The Digestive System

Biological Requirement: All animals must obtain food to survive.
Alimentary Canal: Defined as a one-way tube through which food passes.
The Digestive System Components: Utilizes the mouth, esophagus, stomach, small intestine, and large intestine. It also includes accessory structures such as salivary glands, the pancreas, the liver, and others.
Two Primary Purposes: * Digestion: The process of breaking food into smaller molecules. * Absorption: The process of moving molecules into the bloodstream for delivery to and use by cells.

Step 1: Ingestion: The act of eating or taking food into the body.
Step 2: Digestion: The process of breaking down food into small enough molecules for the body to absorb. It occurs in two phases: * Mechanical Digestion: The physical act of breaking down food by non-chemical means (e.g., chewing). * Chemical Digestion: The chemical breakdown of food through acids and enzymes (enzymatic hydrolysis).
Step 3: Absorption: Nutrient molecules enter the body cells.
Step 4: Elimination: The process and removal of undigested material from the body.

Oral Cavity Components: Includes the teeth and tongue.
Mechanical Processing: Mechanical digestion occurs through the physical break-down of food using the teeth.
Moistening and Mixing: The mouth moistens food and mixes it with salivary secretions.
Salivary Glands: * Includes the Parotid gland, Sublingual gland, and Submandibular gland. * Function: Secretion of lubricating fluid containing enzymes. * Saliva: Contains salivary amylase, an enzyme that breaks carbohydrates into monosaccharides.
Tongue: Responsible for making and pushing the bolus (a ball of food) into the throat.
Supporting Structures: Hard palate, soft palate, and palatine tonsil.

Pharynx: Pharyngeal muscles propel materials into the esophagus.
Larynx: Protected during swallowing by the epiglottis.
Epiglottis: * Closes the trachea (the "air tube") to prevent food from entering the lungs. * Opens the esophagus (the "food tube") to allow the bolus to proceed to the stomach.
Anatomy: Includes the vocal folds and trachea situated near the esophagus.

Function: Transportation of materials from the pharynx to the stomach.
Peristalsis: Defined as the contraction of smooth muscle that pushes food down the esophagus.
Cardiac Sphincter: Also known as the lower esophageal sphincter, it closes the bottom of the esophagus to keep stomach contents from back-flowing.

Mechanical Digestion: Accomplished through churning food with muscular contractions.
Chyme: A mix of fluid and food produced in the stomach.
Chemical Digestion: Gastric glands release various chemicals for breakdown. * Pepsin: An enzyme specifically for the digestion of proteins. * Hydrochloric acid ( $HCl$ ): Released to help activate pepsin.
Anatomy of the Stomach: Includes the Fundus, the lower esophageal sphincter, and the Pyloric sphincter.
Pyloric Valve: Controls the release of chyme into the small intestine.

Liver: * Function: Secretion of bile, which is vital for lipid digestion. * Additional functions: Storage of nutrients and many other vital functions.
Gallbladder: * Function: Storage and concentration of bile. * Bile: Acts as a detergent-like substance that dissolves fats so that enzymes have access to them.
Pancreas: * Exocrine cells: Secrete buffers and digestive enzymes. * Endocrine cells: Secrete hormones that regulate blood sugar. * Digestive Substances: Releases juices into the beginning of the small intestine, including: 1. Sodium bicarbonate: Neutralizes stomach acid. 2. Digestive Enzymes: Including Lipase (breaks down lipids), Trypsin (breaks down proteins), and Amylase (breaks down carbohydrates).
Blood Sugar Regulation: * High Blood Glucose: Pancreas beta cells release Insulin; fat cells take in glucose from the blood. * Low Blood Glucose: Pancreas alpha cells release Glucagon; the liver releases glucose into the blood. * Normal blood glucose levels are maintained by the balance of insulin and glucagon.

General Function: Enzymatic digestion and the absorption of water, organic substrates, vitamins, and ions.
Duodenum (Upper Intestine): Primary site for chemical digestion using enzymes (Amylase, Trypsin, Lipase).
Jejunum and Ileum (Lower Intestine): Primary sites for absorption.
Surface Area Specializations: * Large circular folds: Internal folding increases surface area. * Villi: Larger projections covered by high-density absorptive cells. * Microvilli: Smaller projections on the surface of villi that further increase surface area ( $→$ more absorption).
Absorption Pathways: * Capillaries (Blood vessels) in villi: Absorb carbohydrates and proteins. * Lacteals (Lymph vessels) in villi: Absorb fats.
Internal Anatomy: Components include the lumen, goblet cells, muscularis mucosae, and duodenal glands.

Composition of Chyme: Consists of water, cellulose, and other indigestible substances.
Primary Function: Water absorption, dehydration, and compaction of indigestible materials in preparation for elimination.
Appendix: In some animals (usually herbivores), it uses bacteria to digest cellulose.
Bacterial Action: Bacteria grow on undigested material and aid in final digestion processes.
Structural Pathway (Flow of material): 1. Small intestine 2. Cecum 3. Ascending colon 4. Transverse colon 5. Descending colon 6. Sigmoid colon 7. Rectum (storage before elimination) 8. Anus