Chapter 22: The Digestive System

main functions of the digestive system

1. take in food

2. break it down into molecules

3. absorb molecules into the bloodstream

4. rid body of any indigestible remains

4 groups of macromolecules

1. carbohydrates

2. lipids

3. proteins

4. nucleic acids

organs of the digestive system fall into 2 groups

1. alimentary canal

2. accessory digestive organs

alimentary canal

continuous muscular tube that runs from the mouth to anus

functions of alimentary canal

1. digests food; breaks down into smaller fragments

2. absorbs into blood

accessory digestive organs

1. teeth

2. tongue

3. gallbladder

4. salivary glands

5. liver

6. pancreas

digestive glands

1. produce secretions that break down foods

2. salivary glands

3. liver

4. pancreas

processing of food involves 6 functions

1. ingestion

2. propulsion

3. mechanical breakdown

4. digestion

5. absorption

6. defecation

ingestion

eating

propulsion

movement of food through the AC, which includes

1. swallowing

2. peristalsis

peristalsis

adjacent segments of the AC organs alternately contract and relax; food is moved unidirectionally through canal

segmentation

nonadjacent segments of AC organs contract and relax; food is sloshed back and forth to divide larger pieces into smaller ones

mechanical breakdown

1. chewing

2. churning food in stomach

3. segmentation in intestine

digestion

catabolic steps

absorption

passage of digested fragments from lumen of GI tract into blood or lymph

defecation

elimination of indigestible substances via anus in form of feces

4 tunics of AC

1. mucosa

2. submucosa

3. muscularis externa

4. serosa

muscosa

1. tunic layer that lines lumen

2. functions

   1. secretes mucus, digestive enzymes, and hormones

   2. absorbs end products of digestion

   3. protects against disease

submucosa

1. connective tissue

   1. blood and lymphatic vessels

   2. lymphoid follicles

   3. nerve plexus

muscularis externa

1. segmentation and peristalsis

2. inner circular layer

3. outer longitudinal layer

sphincter

area where circular layer thickens

serosa

1. outermost layer

2. visceral peritoneum

enteric nervous system

1. gut brain

2. more neurons than spinal cord

enteric neurons communicate with each other and control…

1. motility

2. secretions

short reflexes

occur entirely within the gastrointestinal wall

long reflexes

1. CNS

   1. parasympathetic stimulates

   2. sympathetic inhibits

neural reflex pathways in the gastrointestinal tract

1. external stimuli/internal stimuli

2. chemoreceptors, osmoreceptors, mechanoreceptors

3. local (intrinsic) nerve plexus

4. effectors: smooth muscle or glands

5. response: change in contractile or secretory activity

mouth

1. chews food

2. mixed saliva

   1. begins process of digestion

3. swallowing is intiated

associated organs with oral cavity

1. mouth

2. tongue

3. salivary glands

4. teeth

tongue

1. occupies floor of mouth

2. skeletal muscle

functions of tongue

1. gripping, repositioning, mixing of food during chewing

2. forms bolus

3. initiation of swallowing, speech, and taste

bolus

mixture of food and saliva

functions of saliva

1. cleanses mouth

2. dissolves food chemicals for taste

3. moistens food—becomes bolus

4. begins breakdown of starch with enzyme amylase

2 types of secretory cells in salivary glands

1. serous cells

2. mucous cells

serous cells

1. produce watery secretion

2. enzymes

3. ions

4. mucin

mucous cells

produce mucus

composition of saliva

1. mostly water, hypo-osmotic

2. slightly acidic (pH 6.75-7)

3. electrolytes

   1. Na+, K+, Cl-, PO4, HCO3-

4. amylase and lingual lipase

5. proteins: mucin, lysozyme, IgA

control of salivation

activated by parasympathetic NS

ingested food stimulates

1. chemoreceptors

2. mechanoreceptors in mouth

3. send signals to salivatory nuclei—in brain stem that stimulates parasympathetic impulses

strong sympathetic stimulation inhibits…

salivation

pharynx

1. food, fluids, and air

2. strat squamous epithelium

   1. mucous-producing glands

3. skeletal muscle layers

esophagus

1. flat muscular tube that runs to stomach

   1. collapses when not involved in food propulsion

2. pierces diaphragm at esophageal hiatus

3. joins stomach at cardial orifice

gastroesophageal (cardiac) sphincter

1. surrounds cardial orifice

2. keeps orifice closed when food is not swallowed

3. mucus cells protect esophagus from acid reflux

esophagus structure has all..

4 AC layers

esophageal mucosa has…

strat squamous epithelium

submucosa of esophagus

esophageal glands secrete mucus

muscularis externa of esophagus

1. skeletal muscle superiorly (1/3)

2. smooth muscle inferiorly (2/3)

pharynx and esophagus are conduits that pass food from…

mouth to stomach

deglutition

swallowing

two phages of the digestive processes of the mouth

1. buccal phase

2. pharyngeal-esophageal phase

gross anatomy + function of the stomach

1. temporary storage tank

2. starts chemical breakdown of proteins

3. converts bolus to chyme

4. mucosa forms rugae folds

4 parts of stomach

1. cardia

2. fundus

3. body

4. pylorus

cardia

surrounds cardial orifice

fundus

dome-shaped region beneath diaphragm

body

midportion

pylorus

continuous with duodenum through pyloric valve (sphincter controlling stomach emptying)

mucosa layer of stomach

1. simple columnar epithelium

2. mucous cells

3. gastric pits

   1. gastric glands—produce gastric juice

mucous cells

secretes thick alkaline mucus, releases bicarbonate ions

type of glands cells in stomach (includes secretory cells)

1. parietal cells

2. chief cells

3. enteroendocrine

enteroendorcrine cells

regulate secretion of HCl from the parietal cells

G cells

1. release gastrin

2. stimulates parietal cells

D cells

1. release somatostatin

2. inhibits HCl release

ECL cells

1. releases histamine

2. stimulates HCl release

chief cells

secretes pepsinogen

parietal cells

secretes HCl and gastric intrinsic factor

digestive processes in stomach

1. holding area for food

2. breakdown of food

3. delivers chyme to small intestine

4. denatures proteins by HCl

5. pepsin digests proteins

6. lipid-soluble alcohol and aspirin are absorbed

7. essential for life: intrinsic factor for vit B12 absorption

vit B12 function

needed for RBCs to mature

regulation of gastric secretion

1. neural mechanisms

2. vagus nerve increases secretion

3. sympathetic decreases secretion

3 phases of gastric secretions

1. cephalic (reflex) phase

2. gastric phase

3. intestinal phase

cephalic (reflex) phase

1. conditioned reflex triggered by aroma, taste, sight, thought

2. vagus nerve to enteric NS to parietal cells to HCl release

gastric phase

1. lasts 3-4 hours

2. 2/3 of gastric juice released

stimulation of gastric phase

1. distension activates stretch receptors

   1. initiates both long and short reflexes

2. chemical stimuli activates G cells

   1. partially digested protens (peptides and amino acids)

intestinal phase

1. very brief

2. stimulation of intestinal phase

   1. partially digested food enters the small intestine, causing a release of intestinal gastrin

      1. encourages gastric glands of stomach to continue secretory activities

inhibition of gastric phase

1. low pH inhibts

2. D cells secrete somatostatin

3. occurs between meals

4. occurs during digestive as negative feedback

   1. more HCl secreted, decreases pH, inhibits gastrin secretion

inhibition of intestinal phase

1. distention of duodenum due to entry of chyme

   1. inhibitory effects protect intestine from being overwhelmed by too much chyme

duodenal endocrine cells

1. release hormones that inhibit gastric secretion and slow motility in stomach

   1. secretin

   2. cholecystokinin (CCK)

action of ions in the stomach

1. H+ and HCO3- are generated from the dissociation of carbonic acid

2. H+-K+-ATPase pumps H+ into the lumen and K+ into the cell. K+ returns to lumen through mem channels

3. HCO3- leaves the cell (alkaline tide) in exchange for intracellular fluid Cl-

4. Cl- diffuses through mem channels into lumen

regulation of gastric emptying

1. duodenum prevents overfilling by controlling how much chyme enters

2. stomach empties in about 4 hours

   1. fat takes a long time to move

   2. carb-rich chyme moves quickly

accessory organs associated with small intestine

1. liver

2. gallbladder

3. pancreas

liver digestive function

produces bile

bile

fat emulsifer

gallbladder

storage of bile

pancreas function

1. exocrine: pancreatic juice

2. endocrine: secretion of insulin + glucagon

liver hepatocyte functions

1. produces bile

2. store glucose as glycogen

3. make plasma proteins—albumin

4. store fat-soluble vitamins—A, D, E, K

5. perform detox—convert ammonia to urea

pancreatic juice

1. watery, alkaline solution (pH 8) neutralizes acidic chyme

2. electrolytes, HCO3-

3. digestive enzymes: proteases, amylase, lipases, nucleases

proteases

1. targets proteins

2. secreted in inactive form to prevent self-digestion

amylase

targets carbs

lipases

targets lipids n

nucleases

targets nucleic acids

regulation of bile and pancreatic secretions

stimulated by neural and hormonal controls

hormonal controls that regulate bile and pancreatic secretion into the small intestine

1. cholecystokinin (CCK)

2. secretin

mechanisms promoting secretion and release of bile and pancreatic juice

1. CCK and secrein are secreted by duodenal enteroendocrine cells secretin

2. pancreatic secretion

3. bile secretion by liver

4. gallbladder contraction

5. hepatopancreatic sphincter relaxation

CCK and secretin are secreted by duodenal enteroendocrine cells

1. CCK release is stimulated by proteins and fats in chyme

2. secretin release is stimulated by acidic chyme

3. CCK and secretion enter circulation

pancreatic secretion

1. CCK induces secretion by acinar cells of enzyme-rich pancreatic juice

2. secretin causes secretion by duct cells of HCO3- rich pancreatic juice

3. vagus nerve weakly stimulates during cephalic and gastric phases

bile secretion by liver

1. bile salts returning from enterohepatic circulation are the most powerful stimulus for bile secretion

2. secretin is a minor stimulus

gallbladder contraction

1. CCK causes gallbladder contraction

2. vagus nerve stimulates weak gallbladder contraction during cephalic and gastric phases

hepatopancreatic sphincter relaxation

1. CCK causes hepatopancreatic sphincter to relax

2. Bile and pancreatic juice enter duodenum

modifications of the small intestine for absorption

1. circular folds

2. villi

3. microvilli

circular folds

force chyme to slowly spiral through lumen

villi

1. absorption

2. capillary bed

3. lymphatic capillary—lacteal