1. How the food changes consistency and form
Food changes consistency and form via physical and chemical changes and breakdowns facilitated by the organs of the digestive system. As food enters the mouth, mechanically digested from chewing or mastication as it is handled by the tongue, cut by the teeth, and mixed with saliva. Food is chemically broken down by saliva aided by salivary amylase and lingual lipase. The food is consequently reduced to a bolus, which is a soft, pliable mass that is simple to swallow. It is propelled from the mouth to the pharynx, esophagus, and into the stomach via deglutition. Mixing waves occur as food enters the stomach, macerating it and combining it with gastric gland secretions. The stomach's churning activity, assisted by HCI, pepsin, and gastric lipase, continues digestion by turning food into chyme, a soupy liquid. Enzymes in the pancreatic juice secreted by the pancreas also aid in digestion. The liver and gallbladder also help break down food and absorb fats via the secretion of bile. As chyme enters the small intestine, it is mechanically digested by segmentation and migrating motility complex. With intestinal juice, the further digestion of carbohydrates, proteins, and lipids is completed. Digestive nutrients are then passed to the GI tract of lymph through absorption. As chyme enters the large intestine, its consistency and form continue to change via (1) mechanical digestion via movements in the large intestine that mixes chyme with water and other substances (e.g. haustral churning); and (2) chemical digestion via bacterial activity in the lumen. As it remains in the large intestine, chyme then turns into a solid or semisolid substance called feces due to water absorption. It is then eventually pushed into the rectum and is defecated.
2. How the body was able to absorb the nutrients from the foods we eat
As food undergoes mechanical and chemical digestion, it is transformed into forms that can pass past the absorptive epithelial cells lining the mucosa, as well as into the underlying blood and lymphatic vessels. The body can absorb nutrients via absorption, which can be through methods of diffusion, assisted diffusion, osmosis, and active transport. The small intestine accounts for around 90% of total nutrient absorption, while the stomach and large intestine account for the remaining 10%. Anything that is still in the small intestine after being digested or absorbed moves on to the big intestine. Since the stomach's epithelial cells are impenetrable to the majority of substances, only a tiny amount of nutrients are absorbed there.