They provide a slimy layer that acts as a shield against the damaging effects of chemicals like concentrated hydrochloric acid while also aiding lubrication. As the first two chemicals may damage the stomach wall, mucus and bicarbonates are secreted by the stomach. In infants and toddlers, gastric juice also contains rennin to digest milk proteins. Gastric juice mainly contains hydrochloric acid and pepsin. Gastric juice in the stomach starts protein digestion. It will then travel down the esophagus and into the stomach by the action of peristalsis. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. About 30% of starch is hydrolyzed into disaccharide in the oral cavity (mouth). Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food the saliva also contains mucus, which lubricates the food, and hydrogen carbonate, which provides the ideal conditions of pH ( alkaline) for amylase to work, and electrolytes (Na +, K +, Cl −, HCO − 3). In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication (chewing), a form of mechanical digestion, and the wetting contact of saliva. In chemical digestion, enzymes break down food into the small molecules the body can use. Mechanical digestion takes place in the mouth through mastication and in the small intestine through segmentation contractions. The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical digestion. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma.
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