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Lecture 16

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PHGY 210
Ann Wechsler

Lecture 16 Gastrointestinal Tract and its accessory Structures The food enters the digestive system, which is then catabolically broken down into absorbable molecules are formed. Post-absorption, they are then processed for individual cells, mainly to provide energy and raw materials for growth and repair. The structure of the GIT differs between organisms. The simplest of which is that of the earthworm, a straight tube that extends from the anterior to the posterior, the interior of the tube is referred to as the lumen, and are directly exposed to the external environment at both ends. The general format of the earthworm’s GIT is retained in higher organisms. Growth: The tube in humans is elongated, but became limited to a certain section of the body. A 1.5 metre tall person, the digestive track is 4.5m, however in a cadaver, the lack of contraction, the track becomes 9-10m. The surface area of the GIT is massively increased due to the grooved nature of the tube within. The average size of the GIT is 200-250 square metres. Differentiation: The simple tube is replaced with a complex system of connected organs, each of which is specialized for specific aspects of the process of absorbing the food intake. A series of associated structures are not part of the GIT proper, but are functionally related to the GIT. The salivary gland, the liver and the gallbladder system, and the pancreas as well. The outer surface of the tube is covered by a tough layer of connective tissue called the serosa. Which is sometimes continuous with the abdominal wall. Underneath the serosa is a layer of muscularis externa made of 2 layers of muscle, the outer layer of which is parallel to the length of the tube. The rest of the tube is covered by smooth muscle. Undernearth the muscularis externa is the submucosa, a layer of loose connective tissue with the blood vessel, the lymph tracts and other nerves, etc. The muscosa is composed of 3 layers, the outset layer is the muscularis mucosae (smooth), underneath of which is the lamina propria (loose connective tissue with nerves, blood vessel, etc), the innermost is the epithelial layer, a single layer of epithelial cells, which secretes (endocrine and exocrine together). To convey the food along the GIT, allowing it to be disrupted into small molecules which can be absorbed into the circulation. 3 activities: 1. Motility, the muscular activity forces
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