D1.2 - The Digestive System

Digestive System Overview

• The digestive system has two main divisions: accessory organs and digestive tract.
• Accessory organs aid in digestion by secreting enzymes; they don't handle food directly (salivary glands, liver, gallbladder, pancreas).
• Digestive tract structures move food through the body (mouth, esophagus, stomach, small & large intestine, rectum, anus).

Digestive System Structures

• Mouth: Digestion starts here, with teeth breaking down food physically and salivary enzymes (amylase) chemically breaking down starch.
• Esophagus: Transports food (bolus) to the stomach via peristalsis (wave-like muscle contractions).
• Stomach: Undergoes both physical (churning) and chemical (acid, enzymes) digestion. Produces chyme (processed food).
• Small Intestine: Completes digestion and absorbs nutrients. Has villi and microvilli to maximize absorption surface area.
• Large Intestine: Absorbs water and electrolytes; concentrates and eliminates waste.

Accessory Organs Detail

• Pancreas: Delivers enzymes (trypsin, chymotrypsin, amylase, lipase) to the duodenum that complete digestion of carbs, protein, and fats. Produces bicarbonate which neutralizes stomach acid.
• Liver: Produces bile which emulsifies fats, increasing the surface area for lipase enzymes to break them down more efficiently.
• Gallbladder: Stores and releases bile into the small intestine when needed.

Small Intestine Regions

• Duodenum: First part of the small intestine; site of most chemical digestion.
• Jejunum: Middle section; continues breaking down food.
• Ileum: Final section; absorbs nutrients and moves remaining undigested material to the large intestine.

Large Intestine

• The large intestine is shorter but wider than the small intestine, and it absorbs water from the digested food and eliminates wastes. Cellulose, a carbohydrate in plants, is undigested by the human body but adds bulk to the waste materials. Colon cancer correlates to poor diets.

Stomach Digestion

• Stomach has three methods of protection: 1. producing gastric juice only when food is present, 2. secreting mucus to prevent the stomach lining from damage and 3. producing a protein-digesting enzyme called pepsin. Pepsin is active in a highly acidic environment (pH 1-3).
• Gastric juice is made of water, mucus, salts, hydrochloric acid, and enzymes; it helps to soften and break down proteins, and kills bacteria.

Appendix

• The appendix, once believed to be vestigial (having no function), is now hypothesized to aid in digestion by storing "good" bacteria. In cases of bacterial infection, it is surgically removed. Inflammation of the appendix is called appendicitis.

Ingestion Summary

• Saliva contains amylase to start starch digestion.
• Teeth break down food into smaller particles and crush them.
• Peristalsis moves food through the esophagus.
• Sphincters control food movement in and out of the stomach.
• The stomach contains HCI, pepsinogens and mucous.
• Peristalsis mixes food in the stomach.

Protein Digestion

• Pepsin, a protein-digesting enzyme, starts to breakdown protein present in the food in the stomach. A thick mucus prevents the acid from damaging the stomach lining.
• The stomach doesn't usually digest its own proteins.