Digestive System - Basic Divisions• Alimentary Canal

• Accessory Organs

Wave of contraction

Bolus

Relaxation

Wall ofGI tract

Lumen

Basic Digestive Processes

Ingestion Propulsion

Basic Digestive Processes

Mechanical Digestion Chemical Digestion

Microvilli

Epithelium

Basementmembrane

Lymphaticcapillary

Large lipids andlipid-soluble vitamins

are absorbedinto the lymph

Most nutrientsare absorbedinto the blood

capillaries

Capillary

Lamina propria

Lumen

Basic Digestive Processes

Absorption

Defecation

Oral Cavity• Extends from oral orifice to oropharynx• Site of ingestion, mechanical digestion, chemical

digestion, and propulsion (deglutition).

Intrinsic Salivary Glands

Extrinsic Salivary Glands

Parotid Gland

Submandibular Gland

Sublingual Gland

Composition of Saliva• 97-99% water

• Electrolytes

• Salivary amylase

• IgA

• Lysozyme

• Mucin

Functions of Saliva

• Cleanses

• Dissolves and moistens

Sight/smell/thought/taste of food

Stimulates salivary nuclei in pons

Parasympathetic signals to salivary glands

Salivation

Secretion of Saliva

Deglutition

Esophagus

Esophagealhiatus

Stomach

Diaphragm

OropharynxLaryngopharynx

Esophagus• Allows for food to

pass from pharynx to stomach.

• Runs posterior to trachea and heart.

• Goes thru diaphragm

Esophageal Peristalsis

Lumen

Mucosa

Submucosa

Muscularis

Circular layer

Longitudinallayer

Adventitia

Esophageal Histology

Stomach• Enlarged segment of digestive

tract .

• Links esophagus to SI

• Major functions = food storage

• Other functions:• production of intrinsic factor• immune defense• mechanical and chemical digestion

Surface Cells and Gastric Pits

Gastric gland

Surface mucous cell(secretes alkaline fluidcontaining mucin)

Mucous neck cell(secretes acidic fluidcontaining mucin)

Parietal cell (secretesintrinsic factor andhydrochloric acid)

Chief cell (secretespepsinogen andgastric lipase)

G-cell(enteroendocrine cellsthat secrete gastrin intothe blood)

Simplecolumnarepithelium

Gastric pit

Gastric gland

Surface mucous cell(secretes alkaline fluidcontaining mucin)

Mucous neck cell(secretes acidic fluidcontaining mucin)

Parietal cell (secretesintrinsic factor andhydrochloric acid)

Chief cell (secretespepsinogen andgastric lipase)

G-cell(enteroendocrine cellsthat secrete gastrin intothe blood)

Simplecolumnarepithelium

Gastric pit

Gastric Glands

Pepsinogen PepsinHCl

Chief cell

Enteroendocrinecell

Parietal cell

HCl and Pepsinogen• Pepsin = protease; active

form of pepsinogen

• Why not secrete pepsin?

• Note the feedback happening here.

• What other function does the HCl perform?

Parietal Cells and Intrinsic Factor

Gastric Hormones• Gastrin

• released when stomach begins to stretch or due to increased vagal activity.

• stimulates stomach to increase gastric juice secretion

• Ghrelin• released by the empty stomach• acts on hypothalamus to increase appetite/hunger

H+

Muc

us

Mucus

ProteinsHCl

Denaturedproteins

Pepsin(active)

HCl

HCl

Surfacemucouscell

Intrinsicfactor

Mucousneck cell

Chief cellParietal cell

G-cell

Gastrin

Capillary

Pepsinogen(inactive)

Cl–

Mucin

Oligopeptides

Gastric gland

• What about gastric juice is dangerous to the stomach itself?

• How does the stomach protect itself?

Stomach - Danger

• 3 layers instead of 2– Inner oblique – Middle circular – Outer longitudinal

• Extra layer allows for…

Gastric Muscularis Externa

1

2

Contractions of smoothmuscle in stomach wallmix bolus with gastricsecretions to form chyme.

Peristaltic wave resultsin pressure gradient thatmoves stomach contentstoward the pylorus region.

Pyloric sphincterPylorusregion

Peristalticwave

Gastricsecretions

Bolus

Chyme

Gastric Motility

3

4

5

Retropulsion

Duodenum

Pyloric sphincter

Pyloric sphincter closes, and retropulsion occurs.

Pyloric sphincter opens,and a small volume ofchyme enters the duodenum.

Pressure gradientincreases force in pylorusagainst pyloric sphincter.

Chyme

Gastric Motility

Stomach – Absorption

• No nutrient absorption.

• Alcohol and some drugs.

Release of Gastric Juice – Cephalic Phase

Release of Gastric Juice – Gastric Phase

Stomach Distention Long Reflex

Increased gastric juice secretion

Short Reflex

Peptides, caffeine

Increased gastrin release

Inhibition of Gastric Juice Release

Duodenum Distention

Release of CCK and secretin

Decreased gastric juice

secretion

Stress, anxiety

Increased sympathetic release of NE

Low gastric pH

What must happen to chyme as it enters the small intestine:

- in terms of pH?- in terms of nutrients?

What organs play a role in modifying chyme?

Small Intestine

Liver

Gallbladder

Pancreas

Intestinal juice

Bile

Pancreatic juice

Common Bile Duct

Gallbladder & Liver

Pancreas

Hepatopancreatic Ampulla

Duodenum

Main Pancreatic Duct

Stomach Jejunum Ileum

Flow of Bile and Pancreatic Juice

Surface Area of the Small Intestine• Large! (30 m2)• Maximizes enzyme secretion and nutrient absorption• 3 structural features create the large surface area

Plicae Circulares• Big permanent folds of the mucosa and submucosa.

• Surface area and speed bumps.

Villi• Fingerlike mucosal extensions• Blood capillaries and lacteals• Simple columnar absorptive cells • Goblet cells.

Microvilli• Plasma membrane extensions• Brush border• Digestive enzymes

• Mucosal invaginations with endocrine cells• Secrete secretin & cholecystokinin (CCK)

Intestinal Glands (Crypts of Lieberkuhn)

Intestinal Submucosa• Dense connective tissue

• Plumbing and wiring

• Duodenum also has alkaline mucous glands

• Ileum also has lymphatic nodules - Peyer’s patches

Duodenum Ileum

Liver• Functions:

– Carbohydrate metabolism

– Protein metabolism

– Lipid metabolism

– Production of bile

– Synthesis of plasma proteins

– Removal of drugs, toxins etc. from the bloodstream

– Phagocytosis of old/damaged RBCs

– Storage of vitamins and minerals.

Liver• Main digestive output

is bile

• Bile is sent to the gallbladder and the duodenum via ducts:– R&L Hepatic– Common Hepatic– Common bile– Cystic.

From small andlarge intestine

From spleen

Hepatic portalvein

Right and lefthepatic veins

Hepatic artery

Hepaticlobules

Central vein

Vena cava

Blood Supply to the Liver

Liver Histology• Covered by a CT capsule with

inward extending septa that divide the liver into hexagonal lobules.

Hepatic sinusoid

Hepaticlobule

Bile canaliculiCentral vein

Liver Lobules

• 2 blood vessels at each corner.

• Portal arteriole brings O2-rich blood from the hepatic artery.

• Portal venule brings blood from abdominal viscera via the hepatic portal vein

• Both empty into sinusoidal capillaries which flow past hepatocytes towards the central vein.

• Central veins take blood to the hepatic vein.

Liver Lobule

As blood travels thru a liver sinusoid, its:

• Nutrient content will:

• Oxygen content will:

• Carbon dioxide content will:

• Toxin content will:

• Pathogen content will:

• Plasma protein content will:

Bile Canaliculi• Small ducts carrying bile

made by hepatocytes out of the lobules and towards the hepatic ducts

Bile• Produced by the liver

• Watery solution with bile salts and bilirubin

• Amphipathic bile salts emulsify fats

Gallbladder

• Located under the right lobe of the liver

• Stores and concentrates bile

Gallbladder - Histology

• Rugae

• Simple columnar epithelium with microvilli.

• Thick smooth muscle muscularis.

• Serosa

Flow of Bile into and out of the Gallbladder

Fatty chyme enters the duodenum

Duodenal endocrine cells secrete CCK

CCK

Causes GB contraction

Causes HP sphincter relaxation

Inhibits gastric activity

Pancreas

• Deep to stomach. Stretched from spleen to duodenum• Produces alkaline fluid, digestive enzymes, and

glucose-regulating hormones

Pancreatic acinusDuct cell(secretes HCO3

–)

Acinar cell(secretes amylase,lipase, proteases,and nucleases)

99% of Pancreas = Pancreatic Acini

Pancreatic Duct System

Chyme enters the duodenum

Duodenal endocrine cells secrete CCK

CCK

Causes acinar cell

enzyme secretion

Causes HP sphincter relaxation

Inhibits gastric activity

Regulation of Pancreatic Secretion

Chyme enters the duodenum

Duodenal endocrine cells secrete secretin

Secretin

Causes duct cell fluid secretion

Causes HP sphincter relaxation

Inhibits gastric activity

Regulation of Pancreatic Secretion

Pancreaticacini

Pancreaticislet

1% of Pancreas = Pancreatic Islets

Alpha cell(secretes glucagon)

Beta cell(secretes insulin)

VenuleArteriole

Pancreatic Islets = Islets of Langerhans

• Insulin• Decreases blood [glucose] by stimulating uptake of glucose and

glycogenesis by liver and muscle cells• Increases lipogenesis and decreases lipolysis in adipocytes

• Glucagon• Increases blood [glucose] by stimulating liver to perform

glycogenolysis

Horrible Breakfasts

• Extends from ileocecal junction to anus.

• Converts chyme into feces.

• Absorbs water.

• Expels feces.

Large Intestine

Large Intestine Histology

Large Intestine Histology

• Note the abundant goblet cells and the intestinal glands.• How do they relate to the main function of the colon?

Bacterial Flora• Trillions.

• Breakdown of indigestibles.

• Produce vitamin K, vitamin B12, riboflavin (vitamin B2) and thiamine (vitamin B1).

• How do the colonic bacteria get there?

• Slow local movements due to haustral distentionLarge Intestine Motility - Haustral Contractions

• Strong, long waves of peristaltic contractionLarge Intestine Motility - Mass Movements

Gastrocolic reflex

Defecation Reflex

Rectoanal Angle• Which is better?

Feces• 100 g per day.

• 75% water and 25% solids.

• Contain:– Dead bacteria– Shed epithelial cells– Fats, proteins, plant fibers– Bile pigments.