How does the food change in consistency and form?
The first landmark that food has to go through in the gastrointestinal tract would be the mouth or the oral/buccal cavity. Here, voluntary chewing with teeth will begin physical breakdown or breaking down our large food particles into smaller ones. However, what cannot be controlled during these processes is salivary secretion (involuntary) and will be the primary form of chemical breakdown in the mouth (hydrolysis or enzyme-assisted breakdown). Once the food is mixed with saliva, it will now be called a bolus and will be swallowed into the esophagus. Once the esophagus transports the bolus via peristalsis to the stomach, churning (physical breakdown) will occur and gastric juice (chemical breakdown) will be secreted for further digestion. The result will be a melted or a liquid-like substance called chyme. The chyme will move into the small intestine for nutrient absorption from the food, then will proceed to the large intestine for water reabsorption. Because of this water reabsorption, the chyme will harden and turn into a solid waste matter called feces. This feces will be expelled at the anus after passing through the rectum.
How could the body absorb the nutrients from the foods we eat?
The body absorbs the nutrients from food in our small intestine. Enzymes and bile will chemically break down the food and allow the villi/microvilli structure to absorb the nutrients. Materials that are not absorbed or digested (water, ions, vitamin K) will be absorbed through the large intestine. Absorption of lipid-soluble substances such as alcohol or aspirin may take place in the stomach, however.
Carbohydrates and Vitamin B2(with intrinsic factor) can be absorbed through facilitated diffusion or active transport, proteins and nucleic acids are absorbed mainly through active transport; lipids, fat-soluble vitamins, and water-soluble vitamins are absorbed through simple diffusion, and water is absorbed through osmosis.