- How does the food change in consistency and form?
As food enters the GI tract starting at the mouth, it becomes bolus, a soft ball-like mass of food, as it is masticated and broken down by saliva. After being swallowed, it undergoes peristalsis in the esophagus propelling it towards the stomach. Once it enters the stomach, strong muscular contractions in conjunction with the strong acidic environment churn and mix the food with gastric juices; turning it into a semiliquid mixture known as chyme. Then, the chyme enters the small intestine where it undergoes peristalsis once more; where digestive enzymes from the pancreas and bile further breakdown the chyme into a more liquid consistency. Nearing the cecum, the connection between small and large intestine, majority of the nutrients have been absorbed and mostly water remains; which is a job for the large intestine to do. As it travels through the colon, the once liquid mixture is stripped of its remaining water and electrolytes and becomes semi-solid to solid in consistency, getting firmer once it reaches the descending and sigmoid colon. Finally, it is then excreted as feces at the rectum via the anus.
- How could the body absorb the nutrients from the foods we eat?
Before absorption, the body needs to first break down the food into smaller, more manageable pieces. This is done via mechanical (mastication and peristalsis) and chemical digestion (enzymes, juices) throughout the gastrointestinal tract; it begins in the mouth, then transported through the pharynx, esophagus, and further digested in the stomach, where minimal absorption occurs due to the nature of the epithelia within the stomach. As it enters the small intestine, majority of the absorption occurs. Bile, pancreatic juices have prepared the chyme for absorption. This becomes possible because of the villi and microvilli that line the intestinal walls; they facilitate absorption by increasing the surface area that comes into contact with the chyme, allowing the nutrients and other essential biomolecules to enter the bloodstream and/or be stored in the liver for the body to effectively use. It is also important to mention the special bacterial flora that aid in digestion and, subsequently, absorption of important substances and nutrients such as vitamins. Lastly, the remaining undigested matter pass into the large intestine where water and electrolytes are then absorbed, forming solid waste that is to be excreted since absorption is no longer possible.