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Human Digestive System
Digestion is a complex process wherein the large complex food molecules are broken down into simpler food particles containing nutrients that are readily absorbed by the cells of the body, used to maintain vital body functions while the waste particles are expelled out of the body through the excretory system. The process of digestion, rightly, begins with the mouth and ends with the anus. It involves the gastrointestinal tract and other accessory organs—tongue, salivary glands, liver, gallbladder, and pancreas.
The gastrointestinal tract—also known as the digestive tract, alimentary canal, digestion tract, GI tract, GIT, or gut—an organ system within humans and other multicellular organisms that take in food, digest it to extract nutrients and absorb energy, expels the remaining waste as feces. The mouth, throat, esophagus, stomach, small intestine, colon are part of the digestive tract that forms a series of hollow organs joined in a long, twisting tube from the mouth to the anus.
The process of digestion begins at the mouth, the beginning of the digestive tract. As soon as the first bite of a meal is taken, chewing starts breaking down food into smaller pieces, while saliva mixes with food to begin the process of breaking it down into a form a human body can absorb and use.
It's a muscular tube, commonly known as the food pipe or gullet, connecting the throat (i.e., pharynx) with the stomach. Food that is swallowed does not enter the stomach directly through the food pipe, its walls propel food to the stomach by rhythmic waves of muscular contractions called peristalsis. The esophagus runs behind the windpipe called trachea and heart, and in front of the spine.
The stomach is a hollow muscular organ in the GI tract, located between the esophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion and produce gastric chyme—partially digested food. There is a valve called the pyloric sphincter that controls the passage of chyme from the stomach into the duodenum. Here peristalsis movement work.
The intestine is a long, continuous, muscular tube running from the end of the stomach to the anus, an opening at the lower end of the digestive tract. It is also called the bowel, responsible for the absorption of most of the nutrients from food and water. It is divided into three segments—small intestine, large intestine, and rectum.
Small intestine: The small intestine is the longest part of the GI tract, lies between the stomach and large intestine. It has three segments—duodenum, jejunum, and ileum. Duodenum is the first section of the small intestine; jejunum is the middle; and ileum, the last section of the small intestine, continues into the first part of the large intestine (i.e., caecum) joined through the ileocecal sphincter (i.e., a valve). Its function is to absorb nutrients—water, salts, minerals. Its length is about six to seven meters with a surface area of over 200 meters. After death, the length of the small intestine can increase by up to half.
Duodenum: It receives a semisolid sludge of partially digested food called chyme from the stomach. Food enters this segment through the pyloric sphincter. A digestive juice called bile (stored and concentrated in the gallbladder) produced by the liver and enzymes from the pancreas—trypsin, amylase, and lipase— helps further break down the food in the duodenum, helps in the digestion of proteins, carbohydrates, and fats. Some minerals, such as iron and calcium, are absorbed here.
Jejunum: The main function is the absorption of nutrients, that is, sugars, fatty acids, amino acids are absorbed.
Ileum: The ileum is the longest part of the small intestine, absorbs vitamin B12, salts, minerals, and the products that were not absorbed by the jejunum. After absorbing most of the essential nutrients it empties into the large intestine.
The internal walls of the jejunum and ileum contain finger-like projections (i.e., tissues) called villi and each villus is covered with microscopic hair-like structures called microvilli. These villi and microvilli increase the surface area in the intestine available for maximum absorption of nutrients.
Large intestine: The large intestine is much broader than the small intestine. It includes the appendix, cecum, colon, rectum, and anal canal. The appendix is a finger-shaped pouch attached to the cecum, the first part of the large intestine; next is the colon consisting of three parts—ascending colon, transverse colon descending colon—and the rectum is the end of the large intestine. The large intestine is about 5 feet long with an average diameter of 3 inches. The major functions of the large intestine are:
- reabsorption of water and electrolytes (i.e., mineral ions such as sodium and chloride)
- formation and storage of feces (i.e., absorbs water from wastes creating stool)
- fermentation of some of the indigestible food matter by bacteria.
Tongue and salivary Glands
Tongue: It's a muscular organ in the buccal cavity, covered with a mucous membrane called the mucosa. The mucous membrane is an epithelial cell, moist and pink, secretes a gel-like fluid containing mainly water.
The pancreas is a composite organ in the digestive system, located in the abdominal cavity behind the stomach. It weighs about 70 to 100 grams, six to eight inches long. It performs two main functions:
- as an exocrine gland, it secretes pancreatic juice, about 1.5 to 2 liters of digestive juices per day, into the small intestine through a tube known as the pancreatic duct.
- as an endocrine gland, it secretes the hormones insulin, glucagon, somatostatin, and pancreatic polypeptide—directly into the bloodstream—functioning mostly to regulate blood sugar levels.
Pancreatic juice is a clear and colorless fluid, mainly made up of water. It also contains salt, sodium bicarbonate, and digestive enzymes. There are three main types of pancreatic enzymes:
- Lipases: break down fats into fatty acids and cholesterol.
- Trypsin and chymotrypsin: break down proteins.
- Amylases: break down carbohydrates.
The liver looks reddish-brown having soft rubbery tissues, situated above the stomach and towards the left, below the diaphragm. The liver of an adult person weighs about 1.4 kg (i.e., 3.1 pounds). It is an accessory organ required in the human digestive system. It performs various functions that aids in metabolism, digestion, detoxification, immunity, vitamin storage, and many more. The liver filters the blood—carrying nutrients, medication, and also toxic substances—coming from the digestive tract before passing it to the rest of the body while the waste products, even alcohol, are released into the bowels to be eliminated. In this way, the liver can get rid of waste products from the breakdown of medications.
Lobules are the small divisions of the liver containing millions of hepatic cells—the chief functional units of the liver. Blood enters the liver through the hepatic artery which brings oxygen-rich blood. Blood coming from digestive organs enters the liver through the portal vein bringing digested nutrients.
When the meal is taken the liver produces bile—a greenish-yellow, thick and sticky fluid—containing cholesterol and acids with an alkaline compound that helps the breakdown of fat. The bile flows out of the liver through the left and right hepatic ducts, joins together to form the common hepatic duct. This duct then joins the cystic duct, connected to the gallbladder, to form the common bile duct that joins the duodenum at the sphincter of Oddi—a ring-shaped muscle.
The gallbladder is a small, pear-shaped, muscular pouch that holds bile. It is positioned just below the liver, interconnected by biliary ducts. Bile is released each time the meal is taken.
About half the bile secreted between meals directly enters the small intestine through the common bile duct. The rest of the bile is diverted into the gallbladder (via the cystic duct) to be stored. Salts present in the bile aid in digestion by making fats, fat-soluble vitamins, and cholesterol. These are easily absorbed from the intestine. Further, bile helps in the absorption of vitamin K.
Bilirubin, a waste product formed from hemoglobin, is the main pigment in bile and excreted in bile.
More about the liver:
- In the presence of vitamin K, the liver produces proteins required in blood clotting.
- It breaks down old or worn-out cells.
- The liver aids in the metabolism of fats, its cells produce energy. The liver can produce about 800 to 1,000 ml of bile per day.
- The liver is one of the largest solid organs and the largest gland in the human body.
- The liver is the only organ that can regenerate (i.e., it can regrow rapidly).
- Skin is the only organ that is heavier and larger than the liver.
- The iron released from hemoglobin while its breakdown is stored in the liver or bone marrow which is then used to make the next generation of blood cells. It also stores copper.
- The liver acts as a reservoir of blood glucose, stores glucose as glycogen during meals for a later time to be used by the body. It supplies sugar when the body needs it, thus help in maintaining the blood sugar level.
- The liver produces albumin, the most common protein in blood serum, transports fatty acids and steroid hormones to help maintain the correct pressure. Further, it prevents the leaking of blood vessels.
- The liver stores significant amounts of vitamins—vitamins A, D, E, K, and B12.
- The liver converts a toxic substance like ammonia to urea, a less toxic substance. Urea gets released into the blood and then transported to the kidneys and excreted out of the body in the form of urine.
Thus, different organs of the body attached to the GI tract help in digestion, thereby absorption of energy and nutrients to make a multicellular organism work.
- Collins JT, Badireddy M. Anatomy, Abdomen and Pelvis, Small Intestine. [Updated 2019 Apr 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459366/
- InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. How does the liver work? 2009 Sep 17 [Updated 2016 Aug 22]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279393/
- InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. How does the pancreas work? 2009 Nov 11 [Updated 2018 Sep 6]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279306/
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
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