Overview of Normal Digestion

When dealing with a condition like IBS, which affects the digestive system, it can be helpful to understand how the digestion process normally works.

What are the organs involved in the digestive system?

There are several organs in the body that are involved in the digestive system. First, the digestive tract or gastrointestinal (GI) tract is made up of:

  • Mouth
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anus1

The organs in the GI tract are connected in a long, bendy tube, and each of the organs in the GI tract are hollow. Other organs involved in digestion that are solid and not part of the GI tract include:

  • Pancreas, which produces enzymes that break down proteins, carbohydrates, and fats
  • Liver, which produces bile that enables the body to digest fats
  • Gallbladder, which stores the bile and delivers it to the small intestine through bile ducts1,2

How does the digestive process occur?

The digestive process begins in the mouth. Before food is even placed into the mouth, the salivary glands may begin producing saliva (referred to as the "mouth watering" at the sight or smell of food). The teeth and chewing motion break the food into smaller pieces, and the tongue pushes the food around the mouth, mixing it with saliva to become moist and easier to swallow.1,2

With a swallow, the mass of food (now called a bolus) moves from the back of the mouth down into the esophagus. The muscles in the esophagus move the bolus down to the stomach with peristalsis, a rhythmic, automatic movement that squeezes the tube and moves the bolus forward. When the bolus reaches the end of the esophagus, the pressure of the bolus causes the muscles there, the esophageal sphincter, to relax and allow the bolus to enter the stomach.1,2

As food enters the stomach, the muscles of the stomach walls contract to mix the food with digestive juices, stomach acid and other enzymes, which further break down the food. The mixture of digestive juices and food is called chyme, and the stomach slowly releases the well-mixed chyme into the small intestine.1,2

In the small intestine, the chime is mixed with digestive juices from the pancreas, liver, gall bladder, and intestines. The muscles in the small intestine work through peristalsis to mix the food and digestive juices and continue to move the food through the GI tract. The digestive juices further break the food down into nutrients that can be absorbed into the body. The small intestine can be divided into three parts: the duodenum, the jejunum, and the ileum. Each section has its own responsibilities:

  • The duodenum continues the digestion of food particles started by the stomach and is responsible for absorbing iron, calcium, and magnesium.
  • The jejunum is where most of the absorption of nutrients occurs, including carbohydrates, fats, proteins, and vitamins.
  • The ileum, the last section, may absorb nutrients that aren't previously absorbed by the jejunum and is responsible for absorbing bile acids and vitamin B12.1,2

The remaining food and liquid pass through to the large intestine, or colon. The large intestine absorbs water and changes the waste products into stool. The stool is moved along through the large intestine by peristalsis. At the end of the large intestine is the rectum, which stores the stool until it is pushed out of the anus during a bowel movement.1

How does the body use the nutrients from the digestive process?

As the nutrients are absorbed through the lining of the small intestine into the bloodstream, the circulatory system delivers the nutrients to different parts of the body for use or storage. Simple sugars, amino acids, glycerol, some vitamins, and some salts are delivered to the liver, where they are stored until needed. Fatty acids and vitamins are delivered to the lymph system, which is a part of the immune system and protects the body against potential infection and cancer. Many nutrients are used by the body for energy, growth, and cell repair.1

How is the digestive process controlled?

The digestive process is controlled by hormones and the nervous system. The hormones involved in digestion are secreted by the stomach, pancreas, and small intestine, and they stimulate the production of digestive juices, as well as signaling the brain about hunger. Signals travel along the nervous system back and forth from the GI tract to the brain, as well as along the GI tract. These signals cause the salivary glands to produce saliva and manage the speed at which food is moved through the GI tract.1

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