Summary
Highlights
Digestion breaks down large food molecules into simpler ones for absorption. It comprises two main components: chemical digestion, involving enzymes, and mechanical digestion, which is the physical breaking down of food. Both are essential for the digestive process.
Mechanical digestion begins in the mouth with water and mucus from saliva softening food. Saliva production is stimulated by the parasympathetic nervous system, with about 1 to 1.5 liters produced daily to keep the mouth and esophagus moist. Mastication, or chewing, involves the teeth, tongue, and cheek muscles to physically break down food and form a bolus, which is then swallowed in a process called deglutition.
The stomach performs significant mechanical digestion through mixing waves, which churn food into chime. Rippling peristaltic movements occur every 15-25 seconds, mixing food with gastric secretions. Hydrochloric acid, produced by parietal cells, unfolds proteins, increasing their surface area for digestive enzymes.
Bile, produced in the liver and stored in the gallbladder, emulsifies fats in the duodenum. This process breaks down large fat globules into smaller ones, greatly increasing the surface area for lipase enzymes to act upon. The liver produces 800-1000 ml of bile daily.
Segmentation in the small intestine involves localized contractions of circular muscles, which slosh food back and forth. This process mixes the chime thoroughly with digestive enzymes and brings it into contact with the intestinal walls for absorption, rather than propelling it forward.
After segmentation, migrating motility complexes, which are sequential waves of muscular contraction, propel the remaining material through the small intestine towards the colon. This process takes about 90 to 120 minutes to move food from the stomach to the terminal ileum, with chime remaining in the intestine for 3 to 5 hours.
Intestinal juice, produced by the lining of the small intestine (mostly in the duodenum and jejunum), adds 1 to 2 liters of fluid daily. This clear, yellowish fluid contains water and mucus, making the contents fluid for mixing with digestive enzymes and facilitating absorption. It's alkaline, with a pH of about 7.6, and helps dilute the chyme and pancreatic juice.