In the digestive system, organs break down foods, absorb nutrients, and excrete solid wastes. The digestive tract forms a single tube, divided into structural and functional compartments. It includes the mouth, pharynx, esophagus, stomach, small and large intestines, and rectum. Accessory organs produce materials that enter the digestive tract to aid in digestion. The accessory organs are the salivary glands, liver, gallbladder, and pancreas.
Digestion is the breakdown of macronutrients to simple nutrients that can be absorbed. It is divided into two phases that go on together in each compartment of the digestive tract. Mechanical digestion is the physical breakdown of foods. This includes chewing (mastication) and mixing in the stomach and small intestine. Chemical digestion is the breakdown of macronutrients by enzymes. Most of this breakdown occurs by enzymatic addition of water across chemical bonds (hydrolysis).
Once fully digested, nutrients can be absorbed. Absorption is the uptake of simple nutrients into the capillaries and lymphatics (lacteals) of intestinal villi.
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