The mouth includes the tongue, salivary glands and teeth.
The enamel teeth tear, crush and grind food into smaller pieces (mechanical digestion).
The food is mixed with saliva secreted from the salivary glands. Saliva is important because it contains the enzyme amylase, which breaks down carbohydrates.
Saliva also lubricates the food, which has now been formed into a bolus, letting the tongue and soft palate (at the top of the mouth) push the food down through the pharynx, or throat.
The food is pushed through the pharynx and into the oesophagus.
The oesophagus is a muscular tube. It has a diameter of 1 inch and a length of between 10 and 14 inches. At the top lies a ring shaped muscle called the upper oesophageal sphincter, that is consciously controlled, and opens when swallowing.
It pushes the food bolus down to the stomach by alternately contracting and relaxing its muscles, as shown in the diagram. These contractions are called peristalsis, and are present throughout the digestive system.
At the end of the oesophagus, just above the stomach lies the lower oesophageal sphincter. It is also a ring shaped muscle that opens to let food into the stomach, and is otherwise held tightly closed to prevent food being regurgitated.
Food is temporarily stored in the stomach for up to 5 hours. It can expand from its j-shape to contain more food. (up to 1.7liters) Its muscular walls churn up the food to turn it to chyme.
Peristalsis also occurs in the stomach. Gastric juices are also secreted from glands in the stomach lining.
These include hydrochloric acid and pepsinogen, which is turned into pepsin by the hydrochloric acid's acidity. Pepsin (a type of protease) breaks down protein. Water and mucus help protect the stomach lining from the acidic contents.
When the mixture is sufficiently broken down, it is released into the small intestine in a controlled fashion through the pyloric sphincter (pylorus).
The liver is a triangular organ with four lobes that sits above the stomach.
It produces bile, which is stored in the gallbladder (shown in green) and added chyme in the duodenum.
Bile is important to digestion because emulsifies fats and regulates the ph of the small intestine.
Not all of the livers functions are known, but it also metabolises toxins such as alcohol and stores important vitamins.
The small intestine is a tube around one inch in diameter and 20 feet in length. It is divided into three parts, and chyme is moved through it by peristalsis.
In the duodenum, bile from the liver and pancreatic juice from the pancreas is mixed with the chyme. These liquids contain enzymes to further chemical digestion, especially that of lipids.
The jejunum is 3 feet long and serves as the primary site of nutrient absorption. The walls have protrusions called villi, which are covered with microvilli to increase the surface area for nutrient absorption, as shown below.
The ileum completes the nutrient absorption, and, as in the jejunum, nutrients absorbed go into the blood, where they are transported throughout the body.
The large intestine is shorter, around 5 feet long with a diameter of 3 inches. It absorbs water, salt and the remaining nutrients created by good bacteria in the gastrointestinal tract, from the mostly digested food matter, leaving dry stool (faeces).
In addition to the multiple parts of the large intestine, there is an extra organ with no digestive function called the appendix.
Fecal matter is prevented from backing up into the small intestine by a valve called the ileocecal junction/sphincter.
The resulting stool is stored in the last part of the sigmoid colon and rectum until it can be defecated.
The rectum is the final part of the large intestine.
The anus is the cavity through which feces are expelled.
The egestion is controlled by the internal and external sphincter, which, like all the other sphincters, are muscular rings that keep digested food in the right cavity, and control the movement from one part of the gastrointestinal tract to the next.