Digestion is simultaneously a complicated process and a relatively simple one. Chemically, it's quite simple -- it involves breaking the large nutrient compounds in your food down into small nutrient compounds that you can absorb. Logistically, however, this is quite complex. It involves chemicals called enzymes, in addition to hydrochloric acid and bile.
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The process of digestion has two parts: mechanical digestion and chemical digestion. Mechanical digestion begins in the mouth and continues in the stomach -- it involves the physical crushing and churning of large particles of food to produce a soupy mixture called chyme, explains Dr. Lauralee Sherwood in her book "Human Physiology." The purpose of mechanical digestion is to increase the surface area of food and make chemical digestion more efficient. Chemical digestion incorporates enzymes, acid and bile -- it breaks nutrient molecules down into small pieces.
Role of Acid
In theory, you wouldn't need anything but acid to digest your food. The nutrient molecules -- carbohydrates, proteins and fats -- all react with acid and decompose into their smaller constituents. Thus, the role of stomach acid is quite critical in digestion. Sherwood explains that the specific acid you use to digest your food -- hydrochloric acid -- comes from the parietal cells of the stomach. It's a very strong acid and makes the interior of the stomach strongly acidic.
Despite the fact that acid could theoretically digest your food all on its own, fat presents a logistical challenge with regard to digestion. This is because fat doesn't mix with water -- just as oil and water don't mix -- and the acid in your stomach is dissolved in water. As such, fat you eat simply floats on top of stomach acid and doesn't digest. To digest fat, you must pass it through to your intestine. There, bile from the gallbladder surrounds the fat and pulls it into the watery solution in the intestine, allowing digestive enzymes to go to work on it.
Though you could digest carbohydrate and protein -- both of which dissolve in water -- using acid alone, the process would be very slow. For this reason, your digestive system also relies upon enzymes, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Enzymes enhance the rate at which acid from the stomach reacts with the nutrient molecules in your food, ensuring that you're able to digest and absorb your food in a timely manner.