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DIGESTIVE SYSTEM 2, In mouth and digestive tract lumen, In brush border of…
DIGESTIVE SYSTEM 2
Absorption and Digestion
Carbohydrate absorption
- Main source is starch and glycogen
- Enzymes
- Salivary amylase
- Pancreatic amylase -> disaccharides
- Disaccharidases -> monosaccharides
- Cellulose (plant cell wall) is not digestible by humans
- Major end-product is glucose
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Protein Absorption
- Start in the stomach
- Pepsinogen -> gastric acid -> pepsin
- Proteins -> polypeptides
- Continue in the small intestines (main site of digestion is in the duodenum)
- Trypsin, chymotrypsin -> peptides
- Aminopeptidases -> amino acids
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Absorption of Sodium Chloride
Na+ – K+ ATPase at the basolateral membrane creates the electrochemical gradient
(low in the Na+ cell)
- Through sodium channels
- Co-transport carriers
- Na+ – Cl- symporter
- Na+ – H+ antiporter
- Na+ – glucose symporter
- Na+ – amino acid symporter
Absorption of water
Passive absorption
- Active reabsorption of electrolytes and nutrients create osmotic gradient
- Na+ – K+ ATPase also creates a localised concentrated area of high osmotic pressure
- In duodenum, water may flow into lumen due to hyperosmotic chyme
- In jejunum and ileum, water is absorbed
Absorption of vitamins
- Water soluble vitamins
- Vitamin B complex, Vitamin C etc
- Mostly by carrier mediated mechanisms
- Fat soluble vitamins
- Vitamin A, D, E, K etc
- Carried in the micelles and absorbed with other lipids
- Vitamin B12
- Absorbed by receptor-mediated endocytosis
- Requires the formation of a complex by binding with gastric intrinsic factor
Absorption of calcium and iron
- Iron and calcium absorption is regulated
- Iron essential for hemoglobin production
- Slightly < 10% of daily ingested iron (15-25mg) absorbed
- Heme iron > Fe2+ (ferrous iron) > Fe3+ (ferric iron)
- Vit C (Ascorbic acid) ↑ iron absorption by reducing Fe3+ to Fe2+.
- Activation of Vit. D in the kidney or liver is required for the effective absorption of calcium
- ~ 2/3 of daily intake of calcium (~1000mg) is absorbed
Small intestines
- 3 parts
- Duodenum, jejunum and ileum
- Site where most digestion and absorption takes place
- ~ 5 – 6 meters long
- Motility
- Peristalsis
- Segmentation contractions
Peristalsis
- Waves of muscular contractions
- Moves the bolus along the length of the digestive tract
- Oral to anal direction
- Myenteric plexus is essential for this
Myenteric plexus
- Between longitudinal and circular muscle layers
- Controls mainly gastrointestinal movements
- Stimulated by receptors in digestive tract
Migrating Motility
- The migrating motility complex sweeps the intestine clean between meals
- The ileocecal juncture prevents contamination of the small intestine by colonic bacteria
- Ileocecal valve and sphincter
Segmentation Contraction
- Mixes and slowly propels the chyme
- Oscillating, ringlike contractions every few centimeters
Functions
- Mixing chyme with digestive juices
- Exposing chyme to all absorptive surfaces
- Higher rate at the start of small intestines, duodenum (12/min); and lower rate at the end, terminal ileum (9/min)
- Purpose is to move the chyme slowly at ileum end
Secretions
- Note: Pancreatic enzymes perform most of the digestive activities
- Enzymes secreted by the enterocytes in the small intestines are localized at the brush border
- Enteropeptidase
- Disaccharidases (Maltase, lactase, sucrose- isomaltase). Breaks down sugars
- Aminopeptidases. Breaks down peptides to amino acids
Adapted for absorption
Increased surface area
- Folds
- Villus
- Micro villi (brush border)
Mechanisms
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Active Transport
Uses energy, ATP
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Secondary active
Glucose, amino acids and peptides etc
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Absorption
- Most absorbed nutrients immediately pass through the liver for processing
- Anything absorbed into the digestive capillaries first must pass through the hepatic biochemical factory
- Lipid bypass this as they enter the lymphatic system
- Extensive absorption by the small intestine keeps pace with secretion
- Out of 9500ml of fluids secreted by various parts of the GI system, 9000ml is absorbed by the small intestine
Maintaining Chemical Balance
- Biochemical balance among the stomach, pancreas, and small intestine is normally maintained
- Body normally does not experience a net gain or loss of acid or base during digestion
- Diarrhea results in loss of fluid and electrolytes
- Passage of a highly fluid fecal matter, often with increased frequency of defecation
- Dehydration, loss of nutrient and metabolic acidosis etc
- Excessive small intestinal motility
- Excess osmotically active particle in lumen
- Toxins from Vibrio cholera etc
Large Intestines
- Primarily a drying and storage organ
- Motility
Secretion
- Digestion
- No, but bacteria present help digest food and produce Vitamin K, Vitamin B12 and folate etc
- Absorption
Motility
- Contractions of the haustra slowly shuffle the colonic contents back and forth
- Initiated by the rhythmic contractions of the colonic smooth muscles
- Slow and non-propulsive
- Mass movements i.e. peristalsis propel feces long distances into the distal part of the large intestines
- Large segments of the ascending and transverse colon contract simultaneously
Defecation Reflex
- Faeces are eliminated by the defecation reflex
- Initiated by distension of the rectum which stimulates stretch receptors in the rectal wall
- Muscle contraction with relaxation of both the internal and external anal sphincter muscle
- Voluntary straining movements increases intraabdominal pressure
- Contraction of abdominal muscles
- Forcible expiration against closed glottis
2 Feedback Loops
- Short intrinsic myenteric defecation reflex
- Stretch receptors stimulate the plexus to initiate increased local peristaltic contractions in sigmoid colon and rectum
- Parasympathetic defecation reflex (long reflex)
- Stimulate mass movements in descending and sigmoid colon
Constipation
- When feces become too dry
- Abdominal discomfort, dull headache, loss of appetite, nausea etc
- Causes
- Ignoring urge to defecate
- Decreased colonic motility
- Obstruction
- Impairment of defecation reflex
Gases and Secretions
- Intestinal gases are absorbed or expelled
- Flatus: gas passes out
- Swallowed air
- Bacterial fermentation
- Large-intestine secretion is entirely protective
- Colonic secretion consists of an alkaline (NaHCO3) mucus solution
- The colon contains myriad beneficial bacteria
- Microbiota and microbiome
Absorption of Salt and Water
- The large intestine absorbs salt and water, converting the luminal contents into feces
- Some absorption takes place within the colon but not to the same extent as in the small intestine
- Na+ actively absorbed
- Cl- follow down electrical gradient
- Water follows osmotically
Nutrients
Carbohydrates
- Monosaccharides
- Fructose, glucose and galactose
- Disaccharides
- Sucrose (one fructose, one glucose)
- maltose (two glucose)
- lactose (one glucose, one galactose)
- Polysaccharides
Proteins
- Amino acids
- Peptides
- Short chain of amino acid monomers link by peptide bonds
- Polypeptides
- Long chain of amino acids (>10)
- Protein
- 1 (>50 a.a) or more polypeptide chain
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In cytosol of epithelial cell; Sodium-Potassium Pump, Active Secondary Transport of monosaccharides from lumen to cell, facilitated diffusion to get out of cell. Simple diffusion into blood
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