Organs of the Digestive Process

 

Organs of the Digestive Process Mouth Food enters the alimentary canal at the mouth (ingestion). The teeth bite and chew the food to begin its physical breakdown. The tongue aids in tasting, chewing (mastication), and swallowing (deglutition) of food. The hard palate is the anterior roof of the mouth. Unlike the hard palate, the soft palate tissue is not attached to bone on the posterior portion of the mouth. The uvula is a small piece of tissue at the rear of the mouth that prevents food from entering the nasal cavity during swallowing. As food is chewed it is mixed with saliva. Three salivary glands secrete an enzyme (amylase) that begins the chemical portion of the digestive process. An enzyme is a protein that increases the rate of a chemical activity in the body. The three salivary glands are the parotid, sublingual, and submandibular (Figure 15-2). Amylase starts the transformation of starch to sugar. The portion of food mixed with saliva that is swallowed is called a bolus. Figure 15-2  The salivary glands. Pharynx The pharynx or throat is divided into three portions called the nasopharynx (nose), oropharynx (mouth), and laryngopharynx (voice box). Food passes through the oropharynx from the mouth to the esophagus. The epiglottis is located at the junction of the esophagus and the oropharynx. The epiglottis is a small piece of tissue that closes off the trachea to prevent food and moisture from entering the respiratory tract. Esophagus The esophagus is a tubelike structure that carries food from the mouth to the stomach. The bolus of food moves down the esophagus to the stomach with a slow, wavelike motion. Peristalsis is a wave of contraction by which food is moved through the digestive system. Stomach In the stomach, the food bolus mixes with hydrochloric acid and the enzymes pepsin and gastrin to become chyme. The stomach is a saclike muscular organ that churns and squeezes food and continues its physical breakdown. Digestion of protein begins in the stomach. A few substances, such as glucose, some drugs, and alcohol, are absorbed directly into the blood through the stomach walls. The cardiac sphincter is a valve that prevents the chyme from flowing back into the esophagus. The pyloric sphincter controls the flow of the chyme into the intestines. It takes 1 to 4 hours for the stomach to empty the chyme into the intestines. Small Intestine From the stomach, the food enters the small intestine. The small intestine is longer and narrower than the large intestine. It is lined with tiny, threadlike projections of tissue called villi (singular, villus) that increase the area for absorption of nutrients. The three sections of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine produces juices to aid the digestive process. Most absorption of digestive products occurs in the small intestine. When digestion is completed, carbohydrates have been reduced to sugar (monosaccharide and disaccharide). Protein has been changed to amino acid and dipeptide. Fats have been reduced to fatty acid and glyceride. Large Intestine The large intestine has three major portions called the ascending colon, transverse colon, and descending colon. The appendix is a small tube of intestine descending from the side of the intestine with an unknown function in humans. Most of the water from ingested food is absorbed back into the blood through the walls of the large intestine, along with vitamins, electrolytes, and bile salts. The bacteria Escherichia coli normally resides in the large intestine and helps to form feces and to produce vitamin K. The feces, or waste material, is collected in the rectum at the end of the large intestine and eliminated by defecation through the anus. Feces are composed of undigested food, bacteria, mucus, and water. Peritoneum The peritoneum is a flat serous (moist) membrane that surrounds the abdominal cavity. It lubricates and prevents friction between the organs. The mesentery is a fan-shaped projection of peritoneum that contains blood vessels and nerves. It provides support and helps to keep the abdominal organs in place by binding to them.

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