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How does the food change in consistency and form?
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Mechanical Digestion - through the means of different mechanical forces across the gastrointestinal tract, the solid food we intake turns into a churned up and mashed ball of food for easier digestion; from the crushing and tearing of our teeth and mouth, the moisture added by the salivary glands, food is sent down the esophagus as a crushed up ball that is then constricted upon by the muscles around the tube through peristalsis to release the bolus into the stomach; it is the churned up more by the stomach along with the gastric juices to further breakdown the food into simpler components as some nutrients are absorbed as well and moves to the next area of the system as chyme; as it moves along the intestines, more enzymes help with the breakdown along with the absorption of nutrients through the villi and microvilli; lastly, water is absorbed from the remaining mush of undigested food that will then be excreted as waste material, which may either be watery or hard, depending on the absorption of the water within the large intestine.
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Chemical Digestion - not only the mechanical compression and contraction of the muscles along the GI tract helps with the breaking down of food, a lot of the absorption of said nutrients are thanks to the efforts of various enzymes across the different areas of the system; from the amylase that breaks down starch into simpler sugars, the food becomes moist enough to be easily masticated and absorbed; the pepsin and hydrochloric acid of the stomach helps digest the bolus more; lastly, the pancreatic juices and bile and intestinal enzymes further complete the process of digestion to absorb the nutrients and get the valuable components of the food before proceeding to the large intestine and let water be absorbed to defecate feces.
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How could the body absorb the nutrients from the foods we eat?
As food is broken down into smaller and smaller components through both the mechanical and chemical processes of digestion, different areas of the digestive system breaks down and absorb nutrients in different ways as well:
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Mouth - breaks down starch into simpler carbohydrates with salivary amylase
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Stomach - breaks down proteins into amino acids with the help of the gastric acid and digestive enzymes such as pepsin
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Pancreas - helps break down carbohydrates, fats, and proteins through the pancreatic juices it secretes towards the small intestine
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Liver - break down of fats with bile in the small intestine
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Small Intestine - absorbs majority of the nutrients and breaks down starches, proteins, and carbohydrates with additional intestinal enzymes
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Large Intestine - reabsorption of water as the final area for digestion
The nutrients are then transported along the epithelial lining of the GI tract by the means of the following within the walls of the small intestine:
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Active transport - proteins, amino acids as well
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Passive diffusion - short-chain fatty acids, glycerol
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Facilitated diffusion - fructose
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Co-transport - glucose, galactose, amino acids
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Endocytosis - nucleic acid digestion products