The Digestive System

The Digestive System

Definition of the Digestive System

The digestive system comprises of the gastrointestinal tract, also referred to as the digestive tract or merely the GI tract, together with the pancreas, liver, and gallbladder. The gastrointestinal tract is majorly a series of organs that are connected in the form of a long and twisted tube, starting from the mouth to the anus (Denbow, 2015). The organs making up the GI tract include stomach, esophagus, mouth, large intestine, small intestine, and anus. The gallbladder, pancreas, and liver are considered solid organs of the system. The small intestine consists of three sections, namely the duodenum (the first section), jejunum (the middle section), and ileum (located at the end).

The large intestine, on the other hand, consists of the colon, cecum, appendix, and rectum. The first part is the cecum, followed by the colon, and lastly, the rectum situated at the end. The appendix is a small pouch with the shape of a finger, and it is connected to the cecum. The gastrointestinal tract also has some bacteria, known as the microbiome or gut flora that assist in the digestive process (Derrien & van Hylckama Vlieg, 2015). Some parts of the circulatory and nervous system (including the central nervous system and enteric nervous system) also participate in digestion. Therefore, the digestive system organs, blood, bacteria, hormones, and nerves work together in harmony to digest or break down the liquids and foods we eat each day.

Importance of Digestion

For the body to stay healthy and work properly, it is in constant need of nutrients, present in the drinks and foods. Such nutrients include carbohydrates, fats, proteins, minerals, vitamins, and water (Fox, 2015). These nutrients are broken down by the system into portions small enough for the body to absorb or take up for use in growth, cell repair, and energy provision. For instance, carbohydrates are broken down into simple sugars, proteins into amino acids, and fats into glycerol and fatty acids.

How the digestive System Functions

Each component of the digestive system contributes towards the movement of food and liquid through the gastrointestinal tract, breaking down of the liquids and foods into smaller portions or both. Once broken down into parts that are small enough, the body can absorb the nutrients and transport them to where they are required. The waste products resulting from the digestive process is converted into a stool. Hormones, including cholecystokinin, gastrin, gastric inhibitory peptide, and motilin, together with nerves, participate through the control of the process (Rao & Gershon, 2016). The movement of food through the gastrointestinal tract occurs through a process referred to as peristalsis. The hollow and large digestive tract organs have some muscular layers that enable the movement of their walls. Foods and liquids are pushed through the gastrointestinal tract as a result of the movements and, in the process, mix with various contents of the organs. Food is pushed forward due to the contraction of the muscles behind the food and relaxation of the muscles in front of the food.

The movement of food through the GI tract commences at the mouth when a person eats. Food is pushed into the throat by the tongue when swallowing takes place. Epiglottis, a small tissue, prevents choking by folding over the windpipe as food enters the esophagus. Once swallowing begins, peristalsis begins as the brain signals the esophagus muscles. The signaling by the brain makes the process automatic. At the end of the esophagus, the lower sphincter, majorly a ring-like muscle, allows food to enter the stomach through relaxation (Fox, 2015). Usually, the sphincter stays closed to prevent the stomach contents from flowing back into the esophagus. Upon reaching the stomach, the food or liquid is mixed with the digestive juices by the stomach muscles. The resulting contents, called chime, are emptied into the small intestine. The small intestinal muscles mix food with digestive juices produced by the intestine itself, liver, and pancreas, and the mixture pushed forward for further digestion. The digested nutrients and water are absorbed into the bloodstream by the walls of the small intestine, while the waste products pushed into the large intestine.

The process of digestion produces waste products, including undigested parts of fluid, food, and older cells that line the gastrointestinal tract—the large intestine functions in converting these wastes into the stool in addition to absorbing water. The stool moves into the rectum through peristalsis, where it is stool until it is pushed out of the anus during bowel movements. The gastrointestinal organs break down food into smaller portions through processes such as motion (squeezing and chewing) and mixing with enzymes, bile, stomach acid, and other digestive juices (Sherwood, 2015). During chewing in the mouth, a digestive juice called saliva produced by the salivary glands. Saliva has an enzyme called amylase moistens and breaks down starches present in food. In the stomach, the glands present in the stomach lining produce enzymes such as pepsin and stomach acid (gastric acid) that take part in further food breakdown. They are assisted by the stomach muscles for proper mixing with food.

The pancreas produces digestive juice consisting of three major enzymes, including pancreatic lipase, pancreatic amylase, and pancreatic proteases that further break down fats, carbohydrates, and proteins, respectively. The pancreatic juice is delivered through ducts (small tubes) to the small intestine. The liver produces bile, a digestive juice that contributes to the digestion of some vitamins and fats, a process known as emulsification (Denbow, 2015). The bile also acts in the small intestine; however, it is usually stored in the bile duct after production. The digestive juices from the small intestine originate from pits known as crypts of Leiberkuhn situated between the villi. The juice has enzymes such as maltase that mixes with pancreatic juice and bile to further break down the proteins. The small intestine can attract and accumulate water from the bloodstream into the gastrointestinal tract to help in the digestion of food.

Absorption and Transport of Digested Food.

The small intestines also function in absorbing the majority of the nutrients, and the circulatory system transports them to other areas of the body for use or storage. Villi, which are specialized cells that line the intestinal walls, provide a large surface area for the absorption (Sherwood, 2015). The blood through a network of veins carries amino acids, simple sugars, vitamins, and glycerol to the liver where they are stored, processed, and delivered to other parts of the body when required. Fatty acids and vitamins are absorbed into the lymphatic system containing a fluid known as lymph.

Control of Digestion

The digestion process is controlled by the nerves and hormones working together. There is a flow of signals within the gastrointestinal tract and from the tract to the brain in a back and forth fashion. Hormones are released from the cells that line the small intestine and stomach. The pancreas is also involved in the release of hormones. The hormones control the digestive system by regulating the production of digestive juices (Denbow, 2015). They also signal the brain in times of hunger or fullness. Such hormones include gastrin secreted from the stomach by the G cells. It stimulates the release of hydrochloric acid (HCL) that helps in protein digestion. Another hormone called somatostatin stops the release of HCL that changes the acidic environment through a negative feedback mechanism. A hormone known as secretin neutralizes acidic chyme by stimulating the production of alkaline bicarbonate solution from the pancreas. The alkaline solution is released into the duodenum upon production (Sherwood, 2015).  Secretin acts together with cholecystokinin (CCK) that stimulates the production of pancreatic juices. Besides, the hormone CCK also stimulates the release of bile from the gallbladder into the duodenum.

Nerves link the central nervous system (CNS) made up of the brain and spinal cord to the digestive system to control certain digestive functions. For instance, when a person smells or sees food, the brain sends signals that instruct the salivary glands to secret saliva and prepare the person to eat. There is the Enteric Nervous System (ENS) situated within the walls of the gastrointestinal tract (Fox, 2015). They release a wide range of substances to delay or speed up digestive juice production as well as the movement of food, which is critical in certain circumstances, for example, when food stretches the gastrointestinal tract walls. The nerves control the muscles of the gut by sending signals that cause contraction and relaxation, therefore, pushing the food through the intestines.

References

Denbow, D. M. (2015). Gastrointestinal anatomy and physiology. In Sturkie’s avian physiology (pp. 337-366). Academic Press.

Derrien, M., & van Hylckama Vlieg, J. E. (2015). Fate, activity, and impact of ingested bacteria within the human gut microbiota. Trends in microbiology, 23(6), 354-366.

Fox, S. (2015). Human physiology. McGraw-Hill Education.

Rao, M., & Gershon, M. D. (2016). The bowel and beyond: the enteric nervous system in neurological disorders. Nature Reviews Gastroenterology & Hepatology, 13(9), 517.

Sherwood, L. (2015). Human physiology: from cells to systems. Cengage learning.