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  • The digestive system is composed of 2 main components: the gastrointestinal tract, or

  • GI tract, where digestion and absorption take place; and accessory organs which secrete

  • various fluids/enzymes to help with digestion.

  • The GI tract is a continuous chain of organs where food enters at one end and waste gets

  • out from the other.

  • These organs are lined with smooth muscles whose rhythmic contractions generate waves

  • of movement along their walls, known as peristalsis.

  • Peristalsis is the force that propels food down the tract.

  • Digestion is the process of breaking down food into smaller, simpler components, so

  • they can be absorbed by the body.

  • Basically, carbohydrates such as sugars and starch are broken down into glucose, proteins

  • into amino acids, and fat molecules into fatty acids and glycerol.

  • Digestion starts in the oral cavity where the food is moistened with saliva and chewed,

  • food bolus is formed to facilitate swallowing.

  • Saliva is secreted by the salivary glands and contains the enzyme amylase.

  • Amylase breaks down starch into maltose and dextrin which are processed further in the

  • small intestine.

  • The food bolus is propelled down the esophagus into the stomach, the major organ of the GI

  • tract.

  • The stomach produces gastric juice containing pepsin, a protease, and hydrochloric acid

  • which act to digest proteins.

  • At the same time, mechanical churning is performed by muscular contraction of the stomach wall.

  • The result is the formation of chyme, a semi-liquid mass of partially digested food.

  • Chyme is stored in the stomach and is slowly released into the first part of the small

  • intestine, the duodenum.

  • The duodenum receives the following digestive enzymes from accessory organs:

  • - Bile, produced in the liver and stored in the gallbladder; bile emulsifies fats and

  • makes it easier for lipases to break them down.

  • - Pancreatic juice from the pancreas.

  • This mixture contains proteases, lipases and amylase, and plays major role in digestion

  • of proteins and fats.

  • The small intestine also produces its own enzymes: peptidases, sucrase, lactase, and

  • maltase.

  • Intestinal enzymes contribute mainly to the hydrolysis of polysaccharides.

  • The small intestine is where most of digestion and absorption take place.

  • The walls of the small intestine absorb the digested nutrients into the bloodstream, which

  • in turn delivers them to the rest of the body.

  • In the small intestine, the chyme moves more slowly allowing time for thorough digestion

  • and absorption.

  • This is made possible by segmentation contractions of the circular muscles in the intestinal

  • walls.

  • Segmentation contractions move chyme in both directions.

  • This allows a better mixing with digestive juices and a longer contact time with the

  • intestinal walls.

  • The large intestine converts digested left-over into feces.

  • It absorbs water and any remaining nutrients.

  • The bacteria of the colon, known as gut flora, can break down substances in the chyme that

  • are not digestible by the human digestive system.

  • Bacterial fermentation produces various vitamins that are absorbed through the walls of the

  • colon.

  • The semi-solid fecal matter is then stored in the r. until it can be pushed out from

  • the body during a bowel movement.

The digestive system is composed of 2 main components: the gastrointestinal tract, or

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B2 digestion intestine small intestine digestive tract digestive system

Physiology Basics: the Digestive System, Animation

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    Yrchinese posted on 2017/06/04
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