Digestion and Absorption



Nutrients are ingested in the form of large macromolecules, or food. To be useful to the body, these large molecules need to be broken down by a series of mechanical and chemical digestive processes. Mechanical digestion occurs via chewing in the mouth, peristalsis (wave-like contractions that move food along) in the esophagus and stomach, and segmentation (muscle contractions to mix food with secretions) in the intestines. Outside the alimentary canal, which is a tube extending from the mouth to the anus, are accessory organs of the digestive system. These include the salivary glands, pancreas, liver, and gallbladder. These organs secrete enzymes and acids that contribute to chemical digestion. Bacteria in the large intestine also help break down starches and fiber that the human body cannot digest, and produce vitamins that are critical for human health. The digestive process ends with the absorption of nutrients and elimination of waste.

At A Glance

  • Mechanical digestion breaks down food into smaller particles, and chemical digestion breaks food down even further using enzymes and acids.
  • The accessory organs are not part of the digestive system but help in digestion. The salivary glands, pancreas, liver, and gallbladder produce different secretions that contain digestive enzymes
  • After chemical digestion is complete, virtually all nutrients such as vitamins, water, and carbohydrates from food are absorbed into the bloodstream in the small intestine. Nutrients that are unable to cross the membrane of the intestinal lumen of the small intestine and enter the bloodstream are not absorbed.
  • Bacteria living in the large intestine facilitate chemical digestion and absorption and synthesize certain vitamins that are critical for human health.