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lecture 8 - Biology 213 24.1 Digestive System There are...

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Biology 213 Human Organism -2 24.1 Digestive System • There are three organ systems in the body that facilitate the exchange of gases, nutrients, and wastes with the cells and fluids of the body. – Digestive system – Respiratory system – Urinary system • The activity of these systems helps the body maintain homeostasis. Tube-Within-a-Tube Body Plan • The digestive systems of most animals is a complete tube-within-a-tube, meaning that it has a mouth and anus. • This inner tube is separated from the body wall by the coelom. • The basic plan for the digestive system is similar across animal species. Tube-Within-a-Tube Body Plan (cont.) Tube-Within-a-Tube Body Plan (cont.) • The digestive system of animals performs four essential functions. – Ingests food – Breaks down food into smaller molecules – Absorbs nutrients in molecules – Eliminates indigestible waste Mouth • For most animals, the digestive tract begins with the mouth. • While teeth mechanically tear and grind food, the salivary glands release the enzyme amylase into the mouth to degrade carbohydrates. • The ground, partially-digested food material that is swallowed is called a bolus.
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Swallowing • In many animals, the digestive and respiratory systems share a common passage behind the mouth called the pharynx. • The flow of food through the pharynx and into the esophagus is assisted by the epiglottis, which covers the respiratory passage. • The rhythmic contractions of the esophagus, called peristalsis, move food to the stomach. Swallowing (cont.) Stomach • The stomach is the organ in which proteins in food are digested. • The low pH of the stomach and the presence of the enzyme pepsin accomplish the digestion of proteins. • Peristalsis moves food through the stomach while a sphincter regulates the flow of digested food into the intestine. Stomach (cont.) Stomach (cont.) • Ruminant is a term used to describe animals like cattle that have a stomach with a compartment called the rumen. • The rumen contains symbiotic bacteria that help these herbivores digest plants. • When animals chew cud, they are chewing the partially-digested rumen product. Stomach (cont.)
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Small Intestine • The digested food material the exits the stomach and enters the small intestine is called chyme. • The small intestine provides two functions. – Additional digestion of food material – Absorption of nutrients • Three additional organs that assist the small intestine. – Liver – Gallbladder – Pancreas Small Intestine (cont.) • The liver produces bile to emulsify fats for digestion and absorption. • Excess bile is stored in the gallbladder. • The pancreas produces a juice that contains substances that enhance digestion.
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