- thin covering membrane (visceral peritoneum)

Lecture 30 Digestive Flashcards by Sarah underwood

GI Tract

mouth -stratified squamous epithelium 
pharynx 
esophagus 
stomach -simple columnar epithelium 
small intestine
large intestine

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Accessory Organs

salivary glands
liver
pancreas
gallbladder

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Functions of the Digestive System

digestion
absorption
secretion
motility

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digestion

chemical breakdown of food molecules by hydrolysis

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absorption

of nutrients, electrolytes and H2O

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secretion

mucus, digestive enzymes, acid, bicarbonate, electrolytes

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motility

muscular movements of GI tract to mix and propel food
peristalsis - moves material forward
segmental contractions - mix contents

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regional specialization (“assembly line”)

ingestion → mechanical breakdown → chemical digestion → absorption → waste processing

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GI tract structure: 4-layered tube

mucosa
submucosa
muscularis (externa)
serosa

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mucosa

mucosa - epithelium + lamina propria (areolar CT) + muscularis mucosae

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submucosa

submucosa - connective tissue, vascular

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muscularis (externa)

muscularis (externa) - smooth muscle

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serosa

thin covering membrane (visceral peritoneum)

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Mouth, Pharynx and Esophagus

functions:

ingestion, mastication (chewing), deglutition (swallowing)

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salivary glands

secrete saliva: H2O, ions, mucus, enzymes: amylase, lipase

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amylase

begins chemical digestion of starch → disaccharides

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esophagus

swallowing (upper portion), peristalsis (lower portion)

lower esophageal sphincter controls entry into the stomach

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Stomach

functions

storage
mechanical breakdown of food → chyme
sterilization
chemical digestion: acid (HCl) and enzymes (pepsin)

stomach structure

mucosa: simple columnar epithelium, gastric glands
- secrete acidic gastric juice (pH 1-2), 1-3 L/day
- mucous cells secrete alkaline mucus to protect stomach epithelium
muscularis: 3 layers thick
- pyloric sphincter controls passage of chyme from stomach to duodenum

acid secretion

parietal cells secrete hydrochloric acid (HCl)
CO2 + H2O  H2CO3  H+ + HCO3-
H+ is active transported into the lumen, Cl- follows via diffusion through channels
HCO3- is transported back into ECF (countertransport with Cl-)

chief cells

enzyme secretion
chief cells secrete pepsinogen (inactive), activated at low pH to form pepsin
pepsin digests proteins into smaller peptides

Small Intestine, Liver and Pancreas Functions and SI regions

Small Intestine, Liver and Pancreas
functions: chemical digestion and absorption
SI regions: duodenum, jejunum, ileum

Digestion

duodenum receives chyme from stomach, secretions from liver and pancreas

Liver

processes absorbed nutrients (delivered via hepatic portal vein)
secretes bile, stored in gallbladder
bile salts

bile salts

derived from cholesterol, function to emulsify fats → micelles

bile pigments

(bilirubin, biliverdin) - waste products from hemoglobin breakdown

Pancreas

acinar cells secrete digestive enzymes: trypsin, chymotrypsin, carboxypeptidase, amylase, lipase many enzymes are secreted in inactive form (zymogens), activated by trypsin in lumen

duct cells

secrete bicarbonate (NaHCO3) to neutralize acid (pH → 8)

SI (brush border)

enzymes complete digestion | complete digestion of starch

exopeptidase

digests terminal peptide bonds to release amino acids

endopeptidase

digests internal peptide bonds

small intestine

has huge surface area, specialized for absorption (1) length > 3 meters (2) circular folds (3) villi - epithelium (enterocytes and goblet cells) + lamina propria (capillaries and lacteals) (4) microvilli - “brush border” membrane

transport mechanisms of the small intestine

- Na+, Cl - , K+ absorbed via active transport and diffusion through channels - glucose & amino acids - cotransport with Na+ (secondary active transport) - H2O - via osmosis, follows solute transport

water soluble nutrients

are absorbed into intestinal capillaries → liver (via HPV)

chylomicrons

lipids are formed into chylomicrons and absorbed into lymphatic vessels (lacteals)

Large Intestine | functions

fluid absorption, waste packaging and elimination - LI absorbs most remaining water and ions from chyme - intestinal microflora - bacteria in colon, produce some vitamins (K, B12) - defecation reflex

Neural and Hormonal Control

``` enteric ns ans GI peptides hormones phases of digestion ```

. Enteric Nervous System

- submucosal and myenteric plexuses | - local control within the GI tract (short reflex)

Autonomic Nervous System

parasympathetic: vagus nerve - stimulates GI tract motility and secretion (long reflex) sympathetic division mostly inhibits GI tract

gastrin

secreted by G cells in the gastric glands - stimulates gastric acid secretion; stimulates gastric motility and mucosal growth (- acid secretion is also stimulated by histamine secreted by ECL cells in gastric glands)

CCK

(cholecystokinin) - secreted by endocrine cells in intestinal crypts - stimulates bile release from gallbladder and pancreatic enzyme secretion

secretin

- stimulates bicarbonate secretion by pancreas

GIP

(gastric inhibitory peptide) - stimulates insulin secretion by pancreas; - GIP, CCK and secretin all inhibit gastric acid secretion

Phases

1. Cephalic Phase - sensory stimuli and thoughts of food activate autonomic NS (vagus n.) 2. Gastric Phase - vagus n., mechanical & chemical stimuli in stomach stimulate gastric secretion 3. Intestinal Phase - arrival of chyme in duodenum triggers SI endocrine and exocrine secretion; hormonal feedback inhibits gastric acid secretion and slows stomach emptying