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Digestion: Foods
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Digestion begins as you take your first bite of food and ends as your body disposes of unusable products. The primary digestive organs do most of the work of digesting solid foods and the accessory organs help process the nutrients, by producing bile, and secreting digestive enzymes. The physical and chemical modification of your food depends on both the exocrine and endocrine secretions of the accessory organs and the controlled movement of food through the primary digestive tract.
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Digestion begins in the mouth, where saliva lubricates the food and passes it down the esophagus. The dog's teeth -- sharp, jagged, blade-shaped molars -- are designed for gripping, tearing and shredding, and are specifically suited for its digestive capabilities. Other mammals have flat molars, characteristic of grinding.
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Digestion of food that enters the small intestine is usually complete after three to ten hours. Once digestion is essentially finished, waste products leave the ileum with the help of fiber, and these solids then enter the large intestine (the colon). In the colon, water is reabsorbed; some nutrients are produced by friendly bacteria (vitamin K, biotin, vitamin B12); fibers are digested to various acids and gases; and minerals, such as potassium and sodium, are reabsorbed (when needed). Any fiber that is not broken down—and small amounts of other undigested products—are excreted in the feces.
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Chemical digestion begins in the stomach, a large, hollow, pouchlike muscular organ. While food is still in the mouth, the stomach begins its production of gastric juice, which contains hydrochloric acid and pepsin, an enzyme that digests protein. Gastric juice is the material that breaks down the food. Once nerves in the cheeks and tongue are stimulated by the food, they send messages to the brain, which, in turn, alerts nerves in the stomach wall, stimulating the secretion of gastric juice before the bolus itself arrives in the stomach. Once the bolus touches the stomach lining, it triggers a second release of gastric juice, along with mucus that helps protect the stomach lining from the action of the hydrochloric acid. Three layers of powerful stomach muscles churn food into a thick liquid called chyme, which is pumped gradually through the pyloric sphincter, which connects the stomach small intestine.
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In humans, digestion begins in the oral cavity where food is chewed (mastication) with the teeth. The process stimulates exocrine glands in the mouth to release digestive enzymes such as salivary amylase, which aid in the breakdown of carbohydrates. Chewing (mechanical catabolism) ... causes the release of saliva, which helps condense food into a bolus that can be easily passed through the esophagus. The esophagus is about 20 centimeters long. Saliva also begins the process of chemical catabolism, hydrolysis. Once food is chewed properly, the food is swallowed. The bolus is pushed down by the movement called peristalsis, which is an involuntary wave-like contraction of smooth muscle tissue, characteristic of the digestive system.
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Digestion is the breakdown of food into smaller particles or individual nutrients. It is accomplished through six basic processes, with the help of several body fluids—particularly digestive juices that are made up of compounds such as saliva, mucus, enzymes, hydrochloric acid, bicarbonate, and bile.
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