Digestion and Absorption Processes in the Gastrointestinal Tract Lecture (TEST 2) Flashcards
Digestion and Absorption are the ultimate function of the GI
1) DIGESTION begins in the Stomach and is COMPLETED in the Small Intestine
2) ABSORPTION takes place Primarily in the Small Intestine
- Movement of Nutrients, H2O, and Electrolytes from the Lumen of the Intestine into the Blood
Two Main paths of Absorption:
1) CELLULAR:
- Lumen —> Apical Membranes —> Intestinal Epithelial Cell —> Basolateral Membrane —> Blood
- TRANSPORTERS in Membranes
2) PARACELLULAR (Across an Epithelium)
- Intestinal Epithelial Cell —> Lateral Intercellular Space —> Intestinal Epithelial Cell
- TIGHT JUNCTIONS!!!!
Enzymes Secreted along the GI Tract are key for the Digestion of Nutrients
- Amylase
- Pepsin
- Gastric Lipase
- Trypsin
- Chymotrypsin
- Carboxypeptidase
- Elastase
- Enterokinase (ENTEROPEPTIDASE)
- Maltase
- Sucrase
- Aminooligopeptidase
There is two types of Digestive Activity
1) CAVITAL (Luminal) Digestion:
- Refers to Digestion resulting from the action of Enzymes Secreted by the Salivary Glands, Stomach, and Pancreas
2) MEMBRANE (Contact) Digestion
- Refers to Hydrolysis by Enzymes synthesized by Epithelial Cells
Structure of the Intestinal Mucosa is ideal for Absorption of Large amount of Nutrients
- The surface of the Small Intestine is arranged in LONGITUDINAL FOLDS- Folds of KERCKRING!!!!!!!!!!
- Willi and Microvilli INCREASE Surface Area of the Small Intestine
- Villi are Longest in the DUODENUM and Shorter in the Terminal ILEUM!
Villi and Microvilli INCREASE the Intestine Total Surface Area by 600 fold!!!!!!!!!!!!!!**
The Microvillar Surface
- Is the site of activity for a number of DIGESTIVE ENZYMES
- Is also the Barrier that must be TRAVERSE by Nutrients, Water, and Electrolytes on the way to the Blood or Lymph
Several Cell Types compose the Intestinal Epithelium
1) ENTEROCYTES: Epithelial Cells
- Function in Digestion, Absorption, and Secretion
- Turnover Rate; Cells are replaced 3 to 6 days!!!!!!
- Susceptible to Irradiation and Chemotherapy
2) GOBLET CELLS: Mucus secreting Cells
- Physical, Chemical, and Immunologic Protection
3) PANETH CELLS
- Part of the Mucosal Defenses against Infection
- Secrete agents that DESTRUCT BACTERIA or produce Inflammatory responses
The Enterocyte Membrane Control the Flux of Solutes and Fluid between the Lumen and Blood
How the Enterocyte Membrane does it?
1) PINOCYTOSIS
- At the Base of the MICROVILLI
- Major Mechanism in the UPTAKE of Protein
2) PASSIVE DIFFUSION
- Particles move through pores in the Cell Membrane or through Intercellular Spaces
3) FACILITATED DIFFUSION
4) ACTIVE TRANSPORT
Transmural Movement takes place during Absorption
Solute moving across the Enterocyte, from the Lumen to the Blood, must cross SEVERAL BARRIERS:
- Unstirred layer to Fluid
- Glycocalyx
- Apical Membrane
- Cytoplasm of the Cell
- Basolateral Membrane
- Basement Membrane
- Wall of Blood Capillary or Lymphatic Vessel
Digestion and Absorption processes Exhibit Adaptions
- Adaptions are alterations in Function to MAINTAIN HOMEOSTASIS
- The Capacity of the Intestine to adapt is KEY in Several Clinical Scenarios
a) Small Bowel Resection
b) Bypass - Adaptation is limited in Some Instances
a) If Terminal Ileum is resected, Absorption of Vitamin B12 and Bile Salts to abolished - Certain Genetic Abnormalities lead to a LOST ADAPTATION
a) Ex: Lactase Deficiency
Only Monosaccharides are Absorbed by the Enterocytes
- All ingested Carbohydrates must be Digested to MONOSACCHARIDES
- Three ends products of Carbohydrate Digestion:
1) Glucose
2) Galactose
3) Fructose
Co-transport and Facilitated Diffusion are Key Transport Mechanisms in the Absorption of Carbohydrates
APICAL SURFACE:
1) Glucose and Galactose use a SGLT1 Transporter (Secondary Active Transport) to get into the Cell
* Carbohydrate goes against gradient while Na+ goes with Gradient***
2) Fructose uses GLUT 5 (Facilitated Diffusion) to get into the cell
BASOLATERAL SURFACE:
1) Glucose, Galactose, and Fructose all use the GLUT 2 transporter to get into the blood
Lactose Intolerance is a Common Example of Failure to Digest Carbohydrates
- Brush Border LACTASE is Deficient or Lacking
- Undigested Lactose remains in the Lumen and holds H2O, and causes OSMOTIC DIARRHEA!!!!!!!
- Undigested and Unabsorbed LACTOSE is fermented into Methane and Hydrogen Gas, causing EXCESS GAS!!!
Ingested Proteins are digested to Absorbable forms by Proteases in the Stomach and Small Intestine
STOMACH:
1) Pepsinogen –(low pH)–> Pepsin
SMALL INTESTINE
1) Trypsinogen — (ENTEROPEPTIDASE in Brush Border)—–> TRYPSIN
2) Trypsin creates:
a) Trypsin (again)
b) Chymotrypsin
c) Elastase
d) Carboxypeptidase A
e) Carboxypeptidase B
ENDOPEPTIDASES:
- Pepsin
- Trypsin
- Chymotrypsin
- Elastase
EXOPEPTIDASES:
- Carboxypeptidase A
- Carboxypeptidase B
Ingested Proteins are digested to Absorbable forms by Proteases in the Stomach and Small Intestine
1) Protein —-> Amino Acids/ Oligopeptides
- Facilitated by PEPSIN!!!!!!
2a) Protein —-> AA, Dipeptide, Tripeptide
- Facilitated by Trypsin, Chymotrypsin, Elastase, Carboxypeptidaase A, Carboxypeptidase B (Lumen)
2b) Protein —->Oligopeptides
- Facilitated by Trypsin, Chymotrypsin, Elastase, Carboxypeptidaase A, Carboxypeptidase B (Lumen)
3b) Oligopeptides —> AA, Dipeptide, Tripeptide
- Facilitated by PEPTIDASE (Brush Border)
***Trypsin catalyzes the HYDROLYSIS of Trypsinogen (Autocatalysis)
*****The Pancreatic Proteases digest themselves and Each other
Co-transport and Facilitated Diffusion are Key transport Mechanisms in the Absorption of Proteins
***There are 4 separate COTRANSPORTERS: one for each NEUTRAL, ACIDIC, BASIC, and IMINO Amino Acid!!!!!!!!!
***There are 4 separate FACILITATED DIFFUSION Mechanisms: One for each NEUTRAL, ACIDIC, BASIC, and IMINO Amino Acid!!!!!
