Human Nutrition - Alimentary canal

The alimentary canal is a specialized tube for the process of nutrition, including ingestion, digestion (mechanical and chemical), absorption, assimilation, and egestion.

Mechanical digestion is the physical breakdown of food into smaller pieces without changing its chemical nature, facilitated by teeth and the churning of the stomach ($stomach \ muscles$).

Chemical digestion involves the use of enzymes to break large, insoluble molecules into small, soluble molecules that can be absorbed into the bloodstream.

Peristalsis is the wave-like contraction of circular and longitudinal muscles that moves food (a bolus) through the esophagus and the rest of the canal.

The stomach secretes gastric juice containing $HCl$ (hydrochloric acid), which provides an acidic $pH$ of approximately $2.0$. This kills bacteria and provides the optimum $pH$ for the enzyme pepsin.

The small intestine consists of the duodenum and the ileum. It is the primary site for chemical digestion via pancreatic enzymes and the absorption of nutrients.

Villi are finger-like projections in the ileum that increase the surface area for absorption. Each villus contains a capillary network for $C_6H_{12}O_6$ (glucose) and amino acids, and a lacteal for absorbing fatty acids and glycerol.

Bile is produced in the liver and stored in the gallbladder. It neutralizes the acidic chyme from the stomach and emulsifies fats, increasing the surface area for lipase activity.

Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells (e.g., amino acids used for protein synthesis).