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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 hollow organs where food enters at one end and waste gets out from the other. These organs are lined with layers of 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, which breaks down starch into maltose and dextrin that can be further processed in the small intestine. Saliva also contains salivary lipase, which starts the process of fat digestion.
The food bolus is propelled down the esophagus by peristalsis 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 lipase 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 rectum until it can be pushed out from the body during a bowel movement.