Draw a labelled diagram of human digestive system and explain it.
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(i). Food is crushed and cut in the mouth with the help of teeth and is mixed with saliva that is secreted by three salivary glands(one below the tongue and two at the side of the jaw) to make it wet and slippery, this process is known as mastication. (ii) Saliva contains amylase that breaks down complex carbohydrates and the tongue helps in pushing the food to the next part of the alimentary canal. (iii) The soft food then passes through the oesophagus in a wave-like movement known as a peristaltic movement. (iv) In the stomach, food mixes with gastric juices and dil. HCl. The food is broken down to simple substances with the help of digestive enzymes like pepsinogen, while mucus protects the walls of the stomach. (v) From the stomach, the food moves into the small intestines with the help of ring-like muscles called pyloric sphincters which allow only a little food to pass through at a time. (vi) In the small intestine, the carbohydrates, proteins, and fats are broken down with the help of juices secreted by the pancreas, liver and the small intestine itself. (vii) Fat is converted into small globule-like forms with the help of bile juices from the liver. This process is known as emulsification. (viii) Pancreatic juices contain trypsin enzyme that breaks down proteins and lipase that breaks down fats. (ix) The later part of the small intestines is alkaline in nature and helps in the digestion of carbohydrates. (x) The broken-down food is then absorbed by small projections present on the inside walls of the intestine called villi. Villi are surrounded by blood and lymph vessels that absorb the food and transport it to the rest of the body. (xi) Finally, the food moves into the large intestine where most of the water is removed from the food and is then passed out of the body through the anus.