Organ Systems — Definition & Overview

When you eat your lunch, the food enters your mouth first. Glands in your mouth secrete saliva which begins digestion. The food travels through the pharynx to the esophagus, and then to the stomach. Here the chemical and mechanical digestion continues. The resulting mixture enters the small intestine where most digestion takes place, and then move to the large intestine. After this the waste product can be eliminated from the body.

The paragraph above describes the function of which organization level?