10 24 07 - Digestive System II

10 24 07 - Digestive System II - The Digestive System II...

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The Digestive System II The Foregut (continued) 1. Mouth is ectoderm 2. Rest of digestive system is endoderm 3. Foregut- Pharynx, Esophagus, and stomach 4. Midgut-small intestine 5. Hindgut-large intestine and cloaca 6. Cloaca-sinus or large swelling at end of large intestine which receives material from digestive tract, urinary bladder, and reproductive ducts a. Exit for three systems of the body 1. Esophagus - a fairly simple tube in most vertebrates that transports food from the pharynx to the stomach - if heart is more posterior, the esophagus has to be longer i. fish esophagi are almost nonexistant - Peristalsis -series of contractions that starts at top and pushes food down - Main function-moving food down into stomach i. Abrasion-resistant lining - Birds: have a crop - an expansion of the esophagus that acts as a storage chamber, containing plant-digesting bacteria i. Some mucus secretion ii. Secretes other things to help in regurgitation for young 2. Stomach: - Not found in extant jawless vertebrates; first found in gnathostomes. i. Don’t need to store food, when sucking blood or eating small amounts - evolved primarily for storage in fish that ate larger meals, less often. - whenever a stomach is present, a pyloric sphincter is present to prevent food from moving to the intestines prematurely. A cardiac sphincter may be present to prevent food form moving back into (reflux) the esophagus. i. Sphincter- round muscle - digestion-breakdown of food items i. chewing, swallowing, peristalsis - produces pepsin (digests peptides – proteins) - produces HCl to eliminate many bacteria
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10 24 07 - Digestive System II - The Digestive System II...

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