The Digestive System Notes

24-1 Identify the organs of the digestive system, list their major functions, describe the histology of the digestive tract, and outline the mechanisms that regulate digestion.
24-2 Discuss the anatomy of the oral cavity, and list the functions of its major structures and regions.
24-3 Describe the structure and functions of the pharynx and esophagus.
24-4 Describe the anatomy of the stomach, including its histology, and discuss its roles in digestion and absorption.

24-5 Describe the structure, functions, and regulation of the accessory digestive organs.
24-6 Describe the anatomy and histology of the small intestine, and explain the functions and regulation of intestinal secretions.
24-7 Describe the gross and histological structures of the large intestine, including its regional specializations and role in nutrient absorption.
24-8 List the nutrients required by the body, describe the chemical events responsible for the digestion of organic nutrients, and describe the mechanisms involved in the absorption of organic and inorganic nutrients.

24-9 Summarize the effects of aging on the digestive system.
24-10 Give examples of interactions between the digestive system and other body systems studied so far.

Digestive system: Acquires nutrients from the environment
- Used to synthesize essential compounds (anabolism)
- Broken down to provide energy to cells (catabolism)

Digestive system components:
- Digestive tract:
  - Gastrointestinal (GI) tract or alimentary canal
  - Muscular tube extending from oral cavity to anus
- Accessory organs:
  - Teeth, tongue, and various glandular organs
Integrated processes of the digestive system:
- Ingestion
- Mechanical digestion and propulsion
- Chemical digestion
- Secretion
- Absorption
- Defecation

Ingestion: Occurs when food enters oral cavity.
Mechanical digestion and propulsion:
- Involves crushing and shearing of food.
- Propels food along digestive tract.
Chemical digestion:
- Breakdown of food into small organic fragments for absorption by digestive epithelium.
Secretion:
- Release of water, acids, enzymes, buffers, and salts through epithelium of digestive tract, glandular organs, and gallbladder.
Absorption:
- Movement of organic molecules, electrolytes, vitamins, minerals, and water across digestive epithelium into interstitial fluid of digestive tract.
Defecation:
- Elimination of wastes from body; compacted, dehydrated wastes are classified as feces.

Lining of digestive tract safeguards surrounding tissues against:
- Corrosive effects of digestive acids and enzymes.
- Mechanical stresses such as abrasion.
- Bacteria either ingested with food or residing in digestive tract.

Peritoneum: Serous membrane lining the peritoneal cavity.
- Composed of superficial mesothelium covering a layer of areolar tissue.
- Visceral peritoneum (serosa): Covers organs within the peritoneal cavity.
- Parietal peritoneum: Lines inner surfaces of body wall.

Peritoneal fluid:
- Produced by serous membrane lining; allows sliding of parietal and visceral surfaces without friction or irritation.
- Approximately 7 liters produced and absorbed daily; very little fluid in peritoneal cavity at one time.
- Ascites: Abdominal swelling due to buildup of peritoneal fluid.

Mesenteries: Double sheets of peritoneal membrane that suspend portions of the digestive tract within the peritoneal cavity.
- Connect parietal peritoneum with visceral peritoneum.
- Provide route for blood vessels, nerves, and lymphatic vessels; stabilize positions of attached organs; prevent intestines from becoming entangled.

During embryonic development, the digestive tract and accessory organs suspended in peritoneal cavity by:
- Dorsal mesentery
- Ventral mesentery (disappears along most of digestive tract, persists as lesser omentum and falciform ligament).

Lesser omentum: Stabilizes position of stomach and provides access route for blood vessels and other structures entering or leaving the liver.

Falciform ligament: Helps stabilize position of the liver relative to diaphragm and abdominal wall.

Greater omentum: Formed by an expansion of dorsal mesentery; hangs from lateral and inferior borders of stomach, providing insulation to reduce heat loss.
- Contains adipose tissue that pads and protects surfaces of the abdomen.

Mesentery proper: Thick mesenterial sheet that provides stability and permits some independent movement while suspending all but the first 25 cm of small intestine.

Mesocolon: Mesentery associated with parts of large intestine; fuses to posterior body wall locking regions in place during development.

Major layers of digestive tract:
- Mucosa: Inner lining consisting of epithelium, lamina propria (areolar tissue), and muscularis mucosae.
- Submucosa: Layer of dense irregular connective tissue binding mucosa to muscular layer, containing blood vessels and lymphatics, possibly exocrine glands.
- Muscular layer: Dominated by smooth muscle cells, involved in mechanical digestion and moving materials along digestive tract.
- Serosa: Serous membrane covering muscular layer along most portions of the digestive tract enclosed by peritoneal cavity.
Mucosal epithelium varies by region:
- Stratified squamous epithelium in oral cavity, pharynx, esophagus, and anal canal.
- Simple columnar epithelium in stomach, small intestine, and most of the large intestine.

Digestive enzymes: Hydrolysis of large organic molecules (carbohydrates, proteins, lipids, nucleic acids).
Hormonal mechanisms: Enteroendocrine cells in digestive tract produce many peptide hormones that affect almost every aspect of digestion.
Regulation of digestive functions involves local factors (pH, volume, chemical composition), neural mechanisms (local reflexes and higher CNS control), and hormonal mechanisms.

Digestive system's role: Breaks down physical structure of food, disassembling component molecules which are absorbed into blood and utilized for energy or biosynthesis.
Key nutrients include carbohydrates, fats, proteins, vitamins, and minerals.

Age-related changes can include:
- Decline in the division rate of epithelial stem cells.
- Decrease in smooth muscle tone.
- Increase in cancer rates and dehydration prevalence.
- Changes in other body systems affecting digestive functioning.

Digestive system interconnections: Extensively connected to the nervous, cardiovascular, endocrine, and lymphatic systems; functions as an endocrine organ producing hormones and neurotransmitters.