The Digestive System

The digestive system is responsible for the digestion of food and the extraction of nutrients, so the body can later use them. The digestive system has two types of organs: the organs connected to the digestive tract (i.e. stomach, esophagus, etc.) and the subsidiary organs of digestion (i.e. liver, gallbladder, etc.). While the organs that belong to the digestive tract are directly involved with the processing of food, i.e. the nutrition goes through them, the subsidiary organs only help digest the food by producing substances. These substances chemically decompose the food.

The Mouth

The mouth (Latin: os) is the first stop for the food inserted in our body. It’s also the “entrance” for it and is a part of the digestive tract. With the help of the teeth and the tongue, it starts crushing the food mechanically. The salivary glands are connected to the mouth and transport saliva to it, which mixes the crushed food into a mash, so it’s not too difficult to swallow. After the food has been processed, it goes into the food pipe and starts its way to the stomach.

The saliva contains different enzymes (for example, amylase). They initiate the digestion of carbohydrates. There are also oral microorganisms that also help with digestion. The microorganisms are located between the teeth, on the tongue, and in the mucosa.

The taste buds are located on the tongue. They have many receptor cells that make us taste. They perceive the taste after they come in contact with the taste substances that are dissolved in saliva. The saliva also has the task of preventing the mucosa from drying. It also has the function of neutralizing the pH so it doesn’t damage our body.

The Esophagus/The Food Pipe

The esophagus (also spelled esophagus) is a muscular tube. Food goes through it from the mouth to the stomach. The food pipe can contract and expand to allow food to pass through. The location of the esophagus is behind the trachea and the heart and in front of the spine. Both ends are closed off by the sphincters (the anterior and the upper). They are muscular narrowings at both ends of the food pipe.

The upper sphincter is made of circular muscle tissues, which stay closed almost the whole time. They relax if food has to enter the food pipe. Moments after the food enters the esophagus the sphincter closes. Doing this prevents the nutrition from coming back. Contractions of the peristalsis move the food to the stomach. Once it has reached the anterior sphincter, it opens and lets the food enter the stomach, where it can be digested by gastric juices.

The Stomach

The stomach is the organ where a part of the digestion takes place. The gastric juices process the food chemically. After that, it mechanically distributes the food into the intestines.

The human stomach is subdivided into four regions: the cardiac opening (opening from the esophagus into the stomach), the “body” (the central and largest region), the antrum (the lowermost portion), and the pylorus (contraction where food enters the intestine). Each opening (the cardiac and the pyloric) is closed by sphincter muscles, which only open when the food is ready for the final phases of its digestion.

The stomach can contract if there is not enough food in it. The interior walls form numerous folds when the stomach contracts. These folds (rugae) disappear when the organ exits its contracted form.

The stomach’s muscles rarely rest. They knead and mix the food with the gastric juice through constant periodic contractions. The food becomes a semifluid substance called chyme. The muscles also wave the food into the small intestine with the help of peristaltic contractions. These contractions continue even after all food has exited the stomach, which can lead to aches after a significant amount of time, which we perceive as hunger. Those belly aches can also be caused by blood sugar levels.

The absorption of substances in the stomach is negligible, but things like alcohol, drugs and iron are absorbed directly. The stomach is mainly controlled by the vagus nerve and the sympathetic nervous system. Emotions can alter how the stomach works.

Gastric juice (or gastric acid) is an acid, so it has a low pH and it's located in the stomach. This acid activates enzymes (e.g. pepsin). Pepsin cuts protein into amino acids. The gastric acid is neutralized by bicarbonate in the duodenum.

The Small intestine

The small intestine is a long, narrow tube that links the stomach and the large intestine. Most of the digestion and absorption take place in the small intestine. It's about 7 meters long, highly tangled, and placed in the abdominal cavity. The mesentery is a thin membranous (separating from other environments) material that somewhat supports and suspends the intestine. The mesentery contains many blood vessels, as well as fat that retains the temperature of the intestines. The small intestine is also connected to various nerves that generate peristalsis.

The small intestine is subdivided into three successive regions: the duodenum, the jejunum, and the ileum. The three of them make a continuous tube. Although they show differences and unique characteristics, there are no marked separations.

The duodenum is adjacent to the stomach. It has the widest diameter, being 23 to 28 centimeters long, and it’s not supported by the mesentery. Ducts from the liver, the gall bladder, and the pancreas lead to substances inside that neutralize gastric acid. The juices also help digest proteins, fats, and carbohydrates.

The jejunum follows after the duodenum. It’s about two-fifths of the remaining small intestine. Its color is deep red because of the extensive blood supply, and the peristaltic movements are rapid. The mesentery has low-fat levels in this region.

The ileum is the last region of the small intestine. It’s located in the lower abdomen. The blood supply is limited, the peristaltic movements are slower, and the walls are thinner. The mesentery also has more fatty areas.

In mammals the mucous membrane forms finger-like projections known as villi. These villi project into the cavity. These structures highly increase the absorption surface. Peristaltic waves move materials in the small intestines while churning movements are called rhythmic segmentation. These movements mix the food with the enzymes, break materials and push it against the wall that absorbs vital substances.

The Large intestine

The large intestine is the last part of the intestine. It is subdivided into four regions: the cecum (to which the appendix is connected), colon, rectum, and anus. The whole large intestine is sometimes called the colon, which also is one of the regions. The large intestine is shorter than the small intestine (it's 1.5 meters, while the small intestine is almost 7 meters), but it's also wider and has smooth walls. In the upper part, around half of the large intestine, enzymes finish the digestive process, and bacterias produce B and K vitamins.

The main function, however, is the absorption of the remaining water and electrolytes from the digestive residues, the remaining food when it enters the large intestine, and storing the fecal matter, the useless remains, until they are expelled. Churning movements push the digestive residues against the absorbing walls. The gastrocolic reflex propels the residues in the anus, which on average occurs two or three times a day and leads to defecation.

The Liver

The liver is the largest gland in our body. It has many functions. First one being including metabolizing substances like proteins, fats, and carbohydrates. Another one is secreting bile, a digestive fluid, storing substances like glycogen and vitamins. Last one is removing toxic substances from the blood, regulating blood volume, and destroying old blood cells. The liver consists of a mass of cells tunneled by blood vessels and bile ducts. Hepatic cells make up to 60 percent of the tissue and perform more metabolic functions than any other cells. The other cells are the Kupffer cells. They are important in blood formation, antibody production, and ingestion of foreign particles and cellular debris.

The liver secretes bile that contains the necessary salts for the fats. Bile is also responsible for the excretion of some metabolic products, drugs, and toxic substances. The duct that carries bile leads from the liver into the duodenum. The duodenum is connected to the gallbladder, which stores the bile and secretes it if needed. The bile is in a greenish-orange color. Bilirubin, a pigment, causes this. Senescent (worn-out) blood cells are also destroyed in the livers, spleen, or bone marrow.

The liver also synthesizes a lot of enzymes. Beforehand, it filters them out of the blood flowing through the portal vein or hepatic artery. Nutrients can be stored to be used later. Fats are converted into carbohydrates, then taken by the blood to the tissue where they can be further metabolized. Sugar is converted into glycogen, which is stored in the liver until it’s needed for energy production. It is then reconverted into glucose and released into the blood. The liver also manufactures blood serum, the liquid part of the blood. The liver also metabolizes waste products and detoxifies substances, making them ready for the elimination of feces and urine.

The Gall Bladder

The gallbladder is a muscular membranous sac. It stores bile, concentrates it, and secretes it, in case it’s needed for digestion. It has a capacity of around 50 ml. The inner wall has mucous-membrane tissue similar to the one of the small intestine. They have millions of microvilli that increase the area of fluid absorption. It absorbs the water and the salt out of the bile.

The contractions of the gallbladder, which secrete bile into the duodenum, are generated by the vagus nerve and cholecystokinin. This hormone is produced in the gallbladder. The duct that leads the bile is composed of 3 “branches” forming a Y. The lower part leads into the duodenum through the duodenal wall in the small intestine. A sphincter muscle, the sphincter of Oddi, regulates the flow of bile into the small intestine if food is present there. The right branch leads into the liver, the left into the gallbladder.

The Pancreas

The pancreas is a gland that discharges enzymes into the intestine. It also secretes vital insulin and glucagon into the bloodstream. It is adjacent to the duodenum, weighs about 80 grams, and is the shape of a pear. In adults most of the pancreatic tissue has exocrine functions, secreting useful digestion enzymes that are produced by the acinar cells.Their name comes from the Latin word acinus, which means grape. They are called that because of their formations, which look like a cluster of grapes. The islets of Langerhans are located between them and are devoted to the endocrine tasks of the pancreas.

The digestive enzymes help with the digestion of fats, proteins, and carbohydrates. They pass through ducts from the pancreas into the duodenum. Their flow is controlled by the hormones cholecystokinin and secretin, but also by the vagus nerve. The hormones are released into the blood when foods enter the duodenum. When the pancreas receives the hormones, it produces large amounts of water, bicarbonate, and enzymes, which are released into the duodenum.

The endocrine part of the pancreas weighs only around 1 gram and has about 1 million cells. The beta cells produce insulin. About 75% of the endocrine cells are beta cells. The rest produces either glucagon, somatostatin, or pancreatic polypeptide. Each Langerhans islet (they have the hormone-producing cells) is connected to one or two arterioles that branch into numerous capillaries. These capillaries emerge into veins. The main function of the endocrine pancreas is secreting the hormones, which are essential for the regulation of storage or mobilization of glucose, amino acids, and triglycerides in the cells. The functions of the islets are controlled by circulating metabolites or hormones, nerves, or local hormones. While insulin makes the cells store glucose as glycogen, glucagon makes them release glucose into the bloodstream.