Pharynx
The pharynx is the part of the neck and throat situated behind the mouth and nasal cavity, and cranial, or superior, to the oesophagus. It is part of the digestive system and respiratory system. Because both food and air pass through the pharynx, a flap of connective tissue, the epiglottis closes over the trachea when food is swallowed to prevent choking or aspiration.

Oral Cavity
Digestion begins in the oral cavity where food is chewed. Saliva is secreted in large amounts in the oral cavity, and is mixed with the chewed food by the tongue. The saliva serves to clean the oral cavity and moisten the food, and contains digestive enzymes. It also contains mucin, which helps soften the food into a bolus. Swallowing transports the chewed food into the oesophagus.

Oesophagus

The oesophagus is a narrow muscular tube which starts at pharynx at the back of the mouth and ends at the cardiac orifice of the stomach. The wall of the oesophagus is made up of two layers of smooth muscles, which form a continuous layer from the oesophagus to the open and contract slowly, over long periods of time.

At the top of the oesophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the oesophagus to the stomach through peristaltic contraction of these muscles. It takes only about seven seconds for food to pass through the oesophagus and no digestion takes place.

Stomach

The stomach is a small,'C'-shaped pouch with walls made of thick, elastic muscles, which stores and helps break down food. Food enters the stomach through the cardiac orifice where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins.

Small Intestine

After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs here after the milky chyme enters the duodenum. Here it is further mixed with three different liquids Bile, Pancreatic Juice and Intestinal enzymes of the alkaline mucosal membranes.

The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter.

Large Intestine

After the food has been passed through the small intestine, the food enters the large intestine. The large intestine has three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The large intestine absorbs water from the bolus and stores feces until it can be egested.

Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the body and become hard and concentrated feces. The feces is stored in the rectum for a certain period and then the stored feces is eliminated from the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.