The Gastrointestinal Food Journey
The gastrointestinal (GI) system, also known as the digestive system, is a series of tubes that begins with ingestion at the mouth and excretion from the anus. Put simply, the digestive system is responsible for the break down of complex food substances into simple molecules. These molecules can then be absorbed and utilized as energy. The digestive system is composed of series of muscle, (mostly smooth muscle), that is responsible for the movement of food through the process of peristalsis. The digestive system is also responsible for the production of an array of hormones and enzymes necessary for the proper digestion of food, in addition to the elimination of waste products.
Digestion begins with the ingestion of food through the oral cavity (mouth). The mouth is where the process of mastication (chewing) occurs. This process is aided by the secretion of saliva from glands. There are actually two types of salivary secretions. The first is a watery and thin secretion that hydrates the ingested food, while the second is mucous-like thick secretion that lubricates causing the formation of a bolus.
Esophagus and Pharynx
Once food is swallowed, it travels through the pharynx (throat) to the esophagus. Swallowing is a natural reflex initiated by specific receptors located in the pharynx that are activated by the presence of food at the back of the mouth. The esophagus is a muscular tube that connects the mouth to the stomach. The esophagus moves food by utilizing smooth muscle contractions, through the process of peristalsis.
The stomach resembles a small sac, surrounded by muscular walls that are protected by a mucosal lining. When food reaches the stomach, it is further digested by the presence of hydrochloric acid that contains digestive enzymes that are responsible for the further break down of food, particularly proteins.
Accessory Organs: Pancreas, Liver, and Gallbladder
There are three accessory organs associated with the digestive system.
- Pancreas: The pancreas produces a variety of enzymes responsible for the break down of food substances, collectively known as pancreatic juice.
- Liver: The liver produces the substance known as bile, which is responsible for the emulsification of fats for proper absorption.
- Gallbladder: The gallbladder is the storage unit for bile produced by the liver.
The majority of digestion occurs within the first section of the small intestine, known as the duodenum. This is primarily due to the aid of enzymes and bile, secreted into the small intestine by the digestive accessory organs. The jejunum and ileum of the small intestine are responsible for the majority of absorption. Food that can not be utilized is moved from the small intestine to the large intestine by peristalsis.
Large Intestine, Rectum, and Anus
The large intestine is responsible for the absorption of water and the storage of waste products. Left over food enters the large intestine and exits through the rectum and anus as stool. The anal sphincters are responsible for controlling the excretion process, providing the urge to defecate, making it possible to be continent. Sources: http://www.webmd.com/digestive-disorders/digestive-system http://www.mayoclinic.com/health/digestive-system/AN00896