Anatomy Ch.25 Digestive System Part 2

Region of Stomach: Cardia

Where the esophagus enters

Region of Stomach: Fundus

Upper curved portion

Region of Stomach: Body

Main central region

Region of Stomach: Pyloric region

Connects to small intestine

Primary Function of Stomach

Food storage

Secondary Function of Stomach

• Mechanical digestion → churns food into chyme

• Chemical digestion begins:

  • Proteins

  • Fats

Stomach Innervation

• Parasympathetic → vagus nerve

• Sympathetic → celiac ganglia

Stomach Circulation

• Arterial: Branches of celiac trunk

• Venous: Hepatic portal system → liver → heart

Stomach Wall: Mucosa

• Simple columnar epithelium

• Forms gastric rugae (folds when empty)

Stomach Wall: Submucosa

Supportive connective tissue

Stomach Wall: Muscularis externa (3 layers)

• Outer longitudinal

• Middle circular

• Inner oblique

Gastric Glands & Cell Types

Located in gastric pits:

1. Mucous cells

2. Stem (regenerative) cells

3. Parietal cells

4. Chief cells

5. Enteronendocrine cells

Mucous Cells

Secrete mucus

Stem (regenerative) cells

Generate continual supply of new cells

Parietal Cells

Secrete:

• Hydrochloric acid (HCl)

• Intrinsic factor

• Ghrelin (hormone)

Chief Cells

Secrete:

• Pepsinogen

• Gastric lipase

Enteroendocrine Cells

Secrete chemical messengers

Gastric Juice (2-3 L/day)

• Water

• HCl

• Pepsin

Others

Hydrochloric Acid (HCl) Functions

• Activates pepsin and lingual lipase

• Breaks down connective tissue & plant cell walls

• Liquifies food → chyme

• Converts iron → Fe2+ (absorbable form)

• Kill pathogens

Pepsin

• Chief cells release pepsinogen → activated by HCl → pepsin

• Digests dietary protein → smaller peptides

Gastric Lipase

• Digests 10-15% of fats

• Most fat digestion occurs in small intestine

Intrinsic Factor

• Binds vitamin B12 → allows absorption in small intestine

• Needed for hemoglobin production

• Deficiency→ anemia

Only indispensable function of the stomach, → without stomach → digestion continues, but B12 supplements required

Gastric Motility (Stomach Movement)

• Begins as you swallow food

• Food entering stomach → stimulates gastric motility

Pyloric Region (Thick muscularis)

• Acts as a strong pump

• Breaks up semidigested food

Peristaltic Waves

• Occur every 20 seconds

• Churn and break food into small particles (Chyme)

Gastric Emptying

Only 3mL of chyme released into duodenum at a time

• Allows duodenum to:

  • Neutralize stomach acid

  • Digest nutrients gradually

Stomach mainly functions as a food storage organ

Protection of Stomach Mucosa

The stomach protects itself from acid (HCl) and enzymes

1. Mucus Barrier

2. Tight Junctions

3. Rapid Cell Replacement

Stomach Mucus Barrier

• Thick, alkaline mucus coating

• Protects lining from acid and enzymes

Stomach Tight Junctions

• Seal between epithelial cells

• Prevents gastric juice from leaking into deeper tissue

Stomach Rapid Replacement

Epithelia cells replaced every 3-6 days

• Gastritis: Inflammation of stomach wall by acid & pepsin

• Peptic ulcer: erosion of stomach wall by acid & pepsin

Main Cause: Helicobacter pylori (acid-resistant bacteria)

Enteroendocrine Cell Secretions

1. Gastrin (G-cells)

2. Serotonin

3. Histamine

4. Secretin

5. Cholecystokinin (CCK)

Gastrin (G-Cells)

• Increase HCl secretion

• Increases intestinal motility

Serotonin

Neurotransmitter that stimulates gastric motility and secretion

Histamine

Strongly stimulates HCl secretion

Secretin

• Decrease gastric secretion

• Decreases motility

• Slows gastic emptying

Cholecystokinin (CCK)

• Decrease gastric activity

• Delays stomach emptying

Liver

• Largest gland

• Four lobes: Right, left, caudate, quadrate

• Produces bile → emulsifies fats

Hepatocytes

Produce bile → bile canaliculi → R/L hepatic ducts

Hepatic Sinusoids

• Blood-filled channels between hepatocyte plates (in liver lobules)

• Lined with fenestrated endothelium (leaky → allows exchange)

• Blood from stomach & intestines flows directly here → liver processes it first

Functions of Hepatocytes After a Meal

• Absorb nutrients from blood:

  • Glucose

  • Amino acids

  • Iron

  • Vitamins

• Use for metabolism or storage

Functions of Hepatocytes Between Meals

• Break down glycogen → glucose

• Release glucose into bloodstream

Hepatocytes Detoxification

Remove/degrade:

• Hormones

• Drugs

• Toxins

• Bile pigments

Synthesis (What liver makes)

• Albumin

• Lipoproteins

• Clotting factors

• Angiotensinogen

Liver Hepatic Portal Vein

Nutrient-rich blood from digestive organs

Liver Hepatic Artery

• Oxygen supply

• Blood exists → inferior vena cava

Path of Bile to the Small Intestine

1. Right + Left hepatic ducts

2. → common hepatic duct

3. Cyctic duct (from gallbladder)

4. → Bile duct

5. Joins pancreatic duct

6. → enters duodenum at major duodenal papilla

7. Controlled by hepatopancreatic sphincter (closed between meals)

Gallbladder

Pear-shaped sac under liver

• Stores bile

• Concentrates bile (absorbed water & electrolytes)

Bile Components

• Bile salts (from cholesterol)

• Lecithin (phospholipid)

Gallbladder Function in Digestion

Emulsifies fats → improves fat digestion + absorption

Gallstones (Cholelithiasis)

• Cause: excess cholesterol → crystallization

• Can block bile ducts

Gallstones Effects

• Jaundice (yellow skin)

• Poor fat digestion

• Decrease absorption of fat-soluble vitamins (A,D,E,K)

Pancreas

• Retroperitoneal (behind stomach)

• Regions:

  • Head (in duodenum curve)

  • Body

  • Tail

Pancreas Endocrine Function (Hormones)

• Preformed by pancreatic islets

Secretes:

• Insulin

• Glucagon

Mostly located in the tail

Pancreas Exocrine Function (Digestion)

• Makes up 99% of pancreas

• Produces 1,200 - 1,500 mL pancreatic juice/day

Pancreatic Juice Composition

• Water

• Digestive enzymes

• Zymogens (inactive enzymes)

• Sodium bicarbonate

• Electrolytes

Pancreatic Juice - Acini (acinar cells)

Secrete enzymes & zymogens

Pancreatic Juice - Duct cells

Secrete bicarbonate

Main Pancreatic Duct

• Runs through center of the pancreas

• Joins the bile duct → forms hepatopancreatic ampulla

• Empties into duodenum

Accessory pancreatic Duct

• Smaller, secondary duct (present in most people_

• Bypasses the main sphincter

• Allows pancreatic juice to enter duodenum even if bile flow is blocked

Pancreatic Secretion

1. Sodium bicarbonate

2. Zymogens (inactive enzymes)

3. Active Enzymes

Pancreatic Secretions- Sodium bicarbonate

Function: Neutralizes stomach acid (HCl)

• Creates optimal pH for enzymes in small intestine

Pancreatic Secretions- Zymogens

• Trypsinogen → Trypsin

  • Digest proteins

• Chymotrypsinogen → Chymotrypsin

  • Digest proteins

• Procarboxypeptidase → Carboxypeptidase

  • Removes amino acids from peptide ends

Pancreatic Secretions- Active Enzymes

• Pancreatic amylase → digests starch (carbs)

• Pancreatic lipase → digests fats

• Ribonuclease (RNase) → digests RNA

• Deoxyribonuclease (DNase) → digests DNA

Hormonal Control of Pancreatic Juice & Bile

• Acetylcholine (ACh)

• Cholecystokinin (CCK)

• Secretin

Acetylcholine (ACh)

• From vagus nerve & enteric nervous system

• Stimulates enzyme secretion before food arrives

Cholecystokinin (CCK)

• Released when fat enters duodenum

• Stimulates:

  • Pancreatic enzyme secretion

  • Gallbladder contraction → bile release

  • Relaxation of hepatopancreatic sphincter

Secretin

• Released in response to acidic chyme

• Stimulates:

  • Bicarbonate secretion (pancreas + liver)

• Function: raises pH for optimal digestion

Small Intestine

Main site of chemical digestion & nutrient absorption

• Receives: Chyme + pancreatic juice + bile

Duodenum (first 25 cm)

Receives digestive secretions

• Neutralizes stomach acid (via bicaronate)

• Bile emulsifies fat

• Pepsin is inactivated (due to higher pH)

• Pancreatic enzymes begin most chemical digestion

Jejunum

Thick walls, many folds, rid blood supply

• Major site of digestion and absorption

• Most nutrients enter bloodstream here

Illeum

Thinner, less vascular

• Final absorption especially:

  • Vitamin B12

  • Bile acids

Illeocecal Papilla

Formed by thickened muscularis of illeum

• Controls passage of food residue into the large intestine

Lining of the Small Intestine

• The lumen is lined with simple columnar epithelium

• Designed for efficient absorption of nutrients

Small Intestine Surface Area Adaptions

Greatly increases surface area using:

• Circular folds

• Villi

• Microvilli

Small Intestine Circular Folds

• Force chyme to move in a spiral path

• Slow down movement through the intestine

• Increase contact time with the mucosa

Promote:

• Better mixing

• More efficient nutrient absorption

Small Intestine Villi

• Finger-like projections of the mucosa

• Covered by:

  • Absorptive cells (enterocytes) → absorb nutrients

  • Goblet cells → secrete mucus

Villi Structural Features

• Epithelial cells are connected by tight junctions

  • Prevent digestive enzymes and fluids from leaking between cells

• Core of villus= lamina propria

Villi Blood and Lymph Supply

Each villus contains:

• Arteriole

• Capillary network → absorbs most nutrients

• Venule

• Lymphatic capillary (lacteal) → absorbs fats

Microvilli

Tiny projections on the surface of absorptive cells

• Forms the brush border

Microvilli Function

• Contain brush border enzymes attached to the cell membrane

• Carry out the final steps of digestion

• Require direct contact between chyme and the surface

This is called contact digestion

Intestinal Crypts

• Small tubular glands located between villi

• Open into the intestinal lumen

Produce 1-2 liters of intestinal juice per day

• Enterocytes and goblet cells

Intestinal Crypts Cell Renewal

• Lower crypts contain stem cells

• New cells:

  • Divide rapidly

  • Live 3-6 days

  • Migrate upward to villi tips

  • Are eventually shed and digested

Paneth Cells

• Found at base of crypts

• Secrete antimicrobial substances:

  • Lysozyme

  • Phospholipase

  • Defensins

• Help protect against bacterial invasion

Movement of the Small Intestine Contractions

• Mix chyme with intestinal juice, bile, and pancreatic juice

• Neutralize acid and improve nutrient digestion

• Churn chyme and bring it into contact with the mucosa for contact digestion + absorption

• Move residue toward the large intestine

Small Intestine Segmentation

• Main movement during digestion and absorption

• Circular muscle contractions divide chyme into segments

Function:

• Churns and mixes intestinal contents

• Progressively blends chyme with digestive secretions

Small Intestine Peristalsis

• Becomes dominant when most nutrients have been absorbed

• Triggered by the hormone motilin

→ Moves remaining material toward the colon

Small Intestine Migrating Motor Complex

• Series of strong, repeated peristaltic waves

• Each wave moves partially down the intestine

• “Cleans out” remaining contents

Carbohydrate Digestion Mouth

• Begins in the mouth

  • Salivary amylase breaks starch → oligosaccharides

  • Works best at pH 6.8 - 7.0

Carbohydrate Digestion in Stomach

• Amylase is denatured by acid

• Starch digestion stops

Carbohydrate Digestion in Small Intestine

• Pancreatic amylase breaks starch → maltose

• Brush border enzymes (contact digestion) break:

  • oligosaccharides + maltose → glucose

Carbohydrate Absorption in Enterocytes

Transport proteins absorb monosaccharides

• ~80% of absorbed sugar = glucose

• Taken into cells by SGLT (Sodium-glucose transporters)

• Glucose leaves cell by facilitated diffusion

Carbohydrate Absorption Water Movement

• Glucose increases osmolarity of ECF

• Water moves from lumen → ECF via osmosis

• Water carries nutrients by solvent drag

Final Transport:

• Nutrients enter capillaries in villi

• Delivered to liver through hepatic portal system

Lactose Intolerance

• Most people stop producing lactase around age 4

• Without lactase:

  • Lactose passes into large intestine undigested

Protein Digestion Stomach

• Mouth: No chemical digestion

• Stomach:

  • Pepsin hydrolyzes certain peptide bonds

→ Breaking protein down into smaller polypeptides

Protein Digestion Small Intestine

• Trypsin and chymotrypsin hydrolyze other peptide bonds

→ breaking polypeptides down into smaller oligopeptides

• Carboxypeptidase removes one amino acid at a time

Protein Digestion Brush Border Enzymes

• Finish protein digestion via contact digestion

• Produce free amino acids

Protein Absorption

• Amino acids enter enterocytes via:

  • Sodium-dependent cotransporters

• Exit cells via facilitated diffusion

• Enter capillaries → transported via hepatic portal circulation

Lipid Digestion - Lingual and Gastric Lipase

Begin fat breakdown and emulsification

Lipid Digestion- Pancreatic Lipase

Breaks fats → free fatty acids monoglycerides

• Products from micelles (keeps lipids soluble in water)