The food that we consume undergoes many breakdown and absorption processes in our body, this in order to exploit its nutritional value to the fullest. The following video describes these processes, from the moment the food enters our mouth until its absorption in the small intestine.
The digestion process begins already in the mouth by the action of mastication: mechanical breakdown of the food by chewing with our teeth and chemical breakdown through the saliva that is secreted from our salivary glands. Saliva contains the enzyme amylase, which breaks down carbohydrates and allows simple sugars to be absorbed directly into the circulation. Mucus is also secreted in order to stick the chunks of food together and facilitate swallowing. The food continues through the pharynx into the esophagus, where the wave-like contraction of the muscles, called peristalsis, forces it further down.
The food passes through the esophageal sphincter and enters the stomach. There it is further broken down by different digestive enzymes (proteases) and the strong gastric acid (hydrochloric acid, HCl). The acidity level inside the stomach is approximately a pH of 2. These, together with the peristaltic activity that continues in the stomach, break the food down to even smaller particles, which are forced through the pyloric sphincter into the small intestine.
The first segment of the small intestine, called the duodenum, is full of different digestive agents that are secreted from the gallbladder and the pancreas. These include pancreatic enzymes and fat-emulsifying bile from the gallbladder. This allows for the extraction from the food of the basic nutrients that are absorbed in the small intestine. Most absorption occurs in the second part of the small intestine, the jejunum. As the food continues to pass down the digestive tract it is broken into smaller and smaller pieces.
The intestinal wall contains many folds that increase its surface area and allow for enhanced absorption. If we would look at the small intestine wall we would see fingerlike structures called villi. These increase the surface area of the intestine and thus the effectiveness of absorption. A network of small blood vessels lies beneath these structures and transports the absorbed nutrients to the liver. There, further chemical processing occurs: some nutrients remain in the liver for long-term storage, while others are passed along to the rest of the body through the circulation.