The main function of the digestive system is to break down organic nutrients so that they can be absorbed into the body. These organic nutrients can be carbohydrates, proteins, fats, water, vitamins, and minerals. We will discuss how the digestive system performs this function as it applies to a relatively common (but not necessarily nutritious) meal a hamburger, fries, and a soda. The meat within the hamburger (assuming it is lean hamburger) is made up of mostly protein with a little fat (lipid). The hamburger bun and fries are made up of carbohydrates (fries also contain fat). The soda will consist mostly of water, carbohydrates, and some other dissolved substances. The basic processes of the digestive system include the following: secretion involves the release of digestive fluids into the lumen of the digestive tract. Digestion of your meal begins in the mouth, where it is broken down by chewing (or mastication) and is mixed with saliva to form a bolus.