16.18 System: weak chemical fission systems  
     
  The digestive system breaks down food into simple substances that the cells can use. It then absorbs these substances into the bloodstream and eliminates any leftover waste matter. The main part of the digestive system is a long tube called the alimentary canal. This tube consists of (1) the mouth, esophagus, and stomach; and (2) the small intestine and large intestine. Other parts of the digestive system include the gall bladder, liver, pancreas, salivary glands, and teeth.  
 
 
  The mouth, esophagus, and stomach. Digestion begins in the mouth, where the teeth tear and grind food into small pieces. Small pieces of food are more easily broken down during the digestive process than large ones. Therefore, thorough chewing is important. As food is chewed, three pairs of large salivary glands pour saliva into the mouth. Saliva moistens the food, making it easier to swallow. Saliva also contains the first of the system's several digestive enzymes. The digestive enzymes break food down into chemicals the body can use.  
  After the food is swallowed, it enters the esophagus The esophagus is a long, muscular tube that leads to the stomach. Contractions of smooth muscles move the food down the esophagus and into the stomach. The stomach is the widest part of the alimentary canal. It serves as a sort of "holding tank" in which food remains for several hours. During this time, the stomach produces an acid and an enzyme that further break down much of the food. Muscle contractions mix the partly digested food into a thick liquid called chyme.  
  The small intestine and large intestine. Chyme passes from the stomach into the small intestine at a steady rate. Various digestive enzymes complete the breakdown of the food within the first section of the small intestine. The small intestine produces some of these enzymes. The rest are made by the pancreas. The pancreatic enzymes empty into the small intestine through a duct (tube). Bile, a liquid made by the liver and stored in the gall bladder, also enters the small intestine through a duct. Bile does not contain digestive enzymes, but it aids digestion by breaking up large molecules of fatty foods.  
  By the time the food leaves the first section of the small intestine, it has been completely digested. Special cells line the walls of the remainder of the small intestine. These cells absorb useful substances from the digested food. The absorbed substances enter the blood. Some of the substances are carried directly to cells throughout the body. The rest are transported to the liver. The liver stores some of the substances, releasing them as the body requires. It chemically alters the other substances, changing them into forms needed by the body.  
  The substances not absorbed by the small intestine pass to the large intestine. These substances consist of water, minerals, and wastes. The large intestine absorbs most of the water and minerals, which then enter the bloodstream. The wastes move down toward the rectum, the end of the large intestine, and leave the body as faeces.  
     
     
 

 

  
 
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