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How the food changes consistency and form
Food changes consistency and form mainly through mechanical and chemical digestion. The mechanical process of digestion physically breaks down food into smaller fragments, while chemical digestion breaks down large food molecules into subunits that are small enough to be absorbed by the body.
First, food is ingested in the mouth by mastication or chewing, wherein the food is maneuvered by the tongue, pulverized by the teeth, and then mixed with saliva. After chewing, the food is reduced into bolus, a softer and more flexible mass that can be swallowed easier. When the bolus passes through the esophagus and reaches the stomach, the smooth muscles of the stomach and small intestine churn and dissolve the food, while mixing them with digestive enzymes to reduce the food into a liquid called chyme. Large and complex food molecules such as carbohydrates, lipids, proteins, and nucleic acids are broken down into smaller molecules through hydrolysis, which uses digestive enzymes, acids, and bile. For example, when food is mixed with saliva in the mouth, salivary amylase produced by the salivary glands splits starch into smaller fragments, and lingual lipase produced by lingual glands breaks down triglycerides into fatty acids and diglycerides. After digestion, the digested products are absorbed from the gastrointestinal tract into the blood and lymph. Wastes and undigested food residues that are not absorbed by the body are then eliminated through the anus in the form of feces or stool.
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How the body was able to absorb the nutrients from the foods we eat.
After the mechanical and chemical phases of digestion, nutrient molecules go through the process of absorption from the gastrointestinal tract into the blood and lymph to be able to be used by the cells throughout the body. Absorption occurs via diffusion, facilitated diffusion, osmosis, and active transport. Nutrients are mostly absorbed in the small intestine, but some substances are absorbed in the stomach and large intestine. The epithelial lining of the small intestine contains villi and microvilli, which creates a large surface area for absorption. This surface layer transports nutrients from the lumen of the small intestine into a network of blood capillaries and lacteals. Blood in the capillaries absorbs simple sugars, amino acids, glycerol, salts, and water-soluble vitamins, while lymph in the lacteals absorbs fatty acids and fat-soluble vitamins.
References:
Tortora, G. J., & Derrickson, B. H. (2011). Principles of Anatomy & Physiology (13th ed.). Wiley.
Wakim, S., & Grewal, M. (2022). Human Biology. LibreTexts.