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Human Physiology: The Mechanism of Body Function - part 8

Tổng quan: Chức năng của các bộ phận cơ thể tiêu hóa Hình 17-3 trình bày một cái nhìn tổng quan về tiết và chức năng của các cơ quan tiêu hóa. Đường tiêu hóa bắt đầu bằng miệng, tiêu hóa bắt đầu nhai, phá vỡ phần lớn các thực phẩm thành các hạt nhỏ hơn có thể được nuốt. | Vander et al. Human Physiology The Mechanism of Body Function Eighth Edition The McGraw-Hill Companies 2001 III. Coordinated Body Functions 17. The Digestion and Absorption of Food 555 The Digestion and Absorption of Food CHAPTER SEVENTEEN I Motility I FIGURE 17-2 Four processes carried out by the gastrointestinal tract digestion secretion absorption and motility. Overview Functions of the Gastrointestinal Organs Figure 17-3 presents an overview of the secretions and functions of the gastrointestinal organs. The gastrointestinal tract begins with the mouth and digestion starts there with chewing which breaks up large pieces of food into smaller particles that can be swallowed. Saliva secreted by three pairs of salivary glands see Figure 17-1 located in the head drains into the mouth through a series of short ducts. Saliva which contains mucus moistens and lubricates the food particles before swallowing. It also contains the enzyme amylase which partially digests polysaccharides. A third function of saliva is to dissolve some of the food molecules. Only in the dissolved state can these molecules react with chemoreceptors in the mouth giving rise to the sensation of taste Chapter 9 . The next segments of the tract the pharynx and esophagus contribute nothing to digestion but provide the pathway by which ingested materials reach the stomach. The muscles in the walls of these segments control swallowing. The stomach is a saclike organ located between the esophagus and the small intestine. Its functions are to store dissolve and partially digest the macromolecules in food and to regulate the rate at which the stomach s contents empty into the small intestine. The glands lining the stomach wall secrete a strong acid hydrochloric acid and several protein-digesting enzymes collectively known as pepsin actually a precursor of pepsin known as pepsinogen is secreted and converted to pepsin in the lumen of the stomach . The primary function of hydrochloric acid is to dissolve