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
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller THEEXCELLENCELIBRARY. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $18.99. You're not tied to anything after your purchase.
