10.2 The Human Digestion System

The digestive system consists of the digestive tract and accessory organs.
Each component transforms food into energy and materials necessary for survival.

Initiation of Digestion: Begins in the mouth; the smell or sight of food activates salivary glands.
Salivary Glands: Three pairs secrete saliva, which contains the enzyme amylase.
- Amylase: Breaks down starches into simpler sugars.
- Saliva: Dissolves water-soluble food particles, stimulates taste buds, and lubricates food for swallowing.
Mechanical Digestion: Teeth bite, tear, and grind food, increasing surface area exposure to saliva.

As food is chewed, the tongue forms it into a bolus for swallowing.
Epiglottis: Closes off the trachea to prevent food from entering during swallowing.
Peristalsis: Wave-like muscular contractions transport the bolus through the esophagus.
Mucus Production: Keeps the esophagus moist and aids in swallowing.
Esophageal Sphincter: Ring of muscle that prevents stomach contents from backing up, relaxing to allow food into the stomach.

Storage and Digestion: Muscular organ where food is stored and further digested.
Gastric Glands: Secrete gastric juice when food is present.
- Composition: Hydrochloric acid, salts, enzymes, water, and mucus.
Function: Mucus protects the stomach lining from acid; pepsin digests proteins into smaller polypeptides.
Chyme Formation: Mixing of food and gastric juice creates a thick liquid.
Pyloric Sphincter: Regulates passage of chyme into the small intestine.

First Region: U-shaped, receives secretion from pancreas and gall bladder.
Surface Area: Lined with folds, villi, and microvilli for increased nutrient absorption.

Jejunum: About 2.5 m long, continues breaking down proteins and carbohydrates.
Ileum: About 3 m long, absorbs nutrients and shifts undigested materials to the large intestine.

Breakdown of macromolecules into absorbable units by specific enzymes.
Digestive Enzymes: Carbohydrases, lipases, proteases, nucleases for breaking down carbohydrates, fats, proteins, and nucleic acids respectively.

Temperature: Optimal around 37°C; excessive heat causes denaturation.
pH Levels: Each enzyme has an optimal pH range (e.g., pepsin works best in acidic conditions, while trypsin is functional in more neutral conditions).

Monosaccharides: Absorbed into the bloodstream, converted into glucose.
Amino Acids: Enter bloodstream, some become urea, others are used to create proteins.
Glycerol and Fatty Acids: Reassembled into triglycerides for transport into the bloodstream.

Length: About 1.5 m; function is to absorb water and reduce the volume of indigestible matter.
Houses billions of anaerobic bacteria that further process undigested food, producing vitamins.
Remaining material form feces, which are expelled from the rectum.