How the food changes consistency and form?
As we initiate the action of putting food in our oral cavity, we perform mastication. Food starts with how we eat it (solid or in fluid form). Mastication in the mouth crushes the food we eat and mixes it with saliva. Saliva contains enzymes such as lingual lipase and amylase. The previous actions result in the formation of a bolus. Next, the bolus enters the pharynx down the esophagus. The muscular esophagus exhibits peristalsis which involuntarily propels the bolus along the esophagus. The food now enters the stomach where it is the start of chemical digestion. The bolus is now mixed with stomach enzymes and gastric juices ex. HCl. The stomach churns the mixture and creates a chyme. This is where the protein content of food chemically breaks down. This is where some fat-soluble substances also start to be absorbed.
Next, the chyme now enters the small intestine where most of the nutrient contents of food are absorbed. The carbohydrates, lipids, nucleic acids, proteins, vitamins, minerals, and water now enter the bloodstream. There is also some extent of physical digestion in the small intestine (segmentation).
Almost to the final path of food, the large intestine receives the unabsorbed contents of the small intestine. Mostly, the large intestine is where water is reabsorbed. The remaining contents travel down the rectum. The resulting contents here are considered wastes that turn into feces when they exit the anus.
How the body was able to absorb nutrients from the food we eat
Food is made up of different molecules. To answer the question, I’ll answer based on the type of biomolecules and nutrients.
Carbohydrates - Through mechanical digestion (mastication) in the oral cavity, the food we eat is broken down into smaller pieces. Consequently, the amylase in our saliva starts the chemical digestion of carbohydrates that converts large polysaccharides into alpha-dextrin, maltose, and maltotriose.
Protein - Protein is digested by pepsinogen. Pepsinogen is the active form of pepsin. Pepsin is only produced by the stomach. Chemical digestion of protein then only begins in the stomach.
Fats - Fat digestion also starts at the stomach, however, there is also a lingual lipase in saliva which helps break down some forms of fat. To further break down fat, both the gallbladder and pancreas release enzymes that aid in the process. Fun fact, some vitamins like D, E, and K need fat to be absorbed.
The primary absorptive area of the gastrointestinal (GI) tract is the small intestine. The small intestine contains microvilli projections in its internal lining. The special structures increase the absorptive surface area of the organ. Additionally, they grab the nutrients from the chyme to their apical membrane. The brush border enzymes further degrade the nutrients into more absorbable molecules. The absorbed molecules directly enter the bloodstream and lymphatic tissues.
It is important to note that some, although few, nutrient absorption occurs already in the stomach such as non-steroidal anti-inflammatory drugs.