A freshly baked apple pie has just been baked. Even before you see it, the scent of cake has filled your house. The sweet scent of bubbling Honeycrisp apples mixed with the tangy smell of Grannysmith draws you into the kitchen. The cake is too hot to touch, so you stand there looking at it. You watch as the sugary syrup from the apples drips down the sides of the cake. You can't take your eyes off the golden cake filled with apples. The cake seems to call you: “Come here. Come and taste me, just a bite.”Your mouth is watering yet? Even before you eat a bite, your digestive system kicks into action. Just the sight or smell, even a tempting description, of a delicious food is enough to make you start salivating and producing stomach acid. Your digestive system consists of the gastrointestinal (GI) tract, also called the digestive tract, and the liver, pancreas, and gallbladder. The gastrointestinal tract is a series of hollow organs joined together in a long tube from the mouth to the anus. The main purpose of the digestive system is to break down food into smaller substances that the body can use for energy. After eating the first bite of cake, chemical digestion begins in the mouth. The salivary glands here produce enough digestive juices (saliva) to begin chemically breaking down food. Saliva moistens food and helps you chew, taste and swallow. In addition to the salivary glands in the lining of the mouth, there are three pairs of larger salivary glands: the parotid, sublingual, and submandibular glands. Together they produce 2 to 4 liters of saliva per day. The basic secretory units of the salivary glands are groups of cells called acini. These cells secrete a fluid that acts and relaxes in the center of the paper to mix and digest food. The respiratory and digestive systems work closely together to produce energy. The respiratory system provides the oxygen needed to produce energy and the digestive system obtains the nutrients needed to produce energy. The digestive system has the closest relationship with the circulatory system. As the digestive system breaks down food, it absorbs nutrients into the bloodstream. The circulatory system then transports the nutrients obtained through the digestive system to the rest of the body. In a more obscure way, the nervous system, specifically the medulla, controls all involuntary actions performed during the digestive process. Finally, the endocrine system releases many hormones that influence the digestive process. These hormones determine when certain enzymes should be used and when certain nutrients should be absorbed.