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How does the food change in consistency and form?
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As food passes through the digestive system, it undergoes mechanical and chemical digestion, changing its consistency and form. In the mouth, mechanical digestion is done through chewing or mastication where food is broken down into smaller pieces and chemical digestion by salivary amylase breaks down carbohydrates, forming a bolus. In the stomach, gastric juice (containing hydrochloric acid and pepsin) chemically digests proteins, while mechanical churning transforms the bolus into semi-liquid chyme. In the small intestine, pancreatic juice (rich in enzymes like lipase and amylase) and bile from the liver further chemically digest fats, carbohydrates, and proteins, enabling nutrient absorption. In the large intestine, water absorption solidifies the remaining material into feces, which is excreted.
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How could the body absorb the nutrients from the foods we eat?
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The body absorbs nutrients from food through various mechanisms, including diffusion, facilitated diffusion, osmosis, and active transport, facilitated by different parts of the digestive system:
- Stomach: The stomach absorbs small amounts of water, alcohol, aspirin, and other lipid-soluble substances. Most nutrients are not absorbed here because the epithelial cells of the stomach are largely impermeable to other materials.
- Small Intestine: The small intestine is the primary site of nutrient absorption. Carbohydrates are broken down into simple sugars, which enter the bloodstream through active transport and facilitated diffusion. Proteins are digested into amino acids and absorbed similarly. Fats are emulsified by bile salts, broken into fatty acids and glycerol, absorbed by epithelial cells, reassembled into triglycerides, and transported as chylomicrons via the lymphatic system. Fat-soluble vitamins are absorbed alongside fats, while water-soluble vitamins and minerals are absorbed via diffusion, facilitated diffusion, or active transport. Water is absorbed through osmosis, and nutrients travel to the liver for processing or to the rest of the body for use.
- Large Intestine: The large intestine absorbs water, electrolytes, and vitamins. As undigested material moves through, water absorption solidifies the chyme into feces, which is eventually excreted through the anus.