Digestion is one of the most important and complex body functions and transforms the food we eat into nutrients that are used for growth, cell repair and burned as energy. The waste that results through this process is then eliminated.

The gastric tract actually consists of a long tube that starts from the mouth and eventually reaches the anus after many twists. Muscles are positioned along its entire length that control the traffic of food through the tract, as well as the cells involved in the process. These release various chemicals like hormones and enzymes that are required for breaking down food. The entire length of the tract is 30 feet for an average adult person. The liver, pancreas and gallbladder are three key organs placed along the route that facilitate digestion.

In short, digestion transforms food into very small molecules that can enter the bloodstream in order to be transported for use in nutrition. It consists of two main phases. The first one is mechanical and happens inside the mouth, when we chew the food and turn it into smaller parts. The second phase is inside the intestinal tract and uses chemical reactions with the help of digestive enzymes that eventually separate the useful molecules from aliments.

Mammals start the process of digestion inside the mouth through mastication, which involves chewing food with the teeth in order to break it into smaller pieces. The chemical part of digestion also begins in the mouth, as the amylase in saliva triggers the breakdown of starch.

The next step is the descent inside the esophagus, which is a long tube that connects the mount to the stomach. Inside the stomach, proteins start to be separated by the gastric juices present there. Pepsin and hydrochloric acid are the main components of gastric juice, both being very corrosive. In order to protect the stomach from their action, the walls are covered in a layer of mucus. Gastric juice is very acidic, which denatures the proteins in food and breaks them down. Persitalsis is the scientific name for the mechanical digestion that happens at the same time as the chemical one, as the food is mixed with juices and enzymes by the contraction of stomach muscles.

Food remains for one or two hours in the human stomach before travelling to the next part of the tract, the small intestine. It is then broken further by the enzymes present there, as it moves through. Inside the intestine, the molecules of nutrients that result through digestion enter the walls and start to be transported by the blood vessel network. In order to present a surface area as large as possible for nutrient absorption, intestine walls include villi and microvilli, which are projections located in wrinkles and folds.

The key organs in digestion are the pancreas and the liver. Digestion requires bile, which is produced in the liver and travels to the intestine through the bile duct. Pancreatic juices are more complex and play a role in both proper digestion and the absorption of nutrients. Many staple foods, for example rice or bread, consist of complex carbohydrates. These are transformed into glucose and other sugars with a simple structure that are small enough to be transported through the blood stream and then processed inside the cells as energy.

The nutrients and water that still remain in the food are extracted inside the large intestine, which is the final processing stage. The rest is fecal matter that is pushed to the rectum and defecation eventually eliminates this waste from the body.

What is the digestive system?

Scientists consider that the intestinal tract consists of the digestive tube itself, also known as the gastrointestinal (GI) tract, as well as organs like the pancreas, liver and gallbladder. It is actually a long and very twisted tube that runs from the mouth to the anus and connects a series of hollow organs. The components are in order: the mouth, esophagus, stomach, small intestine, large intestine and finally the anus, all of them hollow. The solid ones are the pancreas, liver and gallbladder.

The small intestine is a complex organ and consists of three parts: it starts with the duodenum, the middle part is the jejunum, while the rear is the ileum. The appendix, cecum, colon, and rectum are the elements of the large intestine. The first part is the cecum, while the appendix is a pouch shaped like a finger that is attached to it. It continues through the colon and ends with the rectum.

The gut flora, also known as the micro biome, is the population of healthy bacteria that live in the tract and assist with digestion. The circulatory and nervous systems are also marginally involved in the process. It is a complex system that uses organs, juices, nerves, hormones, bacteria and blood to break down food.

Why is digestion important?

Digestion is obviously very important for human health because we need nutrients in order to stay alive. There are many types of nutrients, for example carbohydrates, proteins, fats, water, vitamins and minerals. All of them are extracted from food and processed into small components that can be used by cells for development, repair and energy.

Digestion processes each of them into usable components: proteins become amino acids, carbohydrates break into simple sugars like glucose and fats turn into fatty acids and glycerol.

How digestion works?

People usually assume that digestion only happens inside the stomach but the process actually starts much earlier. As soon as we see or smell food, even when we just think about it, the production of saliva is initiated inside the mouth. It is produced by glands placed near the jaw and under the tongue, a mental reflex activates them when we detect food or imagine it. As soon as the sensors are stimulated, nervous impulses are sent from the brain to the mouth in order to get ready for a meal and start producing saliva.

Saliva plays several important roles, the main one being to moisturize the food and make it easy to swallow, as the teeth chop it into smaller pieces. It also includes amylase, an enzyme that starts the first phase of the digestion process inside the mouth by breaking down some of the starches and other types of carbohydrates in food.

Muscles located in the mouth and tongue initiate swallowing, which pushed food down the throat. This part of the body, scientifically known as the pharynx, has a length of around 5 inches and allows both food and air to pass through. The epiglottis is a tissue that acts like a switch and prevents choking by closing the route to the lungs when we swallow.

The next organ is the esophagus, a tube lined with muscles that passes through the chest area. The process that pushes food down to the stomach is known as peristalsis and consists of coordinated waves of muscle contraction. The movement of the esophagus, as well as muscular activity of the stomach and intestines, is done automatically, without us being aware of it.

The stomach

The sphincter is a ring of muscles located at the end of the esophagus. Its role is to allow food to enter the stomach but stop it from coming back into the esophagus, which could damage it. Acids and enzymes are part of the juices that are mixed with food inside the stomach, where the muscles in the walls contribute to the process. The acidic juices break down food into smaller components, which are easier to digest. The stomach also houses the glands that produce these juices.

The stomach is a very elastic organ. When it's empty, the stomach is small and has an average volume of about one fifth of a cup. It can expand greatly, depending on the size of the meal, and can reach a volume of 8 cups or more after a very large one.

Some of the nutrients in food are digested in the stomach but most of them need further processing and travel into the intestine. The stomach turns food into chime, which is a liquid with a thick consistency. The pylorus is a muscular tube similar in size to a walnut that regulates the flow of chime, which is not allowed to enter the intestine until it has the right consistency. Inside the small intestine, nutrients from this liquid are absorbed into the bloodstream.

The small intestine

The small intestine consists of three separate segments. The first one, shaped like the letter C, is named the duodenum. The middle section is known as the jejunum and is coiled, while the last one is the ileum that connects with the large intestine.

The small intestine has walls covered with villi, millions of very small projections shaped like fingers. These are used to absorb the nutrients produced by digestion.

The liver

Three organs that are not part of the intestinal tract itself play a critical role in digestion. The first one is the liver, located on the upper abdomen just under the ribs. The gallbladder is found right next to it, while the pancreas is located behind the stomach.

The enzymes required for the digestion of carbohydrates, proteins and fats, as well as a compound used to neutralize stomach acid, are produced by the pancreas. Bile is needed for the absorption of fat and is supplied by the liver, the gallbladder just stores it for future use. Ducts are small channels that allow bile and enzymes to reach the small intestine, where they are used in digestion.

The liver plays further roles in the digestive function, as it is directly involved in processing nutrients. The blood coming from the small intestine carries them to the liver.

The large intestine

Another valve connects the small and large intestines and stops food from travelling in the opposite direction. An amount of water, as well as the food that hasn't been digested yet, reaches the large intestine. By this stage, most of the nutrients have already been extracted. However, the large intestine absorbs the remaining water and turns the undigested material into a solid form that can be excreted as waste.

The large intestine has three parts, just like the small one. It starts with the cecum, a segment shaped like a pouch that connects the small and large intestines and facilitates the transfer of food between them. A smaller pouch, shaped like a finger and known as the appendix, is attached to the cecum. This part doesn't seem to play any role in digestion anymore and scientists suspect it is a remnant from earlier human evolution.

The colon is a large part that connects the cecum to the rectum. It starts on the right part of the abdomen and then extends across the upper and left sides. It can also be divided in three parts: the ascending and transverse segments still play a role in digestion and help absorb salts and water. Friendly bacteria located in the colon allow the digestion of some of the nutrients that remain. The waste is then stored in the descending colon and finally reaches the rectum. Bowel movements eliminate feces from the body, which is the final phase of the digestive process.


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