The digestive system consists of one long, tubular structure responsible for breaking large pieces of food down into their smallest microscopic components. The body can only use nutrients from food when they're small enough to be absorbed into tiny capillaries. If any part of the digestive system malfunctions, you may suffer from malnutrition no matter how much food you eat.
Digestion refers to the process that breaks foods down into their constituent carbohydrates, proteins and fats, and then further deconstructs each nutrient into its component molecules. Muscles lining the walls of the digestive system contract to mix food with enzymes while pushing it through the digestive system. Cells and organs produce enzymes that break down the carbohydrates, proteins and fats. Finally, special structures in the intestine absorb nutrients. While movement, chemical breakdown and absorption are digestive functions, the steps of digestion are defined by their unique roles in the mouth, stomach, small intestine and large intestine.
Digestion begins the minute you put food in your mouth. Saliva softens food so it’s easier for the teeth to crush and grind. Taking the time to properly chew food is important because smaller pieces of food allow the stomach to work more efficiently. Saliva also contains salivary amylase, which is an enzyme that starts breaking down large carbohydrates. Muscles in the tongue help mix food with saliva and push it into the esophagus, where muscles in the wall of the esophagus contract to move food into the stomach. These muscular contractions – called peristalsis – continue to occur at every step through digestion.
Muscles in the stomach wall regularly contract after food enters the stomach. In addition to moving stomach through the digestive tract, stomach muscles work to combine pieces of food with gastric juices to create a mixture called chyme. The gastric juices consist of hydrochloric acid and pepsinogen. Hydrochloric acid dissolves food, kills germs and converts pepsinogen into pepsin. Pepsin begins the process of digesting, or breaking down, proteins.
When chyme leaves the stomach and enters the small intestine, carbohydrates and proteins are partly digested and the breakdown of fats begins. The pancreas secretes sodium bicarbonate to neutralize the acidic chyme and enzymes that start to digest fats while continuing the breakdown of carbohydrates and proteins. The most important job of the small intestine is absorption of nutrients. Small projections called villi line the walls of the small intestine and increase the surface area available to absorb nutrients. They also contain enzymes that ensure proteins are broken down into amino acids and that carbohydrates are in their smallest form called monosaccharides. Then capillaries in the villi absorb amino acids, monosaccharides and digested lipids into the bloodstream.
The large intestine absorbs water, sodium and chloride, then forms, stores and eliminates feces. The large intestine contains healthy bacteria that complete the digestion process of dietary fiber and produce vitamin K. Probiotic products contain live microorganisms similar to those in the large intestine that may assist the digestive process. You'll find probiotics in dairy products such as yogurt and buttermilk, fermented vegetables like sauerkraut and through supplements. Some beverages, cereals and granola or energy bars are enriched with probiotics, but check the label to be sure they contain live organisms or list Lactobacillus or Bifidobacterium in the ingredients.
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