Digestive System Functions and Structure

Study Notes

Alimentary Canal: A muscular tube extending from the mouth to the anus, measuring 5 meters in living individuals and 9 meters outside the body.
- The lumen of the alimentary canal is continuous with the external environment.
Digestive Glands:
- Salivary glands, liver, gall bladder, exocrine pancreas
General Functions of the Gut:
- Food Digestion: Breakdown of complex molecules into simpler ones (e.g., carbohydrates into monosaccharides, proteins into amino acids, fats into fatty acids) for absorption.
  - Requires secretion of digestive juices and wall motility.
- Food Absorption: The process of taking digested nutrients into the bloodstream.
- Excretion of Waste Products: Elimination of undigested and unabsorbed food.
- Immune Function: Defense mechanisms involving saliva, gastric HCl, rapid cell turnover, and lymphoid follicles.

Secretion: Release of digestive juices containing water, electrolytes, and organic constituents (enzymes and mucus) by glands along the gut wall.
- Digestive secretions are reabsorbed back into the blood after participating in digestion.
Motility: Movement of materials through the digestive tract.
- Smooth muscles: Most of the GIT (Gastrointestinal Tract) is composed of smooth muscle, except for the upper third of the esophagus (skeletal muscle), middle third of the esophagus (skeletal and smooth), and external anal sphincter (skeletal muscle).
- Functional Syncytium: Smooth muscle cells of the gut function as a syncytium, allowing action potentials to travel in all directions.
  - This creates a coordinated muscle contraction, controlling peristalsis and segmentation.
Digestion: The breakdown of complex food molecules into simpler ones.
Absorption: The uptake of digested nutrients into the bloodstream.

Tonic Contractions: Sustained mild contraction of smooth muscles in the GIT wall, maintaining constant intraluminal pressure.
- Caused by spike potentials or calcium ion entry.
Propulsive (Peristaltic) Movement: Responsible for moving food contents through the esophagus and intestines.
- Stimulus: Stretch or irritation of the mucosa.
- Response: Contraction behind (by Acetylcholine) and relaxation in front (by Vasoactive Intestinal Peptide [VIP] and Nitric Oxide [NO]) of the stretched segment.
- Control: Auerbach's plexus of the Enteric Nervous System (ENS), influenced by autonomic nerves.
Mixing (Segmenting) Movement: Facilitates food digestion and absorption.
- In the stomach, peristaltic contractions mix food.
- Segmentation contractions: In the small intestines, constrictions divide the gut into equal segments, promoting food mixing.
- Haustrations: In the large intestines, mixing movements are referred to as haustrations.