Human Physiology Chapter on Digestion

Digestion, Absorption, and Metabolism

• This chapter covers the processes of digestion, absorption, and metabolism.
• Digestion breaks down food into small components for absorption.
• Absorption takes these components into the body's interior.
• The gastrointestinal (GI) tract is the hollow tube, including mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus, involved in digestion and absorption.
• A 24-72 hour transit time for food varies based on the type of food (protein, fat, carbohydrates).

Digestion and Absorption

• The process of breaking down food into smaller components for absorption by cells.
• Diagrams show atoms, molecules, cells, and tissues.
• Body systems (Organism, Organ systems, Organs) and their relation to digestion.

Key Terms

• Digestion: Breaking down food into absorbable components.
• Absorption: Taking substances into the interior of the body.
• GI tract: Tube in the body involved in digestion and absorption of food.
• Transit time: Time food takes to pass through the GI tract.

Structure of the GI Tract

• Lumen: Inner portion of the GI tract; food hasn't yet been absorbed.
• Mucosa: Tissue layer lining the lumen; mucosal cells protect and absorb. They have a short lifespan, needing nutrients and produce mucus.
• Connective Tissue: Provides structural support and delivers nutrients to mucosa, aids in nutrient secretion and muscle contractions.
• Layers of smooth muscle: Helps with the movement of food throughout GI tract.
• External layer of connective tissue: Provides strong support to GI Tract.

Overview of the Digestive System

• Mouth: Chews food, mixes it with saliva.
• Salivary glands: Produce saliva with a starch-digesting enzyme.
• Pharynx: Swallowing.
• Esophagus: Moves food to the stomach.
• Stomach: Churns and mixes food, produces acid, and protein-digesting enzymes.
• Liver: Makes bile for fat digestion/absorption.
• Pancreas: Neutralizes intestinal contents, produces enzymes to digest carbs, proteins, and fats.
• Gallbladder: Stores and releases bile.
• Small intestine: Completes digestion and absorbs nutrients.
• Large intestine: Absorbs water and minerals, houses bacteria; passes waste.
• Anus: Opening for waste removal.

Digestive Secretions

• Digestive secretions assist digestion in the GI tract.
• Mucus: Viscous material that protects, lubricates, and moistens the GI tract.
• Enzymes: Protein molecules that speed up chemical reactions; enzymes are not changed during reactions.

Digestive Enzymes

• Table lists specific enzymes, locations, and functions.
• Examples include amylase, pepsin, trypsin, chymotrypsin, etc.

Digestive Hormones

• Hormones regulate digestive processes.
• Examples include gastrin, somatostatin, secretin, and cholecystokinin (CCK).

The GI Tract and Barrier Function

• The GI tract's barrier function limits absorption of harmful substances and pathogens.
• Mucosal cells and immune cells protect and respond to harmful organisms.
• A foreign substance entering the body can stimulate an immune response. Immune cells (phagocytes, lymphocytes, antibodies) participate in this process.

Absorption mechanisms

• Simple diffusion: Movement from higher concentration to lower concentration (e.g., vitamins, fatty acids).
• Osmosis: Movement of water to balance dissolved substances (e.g., water absorption).
• Facilitated diffusion: Movement of substances with the aid of carrier proteins to move from higher to lower concentration (e.g., fructose).
• Active transport: Uses energy to move a substance against a concentration gradient (e.g., various nutrients).

Small Intestine

• The small intestine's inner surface is lined with villi and microvilli, increasing the surface area for nutrient absorption.
• Villi and microvilli increase the efficiency of nutrient absorption into the blood stream from digested food.
• Segments of the small intestine are regulated by segmentation.

Digestion of Large Intestine

• Colon and rectum make up large intestine
• Large intestine absorbs water and minerals, holds/passes waste
• Undigested material is excreted

Stomach

• Stomach walls are thick and have the gastric glands in them. The stomach holds partially digested food.
• Very little absorption happens in the stomach.
• Stomach lining secretes mucus and gastric juice.
• Food entering the stomach triggers a hormone to release gastric juice from glands.

Digestion and Absorption- Swallowing

• Bolus (food mass) forces epiglottis to close off airways (larynx and trachea) to prevent food from going to lungs.
• After the bolus has passed, the epiglottis moves back to allow air movement.

Peristalsis

• Rhythmic contractions propel food through the GI tract.
• Circular muscles contract to push the bolus down the esophagus.
• Acidic contents leaking out of the esophagus are a sign of potential problems.
• Peristalsis is impacted by various factors and can be disruptive.

Stomach Emptying

• Chyme leaves the stomach depending on large versus small meals, solid versus liquid meals, and composition of the food.
• This rate of stomach emptying is crucial to digestion.

Peptic Ulcer

• A sore in the stomach lining.
• Related to HCl, pepsinogen, and digestive activity in the stomach.