Digestive System (Day 2) Flashcards
Ability of bile salts to emulsify fats
conversion of hydrophobic CHOLESTEROL –> amphipathic BILE ACIDS
–> in liver
Digestion and Absorption of Fats
Digestion begins in duodenum: bile stabilizes fat emulsion and lipase (from pancreas) breaks it down into fatty acids and glycerol
Phospholipase A (from pancreas) digests phospholipids into fatty acids and lysolecithin.
Fat Emulsification/Digestion
Digestion produces fats (free FA) incorporated into micelles, producing mixed micelles
These diffuse to absorptive surface and FA pass into cells
Steps:
- Emulsification of fat droplets by bile salts
- Hydrolysis of triglycerides in emulsified fat droplets into FA and monoglycerides
- Dissolving of FA and monoglycerides into micelles to procude “mixed micelles”
Absorption/Transportation of fats
Fatty acids, monoglycerides, and lysolecithin move into bile micelles and are transported to brush border.
The fat molecules then leave the micelles and diffuse into the epithelial cells of the villi.
Inside the epithelial cells, they are regenerated into triglycerides, cholesterol, and phospholipids and combined with proteins to form chylomicrons.
Chylomicrons are secreted by exocytosis into the central lacteal of the villus
Fluid/Electrolyte Secretion and Absorption
Oral fluid intake = 2000 ml/day; digestive secretions = 7000 ml/day; all but 100 ml is reabsorbed, mainly in SI, but also in LI
Mostly water & ions [Na+, K+, Cl-, HCO3-, H+]
Secreted ions are reabsorbed, water follows osmotic gradients created by solute transfer
Control of digestive function occurs at each level of the GI Tract
- Cephalic Phase: stimulation of gastric activiity
- Gastric Phase: stimulatino of gastric activity
- Intestinal Phase: inhibition of gastric activity, stimulation of digestion in intestine
Cephalic Phase: Mechanism
short duration, prepares stomach for arrival of food
Mechanism: neural - reflex signals from medulla oblongata via vagal preganglionic fibers to synapses in submucosal plexus
Cephalic Phase: Actions
- direct neural stimulation of acid, mucus, enzyme secretion
- indirect via stimulation of gastrin release from G cells, gastrin stimulates motility and acid secretion
- indirect via stimulaiton of histamine release from ECL cells
What are conditioned reflexes?
taste smell chewing swallowing hypoglycermia
Gastric Phase: Mechanisms
duration: 3-4 hours, enhances secretion, mix, acidify, and increase SA of chyme, begin hydrolysis of PRO
Mechansims:
- neural reflexes triggered by strech, rising pH
- hormonal (gastrin) release triggered by vagal signals, peptides, AAs
- local release of histamine (stimulates acid secretion) triggered by stretch
Gastric Phase: Actions
increased production and secretion of acid and pepsinogen, increased motility and mixing waves
Gastric Emptying
Chyme metered out into duodenum as stomach empties
Emptying is regulated by intestinal signals to prevent
Liquids empty fastest, solids slowest, nutrient-specific control (senseing of physiochemical properties)
–> ex. H, fat, pi
Intestinal Phase: Mechanisms
duration: hours, acts to control gastric emptying rate,
to limit gastric acid secretion, and to optimize conditions for enzymatic
digestion in intestinal lumen
Mechanisms:
1) NEURAL—short reflex (enterogastric reflex) triggered by distension
of duodenum
2) HORMONAL—stimulation of secretin, CCK, GIP, by presence of H+, CHO, AAs, lipids
Intestinal Phase: Actions
Feedback inhibition of gastric acid/pepsinogen secretion, gastric motility, promotion of intestinal/pancreatic/biliary secretion, intestinal motility
Gastrointestinal Hormones: Gastrin
Secreted by the stomach
stimulates partietal cells to secrete HCl, stimulates chief cells to secrete pepsinogen, maintains structure of gastric mucosa