Mechanical digestion aids chemical digestion by breaking food into smaller pieces with a greater surface area, as well as introducing enzymes to begin the process of chemical digestion. In humans, much of mechanical digestion takes place in the mouth through chewing, during which the teeth break up food items.
Chewing not only breaks up food items, but also mixes them with saliva which contains both enzymes and mucus. The enzymes are primarily for breaking down carbohydrates. The mucus helps stick the chewed food particles together into a semi-solid mass known as a bolus, which is easier to swallow than the original food item would often have been. It travels down to the stomach, where further mechanical and chemical digestion occur.
The stomach mechanically digests the bolus through a process known as peristalsis, a contraction of the stomach walls that churns the bolus and mixes it with stomach acid and more enzymes, particularly pepsin, which breaks down proteins. This process breaks up the bolus into a substance known as chyme. Chyme is released through the bottom of the stomach, and the stomach acids in it are neutralized. The fluids in the top part of the intestines emulsify fat, which is another form of mechanical digestion. The intestines also contain further enzymes to break down carbohydrates and proteins.