The human digestive system is a sequence of organs that use mechanical and chemical means to take in food, break it down, extract nutrients and energy, and eject waste products in the form of urine and feces. This system evolved incrementally over the course of hundreds of millions of years and is the only natural way for humans to obtain energy for movement and thinking. It is capable of handling a variety of food sources, both animal and vegetable, but tends to handle food best when it is cooked. Because cooked food has been around for so long, humanity as a species is slightly “spoiled” in its favor, and many people get sick if they consume food that has not received adequate cooking.
The mouth is the entrance to the human digestive system. Teeth gnash the food, breaking it down mechanically, while the three salivary glands release saliva containing the enzyme amylase, which breaks down starch and fat chemically. Saliva makes food easier to swallow by moistening it, as well as preventing the erosion of tooth enamel by modulating pH.
After entering the body at the back of the throat, food travels down the esophagus, being transported not by gravity but by muscular contractions. This is why it is possible to eat while hanging upside down. The interior of the esophagus is very moist, which helps to further break down food and prevent damage to the rest of the system.
After moving through the esophagus portion of the digestive system, food and drink reaches the stomach, where it is further broken down into manageable pieces. Because the nutrients in food are ultimately meant to be consumed by cells, they must be broken into very small parcels for delivery. The primary agent of digestion in the stomach is gastric juices, which are produced in large amounts and can be very acidic. A secondary agent is muscular contractions within the stomach.
After the stomach, the broken down food moves into the small intestine, the area where most of the nutrient extraction takes place. As the food moves through the small intestine, it is mixed with bile, which is produced by the liver, as well as pancreatic juices, which perhaps unsurprisingly come from the pancreas. These two liquids help further the digestive process, breaking down the nutrients in food to the point where it can be absorbed by the blood. The inner intestine is home to the famous villi, tiny living extrusions which gather nutrients on a fine scale.
The final components of the digestive system are the large intestine or colon, the anus, and the urinary tract, which separate the liquid matter from the solid matter and send them to their respective exit ports. Of course, the human digestive system is not 100% efficient, and there are many nutrients left over in this “waste”, which will be consumed happily by bacteria or sent through a waste processing plant.