1. How does the food change in consistency and form?
-
The food, originally distinguishable, becomes a masticated and digested substance that is eventually expelled from the body. It begins its journey in the mouth, where it undergoes mechanical digestion and some minor chemical digestion due to the salivary amylase. In the mouth, it becomes a bolus that eventually travels down the esophagus and into the stomach where it undergoes further mechanical (churning) and chemical digestion with the help of gastric juices (HCl, pepsinogen, gastrin, and other enzymes). Some absorption of water, ions, certain drugs, and alcohol also occur, which turns the bolus into a thick semi-fluid form called chyme. This travels further down the GI tract and reaches the large intestine. This is where absorption of remaining water and ions from the indigestible components happen and the “food” turns back into a solid or semi-solid substance that is stored in the rectum and is eventually eliminated from the body as stool through the anus.
2. How could the body absorb the nutrients from the foods we eat?
-
The digestive system breaks down the food we eat into smaller substances that are then absorbed into the bloodstream. In the mouth, salivary amylase breaks down starches in the food. The stomach has juices, hormones, and enzymes that break down proteins. Moving into the part of the GI tract where most of the absorption occurs, the small intestine has brush-border enzymes, bile, pancreatic juices, and bacteria that aid in the breakdown of starches, protein, and carbohydrates. The villi, finger-like projections in the mucosa of the small intestine, are rich with capillary beds where the nutrients like water are absorbed. Nutrients are also absorbed all along the small intestine’s plasma membrane. Finally, within the large intestine, more water and nutrients such as vitamin K, some B vitamins, and ions are absorbed.