1. How does the food change in consistency and form?
After we ingest food, mechanical digestion occurs in the mouth as a result of chewing or mastication. Food is manipulated by the tongue, grounded by the teeth, and mixed with saliva which results in an easily swallowed mass called a bolus. Bolus moves from the mouth to the esophagus through the act of deglutition or swallowing, until it reaches the stomach through its involuntary passage in the esophagus. When bolus reaches the stomach, the enzymes turn it into a soupy liquid called chyme, which is then absorbed in the small intestine. About 90% of all absorption of nutrients occurs in the small intestine and these nutrients are carried out through the circulatory system. Any unabsorbed material left from the small intestine passes on to the large intestine.
2. How could the body absorb the nutrients from the foods we eat?
About 90% of all absorption of nutrients occurs in the small intestine; the other 10% occurs in the stomach and large intestine. Because of this, its structure is specially adapted for these functions, specifically its length and components like the circular folds, villi, and microvilli. In the small intestine, absorption of materials occurs via diffusion, facilitated diffusion, osmosis, and active transport. All carbohydrates are absorbed as monosaccharides, most proteins are absorbed as amino acids via active transport processes, and dietary lipids are absorbed via simple diffusion. The fat-soluble vitamins A, D, E, and K are included with ingested dietary lipids and are absorbed via simple diffusion. These nutrients will then be transported and carried out through the circulatory system, while any undigested or unabsorbed material left in the small intestine passes on to the large intestine.