Short answers, pulled from the story.
Digestion is the breakdown of large insoluble food compounds into small water-soluble components so they can be absorbed into the blood plasma. It is a form of catabolism, often divided into mechanical digestion, the physical breakdown of food, and chemical digestion, in which enzymes break food into usable compounds.
Human digestion normally takes between 24 and 72 hours. The process varies between individuals and depends on factors such as the characteristics of the food and the size of the meal. The human gastrointestinal tract is around 9 meters long.
About 95% of nutrient absorption occurs in the small intestine. Its walls are lined with villi, and the epithelial cells carry numerous microvilli that increase surface area. Water and minerals are reabsorbed into the blood in the colon, the large intestine.
External digestion secretes enzymes into the environment surrounding the organism, where they break down organic material, and some products diffuse back; most fungi and nearly all spiders rely on it. Internal digestion occurs inside a tube, the gastrointestinal tract, which is more efficient because more broken-down products can be captured and the chemical environment can be controlled.
Lactose intolerance occurs because roughly 65 percent of the adult population produce only small amounts of lactase, the enzyme that breaks lactose into glucose and galactose. It varies widely by genetic heritage: more than 90 percent of people of east Asian descent are lactose intolerant, compared with about 5 percent of people of northern European descent.
The stomach secretes mucus and bicarbonates that form a slimy layer shielding the stomach wall against chemicals such as concentrated hydrochloric acid. Gastric juice contains hydrochloric acid and pepsin, which could otherwise damage the stomach lining, and the acid provides the acidic pH that pepsin needs.
At least five hormones aid and regulate the digestive system in mammals. Gastrin stimulates secretion of pepsinogen and hydrochloric acid, secretin signals sodium bicarbonate release in the pancreas, cholecystokinin triggers digestive enzymes and bile in response to fat, gastric inhibitory peptide slows stomach emptying and induces insulin secretion, and motilin stimulates the production of pepsin.