1. How does the food change in consistency and form?
In the oral cavity or mouth, food is mechanically broken down into smaller pieces by mastication (chewing), and salivary amylase begins the chemical breakdown of starch into simpler sugars. This turns the food into bolus, a semi-solid mass of chewed food mixed with saliva.
The stomach walls mix the bolus and gastric juices, turning the food into a semi-liquid mixture called chyme (churning). Similar to the oral cavity, chemical digestion also occurs as gastric juices such as hydrochloric acid and pepsin break down the proteins into smaller peptides.
In the small intestine, bile emulsifies fats, while pancreatic enzymes continue the breakdown of proteins, carbohydrates and fats. The brush border enzymes of the jejunum and ileum further digests nutrients into their simplest forms.
2. How do we get nutrients from the foods we eat?
The body absorbs nutrients in the small intestine, primarily in the jejunum and ileum. Its inner surface has finger-like projections called villi to increase the surface area for absorption. Monosaccharides and amino acids are absorbed into the blood capillaries while fatty acids and glycerol are absorbed into the lymphatic capillaries. These nutrients then enter the bloodstream to be utilized by the cells. On the other hand, in the large intestine, water and electrolytes from the indigestible food matter are absorbed.