How Does Your Digestive System Break Down Food? - dummies

How Does Your Digestive System Break Down Food?

By David Terfera, Shereen Jegtvig

The function of the digestive system is to take food into your body, break it down into individual macronutrients (proteins, fats, and carbohydrates) and micronutrients (vitamins and minerals), and then absorb those small bits and pieces of nutrients so the rest of your body can use them as fuel and raw materials for building tissues and structures.

The digestive system includes a very long tube called the digestive tract and a couple additional organs that lend a little help along the way:

  1. Digestion begins in the mouth where the salivary glands make saliva (often before you start eating — just smelling a pumpkin pie can make your mouth water).


    Saliva is a fluid that contains mucin to lubricate foods, buffering agents that help to reduce acidity, antibacterial agents to kill germs, and a digestive enzyme called salivary amylase that begins the job of digesting complex carbohydrates. Who knew saliva contained so much stuff?

    The mouth also contains teeth, which are used to chew food into smaller bits that make the process of digestion easier later on, and a tongue, which helps move foods around in your mouth.

  2. After you chew food in your mouth, it travels through your esophagus, a simple tube that connects the pharynx (throat) with the stomach.

    Between the esophagus and the stomach is a sphincter (a ring-like muscle that surrounds an opening or passageway) called the lower esophageal sphincter that only opens when you swallow foods or liquids or when you need to belch. The rest of the time it stays shut so the delicate lining of the esophagus isn’t damaged by the strong acid found in the digestive juices of the stomach.

    Gastroesophageal reflux disease (GERD) occurs when stomach acid backs up into the esophagus often enough to cause damage to the lining.

    The stomach is a muscular bag located in the upper part of the abdomen, toward the left side. It squeezes and mixes food with digestive juices that are released from specialized cells in the wall of the stomach. Digestive juices include hydrochloric acid, which kills microorganisms and activates an enzyme called pepsin that helps break down proteins, and intrinsic factor, which is a substance that helps the small intestine absorb vitamin B12. Its lining contains cells that produce mucus to help protect the walls of the stomach from damage due to the acid.

  3. Your stomach slowly empties its contents into the small intestine, which is the longest portion of the digestive tract.

    It’s divided into three sections: the duodenum, the jejunum, and the ileum.

    Digestion finishes in the small intestine, and absorption of nutrients begins with a little assistance from a few other organs.

    • Pancreas: This organ is located behind the stomach. It produces digestive enzymes and releases them into the duodenum. These enzymes include lipase, which breaks down fats, pancreatic amylase, which breaks down carbohydrates, proteases, which break down proteins.

    • Gallbladder: This sac-like organ is located near the liver and stores bile, a substance that helps emulsify fats so the lipases have an easier time digesting them.

    • Liver: The liver is located on the right side of the abdomen. It makes the bile that helps emulsify fats, and it processes the nutrients that are absorbed in the small intestine.

  4. After the nutrients are digested, they’re absorbed into the cells of the small intestine. From there, they’re absorbed into the blood and carried away to be processed and metabolized or stored.

    The material that’s left after digestion and absorption in the small intestine travels into the large intestine, or colon. It travels up the right side of the abdomen, across the top, and down again on the left. From there it moves toward the midline of the body and terminates at the rectum with the anus that opens to the outside.

    The main function of the colon is to absorb water, leaving more solid material that’s passed through the rectum as stool.

The appendix is a tube-like pouch of lymphoid tissue that hangs from the large intestine in the right lower quadrant of the abdomen.