Human Physiology Chapter 18

function of the GI tract

:transfers nutrients, water, and electrolytes from ingested food into bodys internal environment

the digestive functions

-motility

-secretion

-digestion

-absorption

motility

:movement of food through digestive tract through the process of;

-ingestion

-mastication

-deglutition

-peristalsis and segmentation

secretion

exocrine and endocrine

digestion

:break down of food molecules into their smaller sub units, which can be absorbed

absorption

:passage of digested end products into blood and lymph

sphincters

allow food to pass uni-directionally

calls lining the tract are replaced

every 3-5 days

the digestive tract includes

mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

accessory digestive organs include

teeth, tounge, salivary glands, liver, gallbladder

teeth

:breakdown of food ingested

functions of chewing

-grind and break down food into smaller pieces to make swallowing easier and increase food surface area

-mix food with saliva

tounge

-movements aid in chewing and swallowing

-plays important role in speech

-taste buds

saliva functions

-salivary amylase begins digestion of carbohydrates

-facilitates swallowing by moistening food

-mucus provides lubrication

-antibacterial action

-solvent for molecules that stimulate taste buds

-helps keep mouth and teeth clean

esophagus - swallowing

-initiated when bolus is forced to rear of mouth

-can be initiated voluntarily but cannot be stopped once it has begun

esophagus

-two esophageal sphincters

-peristalic waves push food through esophagus

secretions in the asophagus

-mucus

function of the stomach

:stores, mixes, and prepares food for digestion

secretions in the stomach

-HCl

-mucus

-pepsin

-gastrin

pyloric sphincter

:controlled release of chyme into small intestine

factors in stomach

:amount of chyme in stomach is main factor that influences strength of contraction

factors in duodenum

-fat

-acid

-hypertonicity

-distension

fat in duodenum

-fat in SI delays gastric emptying

acid in duodenum

-inhibits emptying of acidic gastric contents intil neutralization can be accomplished

hypertonicity in duodenum

-gastric emptying is inhibited when osmolarity of duodenal contents starts to rise

distension is duodenum

-too much chyme in duodenum inhibits emptying of even more gastric contents

small instestine

:site where most digestion and absorption take place

three segments of small intestine

-duodenum

-jejunum

-ileum

small intestine motility includes

-peristalsis

-segmentation

surface area of small intestine

maximize absorption

-fold of intestine

-villi

-microvilli

small intestine secretion

-juice secreted by small intestine does not contain any digestive enzymes

-enzyme are within brush-border membrane of epithelial cells (peptides and disaccharides)

small intestine digestion

-pancreatic enzymes continue carbohydrate and protein

-brush-border enzymes complete digestion of carbohydrates and protein

-fat is digested entirely within small intestine lumen by pancreatic lipase

functions of the large intestine

-absorption of water

-electrolyte absorption

-vitamin synthesis

-formation and storage of fecal matter

large intestine consists of

-colon

-cecum

-appendix

-rectum

-iliocecal sphincter

large intestine is very different from the small intestine

-no digestive enzymes

-no villi/microvilli

-no hormones

-not tightly coiled

-intestinal microflora

movement through GI tract

-substances so not move uniformly

-materials do not leave in the same order they arive

total emptying of stomach takes

4-5 hours

transit through colon takes

30-40 hours

accessory organs

-pancreas

-liver

-gallblader

pancreas

-exocrine and endocrine function

endocrine function of the pancreas

-secrete insulin and glucagon

exocrine function of pancreas

-secrets pancreatic juices consisting of: amylase, lipase, and proteases

liver

:largest and most important metabolic organ in the body

-first organ to receive absorbed nutrients from hepatic portal vein

liver functions

-produces proteins

-manufactures bile salts

-alcohol metabolism

-stores nutrients

-removes and degrades toxins

liver - bile

-made by liver

-stored and concentrated in gallbladder

-after a meal, bile enters duodenum

-(enterohepatic circulation)

liver - bile salts

-derivatives from cholesterol

-fat emulsifier

-after participation in fat digestion and absorption, most are reabsorbed into the blood

enterohepatic circulation

:substances secreted in the bile may be absorbed by the intestinal epithelium and recycled to the liver via the hepatic portal vein

regulation of GI secretions

1. mechanoreceptors =distension of wall

2. osmoreceptors = osmolality of chyme

3. chemoreceptors = acidity of chyme

4. chyme concentration of digestive products

: monosaccharides, fatty acids, peptides, amino acids

gastrin

secreted by: stomach

-secretes HCl, pepsinogen, maintains gastric mucosa

Cholecystokin (CCK)

secreted by: small intestine

-contraction of gallbladder, secretion of pancreatic juices, inhibits motility and secretion

Gastric Inhibitory Peptide (GIP)

secreted by: small intestine

-inhibits motility and secretion, stimulates secretion of insulin

secretin

secreted by: small intestine

-stimulates water and bicarbonate secretion in pancreas, increased CCK action

phases of gastric secretion

-cephalic phase

-gastric phase

-intestinal phase

cephalic phase

:initiated when receptors in head stimulated by senses

gastric phase

-begins when food actually reaches stomach

-presence of protein increased gastric secretions

intestinal phase

-initiated by stimuli in intestinal tract (distension, acidity, osmolarity, digestive products)

enzymes for digesting

-amylase

-disaccharides

-pepsin

-pancreatic enzymes

-brush border enzymes

-lipase

-bile salts

amylase

source: saliva

action: starch

sub units: glucose

disaccharides

source: brush border

action: sucrose, maltose, lactose

sub units: glucose

pepsin

source: gastric glands

action: proteins

sub units: amino acids

pancreatic enzymes

source: pancreatic juices

action: amylase, lipase, trypsin

sub units: CHO, fat, protein

brush border enzymes

source: brush border

action: peptides, disaccharides

sub units: amino acids, glucose

lipase

source: pancreatic juices

action: triglycerides

sub units: free fatty acids

bile salts

source: lover, gallbladder

action: triglycerides

sub units: free fatty acids

hypothalamus

:regulates hunger

-effects hunger and satiety, energy expenditure, growth and reproduction