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How does the food change in consistency and form?
As food moves through the gastrointestinal tract, it undergoes various processes that change its consistency and form. When food is ingested or taken into the mouth, food is broken down into smaller pieces when it is chewed. Along with this, enzymes, like salivary amylase and lingual lipase, help with the chemical breakdown of food which reduces it to a soft and flexible mass that can be easily swallowed called a bolus. This then goes through the esophagus and to the stomach where churning and hydrolysis happens to further breakdown the food with digestive enzymes. This process transforms the food into a fluid substance called chyme which is then passed through to the intestines. In the small intestines, the chyme is acted upon by enzymes and it is also physically digested in a process called segmentation. When the digested food reaches the large intestines, water moves from the GI tract into the bloodstream and waste products from the food forms a stool, usually having a soft to firm consistency, that is eliminated from the body through defecation.
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How could the body absorb the nutrients from the foods we eat?
As food goes through different processes in the gastrointestinal tract, both mechanical and chemical digestion, it is broken down into smaller components that can be easily absorbed by the body. The small intestines absorb most of the nutrients from the food that we eat. When chyme enters the small intestine, it is then mixed with digestive enzymes from the pancreas and bile from the liver in the duodenum. As it is squeezed into the jejunum and ileum, finger-like projections called villi aid in the absorption of nutrients from the GI tract into the blood or lymph via diffusion, facilitated diffusion, osmosis, and active transport. Monosaccharides, amino acids, dipeptides, tripeptides, lipids, electrolytes, vitamins and water are absorbed in the small intestine. Undigested food material is then passed on to the large intestine where water, ions, and some vitamins are absorbed and the remaining indigestible parts of food are eliminated out of the body through defecation.