Home Digestion Mechanism in Human-- Once the food is ingested into the mouth the action of the upper and the lower jaw crushes and grinds the food. This process is called mastification. This also stimulates the production of saliva. Saliva is thoroughly mixed with the food with the action of the jaws, teeth and tongue. The food is formed into a bolus. Digestion of food begins in the mouth. Due to the presence of salivary amylase, partial digestion of starch takes place here. The digestion is helped by the chewing action that breaks down the food into smaller particles.The mucus present in the saliva moistens and lubricates the food making it easier to swallow. The bolus is then passed into the pharynx by an involuntary swallowing action. It is made easier by the presence of mucus along the walls of the alimentary canal. Bolus next moves into the oesophagus. It moves along the oesophagus by contraction of the portion of the oesophagus above the bolus and expansion of the portion below. Thus it is pushed forward and then the next region is contracted and expanded. This alternate wave of contraction and expansion is called peristalsis. It is by this process that the food moves along the alimentary canal. No digestion takes place in the oesophagus. The food then moves into the stomach. The cardiac sphincter guards the opening of the stomach, which is a muscular ring. The stomach opens only in response to swallowing. The stomach has an acidic pH due to the HCl secreted by the gastric glands. Here the proteins are broken down into proteoses and peptones by the action of pepsin and the soluble milk protein casein is degraded into insoluble paracasein in children. It then coagulates into calcium paracaseinate. The bolus thus partially digested is now called the chyme. Chyme is then passed into the small intestine, the opening of which is guarded by the pyloric sphincter. The first portion of the small intestine, the duodenum gets the pancreatic and the bile juice. The intestinal lumen has an alkaline pH due to the bicarbonate salts present in these secretions. The chyme is thus acted upon by the enzymes and salts present in these two secretions. The starch is converted into maltose by the pancreatic amylase. Trypsinogen of pancreatic juice is converted into active trypsin. Tyrpsin then converts the other proteases like the chymotrypsin and carboxypeptidase into their active forms. Thus, the remaining proteins, proteoses and peptones are digested rapidly into peptides and amino acids by the action of different proteases. The bile juice emulsifies the fats, which helps in their conversion into fatty acids, glycerol and monoglycerides by the action of lipase. Some of the triglycerides are absorbed as such into the intestinal cells. In jejunum, there is no digestion. In ileum, the food is completely broken down into the simplest of forms - proteins into amino acids and carbohydrates into monosaccharides. This digested mass is now called the chyle and it is in a liquid form.
Posted on: Fri, 14 Mar 2014 05:27:44 +0000
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