Introduction

Digestion begins in the oral cavity with both mechanical and chemical breakdown of food. Chewing motion breaks down food particles and mixes food with saliva secreted from salivary glands. There are three salivary glands that contribute secretions to saliva. The parotid glands contribute a serous fluid rich in amylase. Amylase is a digestive enzyme that begins carbohydrate digestion. The submandibular glands secrete a primarily serous fluid that also contains some mucus. Mucus, secreted from the sublingual glands, helps bind food together and acts as a lubricant during swallowing. As food is swallowed, it enters the esophagus and is moved downward by a wavelike muscular contraction called peristalsis. The contractions progress from the superior end of the esophagus to the gastroesophageal junction, where the contents of the esophagus empty into the cardiac region of the stomach. The lining of the stomach secretes gastric juices that contain digestive enzymes. Peristalsis churns and mixes the food with gastric juices while moving the food toward the first part of the small intestine, the duodenum. In the duodenum, food is digested even further. The bile duct from the liver and gallbladder, and the pancreatic duct from the pancreas all empty their secretions in the duodenum. The bile works to break down fats, while the pancreatic enzymes aid in the digestion of other food particles. Together, these digestive juices help prepare the food so that the small intestine can absorb as many nutrients from the food as possible. The majority of the absorption takes place in the second portion of the small intestine, a three foot section called the jejunum. As the food travels through the jejunum, it's gradually broken down into smaller and smaller particles. Folds line the walls of the intestine and act to increase the absorptive surface area. The green nutrient particles and the yellow fat particles get caught in these folds and are absorbed into the intestinal wall. Larger, less digested particles are pushed on to the large intestine where much of the water they contain is reabsorbed before being expelled from the body. A closer inspection of the folds reveals that the intestinal wall is covered with thousands of fingerlike villi. A network of tiny blood vessels called capillaries runs just below the surface of each villus. These capillaries absorb the available nutrients in the intestine and transport them to the liver. The yellow fat nutrients are absorbed into a vessel called a lacteal that carries them to the circulatory system. Nutrients absorbed by the blood in the villi are carried to the liver by way of the portal vein. In the liver, the blood is filtered and the nutrients are extracted and processed. After processing, the nutrients are either stored for future use or released into the bloodstream to be used throughout the body.


Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.