• 6 Processes of Digestion– Ingestion– Mechanical Digestion

• Chewing, swallowing• Peristalsis• Segmentation

– Chemical Digestion• Enzymes breaking

chemical bonds– Secretion

• Goblet cells secreting mucus

• Organs secreting enzymes

– Absorption• Nutrients pass into blood

stream– Defecation

• Removal of solid waste

FoodIngestion

PropulsionEsophagus

Stomach

PharynxMechanicaldigestion

Chemicaldigestion

• Chewing (mouth)• Churning (stomach)• Segmentation (small intestine)

Smallintestine Largeintestine

Defecation Anus

Feces

Bloodvessel

Lymphvessel

Absorption

• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)

Mainly H2O

Salivary glands

stomach Small Intestine

Accessory Organs to S. Intestines

Large Intestine

Blood stream

Histology of Digestive Tract

• Stratified squamous– Mouth, pharynx &

esophagus• Simple Columnar

– Stomach thru L.I.– Goblet cells

• VilliMicrovilli

Mouth

• Main Functions– Ingestion– Physical digestion

• Tongue, teeth

– Chemical digestion• Saliva and salivary

glands

Accessory Organs of Mouth

• Tongue– Taste buds– Responsible for

gripping food– Mixes food with saliva– Initiates swallowing

Frommouth

(b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs.

(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.

Peristalsis vs. Segmentation

Salivary GlandsParotid, submandibular and sublingual

• Functions– Produce saliva

• 99% water• Contains enzyme amylase• Mucin – forms mucus• Growth Factor

• Functions of Saliva– Cleanses mouth

• Growth factor and lysosomes– Dissolves food for taste– Moistens food; creates bolus– Enzyme amylase begins the

chemical digestion of starch

Amylase

• Starch (polysaccharide) disaccharides

Pharynx and Esophagus

• Pharynx– Back of throat– Passage way for

food• Esophagus

– Food chute– Stratified squamous

tissue

Stomach

• Four Primary Functions– Temporary storage tank for food– Mechanically break down food

• Food is converted into creamy paste called chyme

– Chemical breakdown of protein begins– Production of intrinsic factor; necessary for

absorbing Vitamin B12.

Regions of the Stomach• Cardiac region

– Upper area around cardiac sphincter

• Fundus– Dome-shaped part of

stomach

• Body• Pylorus- inferior

portion• Pyloric sphincter:

controls the emptying of the stomach into the S.I.

Stomach Physiology

• Rugae• Peristalsis

– Controlled by pacemaker cells

– Becomes more forceful closer to the pyloric region

• Gastric Pit– Gastric gland

• Secretes mucus and enzyme pepsinogen

• Parietal cells– Secrete HCl and

instrinsic factor– HCl is needed to

convert pepsinogen into pepsin

– Kills bacteria that is ingested with food

– Rennin

• Chief cells – produce pepsinogen; inactive form of pepsin

Converting Pepsinogen to Pepsin

(c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland

Pepsinogen

Mitochondria

PepsinHCl

Chief cell

Enteroendocrinecell

Parietal cell

Converting pepsinogen to pepsin

Regulation of Gastric Activity

1. Cephalic phase – prepares stomach to receive food– Sight, smell, taste of food– Controlled by vagus nerve and

parasympathetic system– Only lasts a short time

2. Gastric Phase– Stimulation of stretch receptors and

chemoreceptors stimulate the release of hormone gastrin

– Gastrin causes release of HCl and pepsinogen

– Increase in stomach contraction intensity– Small amounts of caffeine and alcohol

stimulate chemoreceptors causing an increase in gastric secretions

3. Intestinal phase– Begins when food starts to enter S.I.– Controls the rate of gastric emptying– Slows down gastric activities by releasing

three hormones• Cholecystokinin (CCK) • Secretin• Gastric inhibitory peptide (GIP)

Small Intestine

• Digesting & Absorbing Organ

• Anatomy– Consists of 3

sections• Duodenum• Jejunum• Ileum

– ileocecal spincter: controls flow into colon

– Mesentary

Modifications for Absorption

• Folds– Mixes chyme

and slows down movement

• Villi– Fingerlike

projections increases surface area

• Microvilli

Lining of Small Intestine

Regulation with Intestinal Hormones• Secretin: released when pH falls in duodenum

(Why would this happen?) – Result: causes water and buffers to be secreted into

S.I. by pancreas

• Cholecystokinin (CCK): secreted when protein and lipid dense chyme arrives in S.I.– Result: increases the secretion of digestive enzymes

from the pancreas and bile from the liver

• Gastric Inhibitory Peptide (GIP): released when food begins to enter S.I.– Result: inhibit digestive activity in stomach; causes

release of insulin from pancreas

1

2

3

4

5

6

Chyme enter-ing duodenum causes release ofcholecystokinin (CCK) and secretin from duodenal cells.

CCK (red dots) and secretin (yellow dots) enter the bloodstream.

CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3

–-rich pancreatic juice.

Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly.

CCK (via bloodstream) causes gallbladder to contract ; bile enters duodenum.

During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.

Organs affected by digestive hormones

Chemical Digestion • Brush Border

Enzymes– Intestinal juice is

water, mucus and enzymes

– Sucrase– Lactase– Maltase– Peptidases/proteases

Lactose Intolerance

• Segmentation: movement of food through S.I.; moves very slowly so chemical digestion can occur

• Chemical Digestion:– Very few Enzymes come from S.I.– Need assistance from accessory organs

• Absorption: the building blocks of food are passed into blood stream by Active Transport– Most absorption is completed b4 it reaches the ileum

• Ilieum’s main function is to return bile salts to liver

– Mesentary

Accessory Organs: Pancreas• Endocrine Gland

– Produces hormones• Exocrine Gland

– Produces digestive enzymes

• Empties into duodenum

• Pancreatic duct fuses with Common bile duct and enters the duodenum

Pancreatic Secretions

• Secretes (Exocrine Gland)– Bicarbonate ions (NaHCO3

-); turns acidic chyme from stomach into basic solution (ph 7-8)

– Enzymes• Trypsinogen (inactive form)• Amylase• Lipase• Nucleases

Conversion of Trypsinogen into Trypsin

• Increase in pH from bicarbonate ions and another enzymes converts trypsinogen into its active form trypsin

• Trypsin is a protease and continues protein digestion

Digestion of Lipids

•

Lipase breaks down lipids into fatty acids and glycerol.

Fatty acids and glycerol can then be absorbed into the blood stream

Bile assists in this process by physically breaking down fats into smaller droplets.

• Endocrine Gland– Islets of Langerhans

are specialized cells • Responsible for

secreting 2 hormones– Glucagon– Insulin

Anatomy of Liver and gall bladder• Liver

– Hepatic duct allows bile to leave the liver

• Gall Bladder– Lies under liver;

appears like a slightly inflated green balloon

– Stores bile• Bile leaves the liver via

the cystic duct• Cystic duct and hepatic

duct join to make Common Bile Duct.

• Bile Production– Only digestive process– Physically separates

fats into smaller droplets; allows lipase to digest faster

– (Remember fats don’t dissolve in water)• I.e. scatters fat droplets

throughout solution

• Metabolic Regulation– Processes all nutrients absorbed into blood– Helps regulate cholesterol levels– Forms nonessential amino acids; byproduct is

ammonia (toxic)– Converts ammonia into urea (less toxic)– Stores glucose as glycogen (polysaccharide)– Stores fat soluble vitamins (A,D,E,K)– Detoxifies alcohol and drugs (both legal and illegal)

• Blood leaving liver has fewer waste products than the blood that enters it.

• Assists in Regulating Blood– Produces plasma

proteins– Conserves iron from

old RBC’s

• Cirrhosis– Chronic inflammation of

liver– Results from severe

alcoholism or chronic sever hepatitis

– Liver cells die are replaced scar tissue and fat deposits.

• Hepatitis– Inflammation of liver– Caused by a virus, sewage

contaminated water or feces-mouth

• Cholesterol in Bile salts crystallize;

• Gets stuck in duct; causes extreme pain

Protein digestion

• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+

and hydrolyzed to amino acids within the cells.

+

• Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Stomach

Foodstuff

Protein

Large polypeptides

Pepsin(stomach glands)in presence of HCl

Small polypeptides,small peptides

Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)

Amino acids(some dipeptidesand tripeptides)

Brush border enzymes(peptidases, proteases)

Path of absorptionEnzyme(s)and source

Site ofaction

Chemical Digestion and Absorption of Proteins

Carbohydrate digestion

• Glucose and galactose are absorbed via cotransport with sodium ions.• Fructose passes via facilitated diffusion.• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Starch and disaccharides

Oligosaccharidesand disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivaryamylase

Mouth

Pancreaticamylase

Brush borderenzymes in small intestine(lactase,maltaseand sucrase)

Smallintestine

Smallintestine

Foodstuff

Galactose

Path of absorptionEnzyme(s)and source

Site ofaction

Chemical Digestion and Absorption of Carbohydrates

Fat digestion

Small intestine

Small intestine

Foodstuff

Unemulsifiedfats

Emulsification by the detergent action of bile salts ductedin from the liver

Pancreatic lipases

Monoglyceridesand fatty acids

Glyceroland

fatty acids

Path of absorptionEnzyme(s)and source

Site ofaction

• Fatty acids and monoglycerides enter the intestinal cells via diffusion.

• Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells.

• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.

Chemical Digestion and Absorption of Fats

Nucleic acid digestion

• Units enter intestinal cells by active transport via membrane carriers.

• Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Foodstuff

Nucleic acids

Pancreatic ribo-nuclease and deoxyribonuclease

Brush borderenzymes(nucleosidasesand phosphatases)

Pentose sugars,N-containing bases,

phosphate ions

Path of absorptionEnzyme(s)and source

Site ofaction

Chemical Digestion and Absorption of Nucleic Acids

What remains in Small Intestine?

• Indigestible food materials (cellulose)• Lots of bacteria• Water• Passes through ileocecal valve into Large

Intestine

Large Intestine/ColonAnatomy• Cecum• Appendix• Colon

– Ascending– Transverse– Descending– Sigmoid

• Rectum• Anal Canal

Microscopic Anatomy• No villi or microvilli• No cells which secrete

enzymes• Many goblet cells lining

the simple columnar tissue for lubrication

• Rectum tissue becomes stratified squamous

Main functions• Absorbs water and

electrolytes (Na, Cl, K)

• Absorbs water soluble vitamins

• Eliminates undigested food; defecation

Haustra = wall pockets (seen in picture)

Other Inhabitants• Bacteria

– Responsible for fermenting undigested food

– Product of these reactions is gas (H2, N2, CO2, H2S, CH4)

– Assist in synthesizing vitamin B and vitamin K.