cells
- THEY SECRETE MUCOUS
- Mucous forms a protective barrier against acidic chyme
THE SIGNIFICANT CELLS OF THE SMALL INTESTINE AND THEIR SECRETIONS
Paneth Cells
- Secrete lysozome
The Enteroendocrine Cells
- S Cells
- Secrete secretin in response to acidic chyme, promoting release of HCO3- rich pancreatic
juice; secondarily, secretin slows down secretion of gastric juice.
- K Cells
- Secrete GIP (glucose-dependent insulinotropic peptide)
- slows down secretion of gastric juice
- stimulates insulin secretion
- CCK Cells
- Secrete cholecystokinin in response to amino acid and fatty acid
- CCK stimulates digestive enzyme-rich pancreatic juice
- relaxes the sphincter of Oddi
- slows gastric emptying
- promotes a feeling of satiety
- causes contraction of the gallbladder
- CCK and Secretin enhance each other
Brunner’s Glands
- Secrete alkaline rich mucus to counter acidic chyme
- Located in the duodenum
The Brush-border
- Brush-border enzymes are inserted into the plasma membranes of the microvilli in the small intestine
Alpha-dextrinase
- Alpha-dextrins to glucose
Maltase
- Maltose to glucose
Lactase
- Lactose to glucose and galactose
3
, Sucrase
- Sucrose to glucose and fructose
Enterokinase
- Converts trypsinogen to trypsin
Aminopeptidase
- Further breaks down amino acids and peptides
Dipeptidase
- Dipeptides to amino acids
Phosphatases & Nucleosidases
- Nucleotides to nitrogenous bases
PANCREATIC JUICE
Pancreatic Amylase
- Breaks down carbohydrates
- Converts polysaccharides to a-dextrose, maltose and maltotriose
Pancreatic Lipase
- Breaks down lipids
- Converts triglycerides to fatty acids and monoglycerides
Trypsin
- Breaks down proteins
- Converts proteins to peptides
Chymotrypsin
- Breaks down proteins
- Converts proteins to peptides
Carboxypeptidase
- Breaks down proteins
- Removes amino acid at carboxyl end of peptides
Elastase
- Breaks down proteins
- Converts proteins to peptides
Ribonuclease
- RNA to nucleotides
Deoxyribonuclease
- DNA to nucleotides
ABSORPTION OF NUTRIENTS
- Most absorption (90%) occurs in small intestine, specifically duodenum.
- Microvilli have a capillary bed and a lacteal to allow for absorption into the general circulation.
The shape and structure of villi and the pliae circulares (circular folds) increase surface area,
which allows for more absorption.
- Lipids
- Lipids need to be emulsified by amphipathic bile salts and then they are surrounded by bile
salts in a spherical structure called a micelle. The micelle transports the dietary lipids and
other hydrophobic particles in the chyme to the absorptive cells where they enter via simple
diffusion
- Most of the lipids recombine into larger structures, lipoproteins called chylomicrons. The
chylomicrons are too big to diffuse into blood capillaries, thus they are taken up into
4
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