Endocrine System
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The Digestive Tract (Alimentary Canal)
7.2 The digestive tract is a long tube running from the mouth down to the neck and trunk of the body, finally ending at the anus opening. If the human digestive tract were stretched out in a straight line, it would be about 95 cm. long. This long tube is also called the alimentary canal. It consists of the mouth (pharynx), foodpipe (oesophagus), the stomach, the small intestine and the large intestine. During an average lifetime about 65 tonnes of food and drink passes through the alimentary canal.
The Mouth (Pharynx) and Saliva
7.3 Saliva is produced chiefly by three pairs of salivary glands in the body. One pair is in front of the ears, one under the tongue, and one under the lower jaw. The largest glands, in front of the ears, secrete large quantities of watery saliva to moisten and to soften the food for easy chewing. The glands near the lower jaw secrete saliva to make the food slippery for easy swallowing. Which of the glands will produce the most saliva depends on the food we take in. If we bite a juicy apple, our lower glands will function to make the food slippery, as it need not be moistened. If we eat dry crackers, the glands in front of the ears will work to produce large quantities of watery saliva. The nerves in our lips, tongue, teeth and jaw muscles help us to know what we are eating. They send messages to the brain about how cold or hot our food is and whether it is rough or smooth, hard or soft.
7.4 The digestion of food begins in the mouth with the three pairs of salivary glands and continues in the stomach and the small intestines. The smell, and even the thought of food causes these glands to increase their production of saliva and our mouths start watering. Saliva contains enzymes that breakdown starches and other complex carbohydrates in food. As a person chews, food is broken into small pieces and mixed with the saliva, which moistens it and/or makes it slippery for easier swallowing. Proper chewing is essential for optimal digestion. In addition to stimulating the digestive juices in the saliva, proper chewing breaks down food, massively increasing the surface area available for contact with digestive juices. This increases the efficiency of digestion by giving stomach acid and digestive enzymes the opportunity to penetrate the food and do the digestive work. Each mouthful should ideally be chewed to a cream before swallowing.
The Stomach
7.5 After food is properly chewed and swallowed, it passes through a muscular 25 cm. long foodpipe (oesophagus or gullet) and enters the J-shaped muscular stomach. The stomach wall has three muscle layers and a mucus membrane that line it. An average adult holds about one litre of mixed food. The glands in the stomach lining secrete a variety of enzymes including pepsin, hydrochloric acid and other chemical compounds. These secretions aid in digesting the food and are collectively called gastric juices. Most of the protein and carbohydrates in the food are broken down in the stomach. The churning action of the stomach muscles furthers the digestion process by mixing the food and gastric juices. After about 3-5 hours, depending on the type, the food becomes a liquid called chyme. The chyme then passes into the small intestines.
