Hi, I'm Jim Shields for About.com, and today I'm going to teach you about the digestive system. Our cells need energy to function, and they make energy through a reaction between oxygen and nutrients, such as carbohydrates, proteins and fats. The respiratory system gets the oxygen to the cells, while it's the job of the digestive system to provide the nutrients. So how does the digestive system take large pieces of food, like a slice of pizza or an apple, and make it small enough for the nutrients to be extracted for delivery to the tiny cells? Much of the work of the digestive system is to take large pieces of food and break it down. That starts in the mouth, where we place food to be roughly smashed and chopped by chewing. The tongue helps this process by moving and turning the food while we chew. But the digestive process begins even before we start eating. Ever feel your mouth "watering" over a delicious smell of fresh baked cookies? That "water" is saliva, which is excreted into your mouth by your salivary glands in anticipation that digestion is set to begin. The saliva helps take the food your teeth have ground up and make it softer for its trip to the rest of the digestive system. The soft, chewed food now moves from the mouth through the esophagus and into the stomach. In the stomach, the food waits to be broken down by gastric juices excreted by the stomach walls. The churning stomach makes the food even smaller, to fit into the small intestine. Three organs work with the small intestine to deliver nutrients from the food: the pancreas, the liver and the gall bladder. The pancreas helps digest proteins and fats. The liver sends liquid called bile to the small intestine to help absorb the fats into the bloodstream, and the gall bladder stores the bile until the liver needs it. Furthermore, the liver filters harmful toxins from the blood and regulates the delivery of vitamins to the bloodstream. The food continues on its journey, entering the large intestine. The large intestine includes the appendix, which is an essentially useless organ, and the colon, which helps remove the last bit of water and nutrients from the food mixture. As it does this, what is left is waste products with the water removed, known as feces, which is stored in the rectum, the last stop along the digestive system. The waste that the body can't use is squeezed out of the body through the anus. Thanks for watching. To learn more about biology, visit us on the web at About.com.