Overview of the Digestive System

Exam Tip: Remember that the small intestine is considered the "workhorse" of the digestive system. This is where the majority of both digestion and nutrient absorption occurs.

The Process of Digestion

Digestion is not a single event but a series of functional stages that move food through the body.

1. Ingestion: The process of taking food into the mouth.
2. Mechanical Digestion: The physical breakdown of food into smaller pieces (e.g., chewing in the mouth).
3. Chemical Digestion: The use of enzymes and secretions to break down food at a molecular level.
4. Propulsion: The movement of food through the digestive tract.
5. Absorption: The process where released nutrients are taken up into the blood or lymph.
6. Defecation: The elimination of indigestible substances from the body.

Histology of the Alimentary Canal

The walls of the alimentary canal are not uniform; they are composed of four distinct tissue layers that allow for specialized functions like secretion and movement.

• Mucosa: The innermost layer, consisting of a mucous membrane (epithelium), a layer of loose connective tissue called the lamina propria, and a thin muscle layer.
• Submucosa: A layer of dense connective tissue.
• Muscularis: Composed of two muscle layers responsible for movement.
• Serosa: The outermost layer, which is the visceral peritoneum found around abdominal organs.

Nervous System Regulation

The digestive system is regulated by two interacting nervous systems that control everything from secretions to the movement of food.

The Enteric Nervous System (ENS)

The ENS provides "intrinsic" innervation, meaning it is built directly into the walls of the alimentary canal, running from the esophagus to the anus. It contains approximately 100 million neurons and is organized into two main plexuses:

• Submucosal plexus: Primarily controls digestive secretions.
• Myenteric plexus: Primarily controls gastrointestinal motility (movement).

The Autonomic Nervous System (ANS)

The ANS provides "extrinsic" innervation, allowing the brain to influence digestive activity:

• Parasympathetic impulses: Stimulate the enteric nervous system to increase digestive activities.
• Sympathetic impulses: Inhibit mixing and propelling movements.

The Peritoneum

The organs within the abdominal cavity are not just floating loosely; they are held in place by the peritoneum. The peritoneum is a broad, serous membranous sac made of squamous epithelial tissue surrounded by connective tissue.

It consists of two main regions:

1. Parietal peritoneum: The layer that lines the abdominal wall.
2. Visceral peritoneum: The layer that envelopes the actual abdominal organs.

The space between these layers is known as the peritoneal cavity.