BIO 252 Anatomy and Physiology Module 9
The Digestive System
6.1 – Physiology of the Digestive System
Primary Functions of the Digestive System ●
The digestive system works
continuously to:
○ Ingest food
○ Break down food mechanically and chemically
○ Absorb nutrients into the bloodstream or lymph
○ Eliminate waste (defecation)
● The small intestine is the primary site of digestion and absorption
Alimentary Canal (GI Tract)
● One-way muscular tube from mouth → anus
● Food inside the canal is technically outside the body until absorbed
Organs (In Order)
1. Mouth (oral cavity)
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
a. Duodenum
b. Jejunum
c. Ileum
6. Large intestine
a. Cecum
b. Ascending colon
c. Transverse colon
d. Descending colon
e. Sigmoid colon
7. Rectum
8. Anal canal / anus
Accessory Digestive Organs
● Assist digestion but food does not pass through them:
, ○ Teeth and tongue → mechanical
digestion
○ Salivary glands → saliva + enzymes
○ Liver → bile production
○ Gallbladder → bile storage
○ Pancreas → digestive enzymes & bicarbonate
Histology of the Alimentary Canal
● The GI tract has 4 tissue layers (lumen → outward)
1. Mucosa
a. Epithelium
i. Mostly simple squamous columnar
epithelium
ii. Contains goblet cells → mucus
iii. Enteroendocrine cells → hormones
iv. High cell turnover (protects against
wear)
b. Lamina Propria
i. Loose connective tissue
ii. Blood and lymph vessels
iii. Immune function (Peyer’s patches in
distal ileum)
c. Muscularis Mucosae
i. Thin smooth muscle
ii. Creates folds → ↑ surface area
2. Submucosa
a. Dense connective tissue
b. Blood & lymph vessels
c. Submucosal glands
d. Submucosal nerve plexus → regulates secretions
3. Muscularis Externa
a. Smooth muscle layers
i. Inner circular
ii. Outer longitudinal
b. Responsible for peristalsis and segmentation
c. Modifications
i. Stomach: additional oblique layer
ii. Colon: longitudinal layer forms tenia coli
d. Exceptions
, i. Mouth, pharynx, upper esophagus →
skeletal muscle
4. Serosa / Adventita
a. Serosa (intraparietal organs): visceral peritoneum
b. Adventitia (retroperitoneal organs): connective tissue anchor
Enteric Nervous System (ENS)
● Intrinsic nervous system of the GI tract (esophagus → anus)
● Two major plexuses
○ Myenteric plexus
■ Located in the muscularis externa
■ Controls motility
○ Submucosal plexus
■ Located in submucosa
■ Controls secretions and blow flow
● Extrinsic Control
○ Parasympathetic (rest and digest)
■ ↑ motility and secretions
○ Sympathetic (fight or flight)
■ ↓ motility and secretions
Splanchnic Circulation (Blood Supply)
● Supplies digestive organs with blood
● Major arteries
○ Celiac trunk
○ Superior mesenteric artery
○ Inferior mesenteric artery
● Venous drainage
○ Nutrient-rich blood → hepatic portal vein
○ Liver processes nutrients and detoxifies blood
○ Blood exits liver → hepatic veins → inferior vena cava
● During digestion, ~25% of cardiac output goes to GI organs
Peritoneum and Digestive Organ Relationships
● A serous membrane that supports abdominal organs
● Layers
○ Parietal peritoneum → lines abdominal wall
○ Visceral peritoneum → covers organs
○ Peritoneal cavity → lubricated space between layers
, Peritoneal Folds
Fold Function
Greater Omentum Fatty apron from stomach: immune & fat storage
Lesser Omentum Connects stomach to liver
Falciform Ligament Anchors liver to anterior abdominal wall
Mesentery Attaches small intestine; contains vessels
Mesocolon Attaches portions of large intestine
Retroperitoneal Organs
● Located behind peritoneum (e.g. duodenum, pancreas, parts of colon)
Digestive Process– 6 Parts
1. Ingestion
a. Entry of food into mouth
b. Chewing + saliva begin digestion
2. Propulsion
a. Swallowing
b. Peristalsis: wave-like muscle contractions moving food forward
3. Mechanical Digestion
a. Physical breakdown:
i. Chewing
ii. Stomach churning
iii. Segmentation (small intestine mixing)
4. Chemical Digestion
a. Enzymes break macromolecules into monomers
b. Begins in mouth, mostly completed in small intestine
5. Absorption
a. Nutrients enter bloodstream or lymph
b. Lipids → lacteals → lymph → blood
6. Defecation
a. Elimination of indigestible material as feces
Peristalsis vs Segmentation
Feature Peristalsis Segmentation
The Digestive System
6.1 – Physiology of the Digestive System
Primary Functions of the Digestive System ●
The digestive system works
continuously to:
○ Ingest food
○ Break down food mechanically and chemically
○ Absorb nutrients into the bloodstream or lymph
○ Eliminate waste (defecation)
● The small intestine is the primary site of digestion and absorption
Alimentary Canal (GI Tract)
● One-way muscular tube from mouth → anus
● Food inside the canal is technically outside the body until absorbed
Organs (In Order)
1. Mouth (oral cavity)
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
a. Duodenum
b. Jejunum
c. Ileum
6. Large intestine
a. Cecum
b. Ascending colon
c. Transverse colon
d. Descending colon
e. Sigmoid colon
7. Rectum
8. Anal canal / anus
Accessory Digestive Organs
● Assist digestion but food does not pass through them:
, ○ Teeth and tongue → mechanical
digestion
○ Salivary glands → saliva + enzymes
○ Liver → bile production
○ Gallbladder → bile storage
○ Pancreas → digestive enzymes & bicarbonate
Histology of the Alimentary Canal
● The GI tract has 4 tissue layers (lumen → outward)
1. Mucosa
a. Epithelium
i. Mostly simple squamous columnar
epithelium
ii. Contains goblet cells → mucus
iii. Enteroendocrine cells → hormones
iv. High cell turnover (protects against
wear)
b. Lamina Propria
i. Loose connective tissue
ii. Blood and lymph vessels
iii. Immune function (Peyer’s patches in
distal ileum)
c. Muscularis Mucosae
i. Thin smooth muscle
ii. Creates folds → ↑ surface area
2. Submucosa
a. Dense connective tissue
b. Blood & lymph vessels
c. Submucosal glands
d. Submucosal nerve plexus → regulates secretions
3. Muscularis Externa
a. Smooth muscle layers
i. Inner circular
ii. Outer longitudinal
b. Responsible for peristalsis and segmentation
c. Modifications
i. Stomach: additional oblique layer
ii. Colon: longitudinal layer forms tenia coli
d. Exceptions
, i. Mouth, pharynx, upper esophagus →
skeletal muscle
4. Serosa / Adventita
a. Serosa (intraparietal organs): visceral peritoneum
b. Adventitia (retroperitoneal organs): connective tissue anchor
Enteric Nervous System (ENS)
● Intrinsic nervous system of the GI tract (esophagus → anus)
● Two major plexuses
○ Myenteric plexus
■ Located in the muscularis externa
■ Controls motility
○ Submucosal plexus
■ Located in submucosa
■ Controls secretions and blow flow
● Extrinsic Control
○ Parasympathetic (rest and digest)
■ ↑ motility and secretions
○ Sympathetic (fight or flight)
■ ↓ motility and secretions
Splanchnic Circulation (Blood Supply)
● Supplies digestive organs with blood
● Major arteries
○ Celiac trunk
○ Superior mesenteric artery
○ Inferior mesenteric artery
● Venous drainage
○ Nutrient-rich blood → hepatic portal vein
○ Liver processes nutrients and detoxifies blood
○ Blood exits liver → hepatic veins → inferior vena cava
● During digestion, ~25% of cardiac output goes to GI organs
Peritoneum and Digestive Organ Relationships
● A serous membrane that supports abdominal organs
● Layers
○ Parietal peritoneum → lines abdominal wall
○ Visceral peritoneum → covers organs
○ Peritoneal cavity → lubricated space between layers
, Peritoneal Folds
Fold Function
Greater Omentum Fatty apron from stomach: immune & fat storage
Lesser Omentum Connects stomach to liver
Falciform Ligament Anchors liver to anterior abdominal wall
Mesentery Attaches small intestine; contains vessels
Mesocolon Attaches portions of large intestine
Retroperitoneal Organs
● Located behind peritoneum (e.g. duodenum, pancreas, parts of colon)
Digestive Process– 6 Parts
1. Ingestion
a. Entry of food into mouth
b. Chewing + saliva begin digestion
2. Propulsion
a. Swallowing
b. Peristalsis: wave-like muscle contractions moving food forward
3. Mechanical Digestion
a. Physical breakdown:
i. Chewing
ii. Stomach churning
iii. Segmentation (small intestine mixing)
4. Chemical Digestion
a. Enzymes break macromolecules into monomers
b. Begins in mouth, mostly completed in small intestine
5. Absorption
a. Nutrients enter bloodstream or lymph
b. Lipids → lacteals → lymph → blood
6. Defecation
a. Elimination of indigestible material as feces
Peristalsis vs Segmentation
Feature Peristalsis Segmentation
