23.2

23.2 Digestive System Processes and Regulation

Learning Objectives

• By the end of this section, you will be able to:
    - Discuss six fundamental activities of the digestive system, giving an example of each.
    - Compare and contrast the neural and hormonal controls involved in digestion.

Overview of Digestion

• The digestive system employs mechanical and chemical activities to decompose food into absorbable substances during its journey through the digestive tract.

• Table 23.3 provides an overview of the basic functions of the digestive organs.

• Interactive Resource:
    - Visit OpenStax Food Digestion for an overview of digestion in various regions of the digestive tract.
    - Note the pathway of non-fat nutrients from the small intestine to their utilization in the body.

Functions of Digestive Organs (Table 23.3)

Mouth

• Major Functions:
    - Ingests food.
    - Chews and mixes food.
    - Begins chemical breakdown of carbohydrates.
    - Moves food into the pharynx.
    - Begins breakdown of lipids via lingual lipase.
    - Moistens and dissolves food for taste.
    - Cleans and lubricates the teeth and oral cavity.
    - Exhibits some antimicrobial activity.

Pharynx

• Major Functions:
    - Propels food from the oral cavity to the esophagus.
    - Lubricates food and passageways.

Esophagus

• Major Functions:
    - Propels food to the stomach.
    - Lubricates food and passageways.

Stomach

• Major Functions:
    - Mixes and churns food with gastric juices to form chyme.
    - Begins chemical breakdown of proteins.
    - Releases food into the duodenum as chyme.
    - Absorbs some fat-soluble substances (e.g., alcohol, aspirin).
    - Possesses antimicrobial functions.
    - Stimulates protein-digesting enzymes.
    - Secretes intrinsic factor required for vitamin B12 absorption in the small intestine.

Small Intestine

• Major Functions:
    - Mixes chyme with digestive juices.
    - Propels food slowly for digestion and absorption.
    - Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, as well as vitamins, minerals, and water.
    - Performs physical digestion via segmentation.
    - Provides an optimal medium for enzymatic activity.

Accessory Organs

• Liver:
    - Produces bile salts for emulsification of lipids, aiding digestion and absorption.

• Gallbladder:
    - Stores, concentrates, and releases bile.

• Pancreas:
    - Produces digestive enzymes and bicarbonate.
    - Bicarbonate-rich pancreatic juices help neutralize acidic chyme, providing an optimal environment for enzymatic activity.

Large Intestine

• Major Functions:
    - Further breaks down food residues.
    - Absorbs residual water, electrolytes, and vitamins produced by enteric bacteria.
    - Propels feces toward the rectum.
    - Eliminates feces.
    - Concentrates food residue and temporarily stores it prior to defecation.
    - Mucus eases the passage of feces through the colon.

Digestive Processes

• The digestion process includes six main activities:
    1. Ingestion:
        - Entry of food into the alimentary canal through the mouth.
        - Involves chewing and mixing food with saliva, which initiates carbohydrate breakdown and some lipid digestion.
    2. Propulsion:
        - Movement of food through the digestive tract; includes voluntary swallowing and involuntary peristalsis.
        - Peristalsis involves waves of contraction and relaxation, propelling food along the tract and mixing it with digestive juices.
    3. Mechanical Digestion:
        - Purely physical process that makes food smaller without changing its chemical nature.
        - Includes chewing, churning in the stomach, and segmentation in the small intestine.
        - Segmentation isolates small sections of the intestine, mixing food throughout the intestinal lumen and aiding absorption.
    4. Chemical Digestion:
        - Involves digestive secretions that break down complex molecules into their building blocks (e.g., proteins into amino acids).
        - Food must be entered into the bloodstream for nutrient use, completing primarily in the small intestine.
    5. Absorption:
        - Primarily occurs in the small intestine where nutrients are absorbed into the bloodstream via epithelial cells.
        - Lipids are absorbed into lacteals, transported via lymphatic vessels to the bloodstream.
    6. Defecation:
        - The final step where undigested materials exit the body as feces.

Aging and the Digestive System

• Age-related changes in the digestive system start in the mouth and affect nearly all aspects of digestive function.

• Changes Include:
    - Reduced taste bud sensitivity decreases food appeal.
    - Dental issues, gum disease, and decreased saliva production complicate swallowing.
    - Food moves more slowly through the alimentary canal due to decreased muscular strength and tone.
    - Pathologies affecting digestive organs (e.g., hiatal hernia, gastritis, peptic ulcer disease) occur more frequently with age, alongside symptoms like maldigestion and constipation.

Regulatory Mechanisms

Neural Controls

• Sensor Types in Alimentary Canal:
    - Mechanoreceptors, chemoreceptors, and osmoreceptors detect various stimuli (mechanical, chemical, osmotic).

• Functions include:
    - Sensing stomach distension and liquid presence, signaling digestive processes.
    - Sending messages to activate glands for digestive juices or stimulate muscle contractions.

• Nerve Plexuses:
    - Embedded throughout the alimentary canal, interact with the central nervous system.
    - Extrinsic nerve plexuses manage long reflexes involving the central and autonomic nervous systems.
    - Short reflexes are regulated by internal plexuses within the canal, coordinating local peristaltic movements and secretions.
    - Example: Sight, smell, or taste of food triggers long reflexes through sensory neurons to the medulla, stimulating digestive secretions in anticipation of incoming food.

Hormonal Controls

• Various hormones are crucial for the digestive process.

• Key Hormones:
    - Gastrin:
      - Secreted in response to food presence, stimulates gastric acid secretion by parietal cells.
    - Duodenum Hormones:
      - Secretin:
        - Stimulates bicarbonate secretion by the pancreas.
      - Cholecystokinin (CCK):
        - Stimulates secretion of pancreatic enzymes and bile from the liver; triggers gallbladder release of bile.
      - Gastric Inhibitory Peptide (GIP):
        - Inhibits gastric secretion, slowing gastric emptying and motility.

• Source of Hormones:
    - Produced by specialized epithelial cells (endocrinocytes) in mucosal epithelium of the stomach and small intestine; enter bloodstream to reach target organs.