Digestive system is a group of organs works together to perform the most essential functions of the human body. From ingestion to excretion are all functions included in this system.
It consists of a digestive tract which starts from mouth and ends at anus. It consists of oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
Digestion is the breakdown of food into small molecules, which are then absorbed into the body. The digestive enzymes produced by digestion system organs, plays a vital role in digestion. The internal linings of digestive tract also help food to absorb and assimilate in the body.
Though, the function of the digestive system is digestion and absorption majorly, the other functions are mentioned here.
- Ingestion is the process of eating. The process begins from mouth.
- Propulsion is the movement of food along the digestive tract. The major means of propulsion is peristalsis, a series of alternating contractions and relaxations of smooth muscle that lines the walls of the digestive organs and that forces food to move forward.
- Secretion of digestive enzymes and other substances liquefies, adjusts the pH of, and chemically breaks down the food.
- Mechanical digestion is the process of physically breaking down food into smaller pieces. This process begins with the chewing of food and continues with the muscular churning of the stomach. Additional churning occurs in the small intestine through muscular constriction of the intestinal wall. This process, called segmentation, is similar to peristalsis, except that the rhythmic timing of the muscle constrictions forces the food backward and forward rather than forward only.
- Chemical digestion is the process of chemically breaking down food into simpler molecules. The process is carried out by enzymes in the stomach and small intestines.
- Absorption is the movement of molecules (by passive diffusion or active transport) from the digestive tract to adjacent blood and lymphatic vessels. Absorption is the entrance of the digested food (now called nutrients) into the body.
- Defecation is the process of eliminating undigested material through the anus.
These are seven processes through which the food gets treated once we consume it.
The mouth is one of the most used parts of the body, mainly for eating, speaking and making facial expressions. At the same time, the mouth is the beginning of the digestive system as well.
The mouth consists of oral cavity and buccal cavity. This write-up will brief you about various parts of mouth.
There are 32 teeth in an adult person including incisors, cuspids, premolars, and molars. The teeth are embedded within sockets of the upper and lower jawbones. Upper jawbone is maxillae and lower jawbone is termed as mandible.
Each tooth is surrounded by gum, or gingival, and held in its socket by a periodontal ligament. A tooth has the following structural features like dentin, crown, enamel, and pulp cavity.
- The bulk of the mouth is called as Dentin. It is is a calcified tissue like bone.
- The crown is the portion of the tooth embedded in the bone.
- The neck is the region at the gum line where the crown and root meet.
- Enamel is the hard, nonliving material that covers the crown. Calcium compounds make the enamel the hardest substance in the body.
- Cementum is the bonelike substance that covers the root and binds it to the periodontal ligament.
- The pulp cavity is the central cavity inside the tooth. It contains blood vessels, nerves, and connective tissue.
The area between the cheeks and teeth and between the lips and teeth is called as vestibule.
The tongue is the lower boundary of the mouth. It helps position the food during mastication (chewing) and gathers the chewed food into a ball, or bolus, in preparation for swallowing.
The tongue is covered with papillae, small projections that help the tongue grip food. Many of the papillae bear taste buds.
The palate defines the upper boundary of the mouth. The forward portion is the hard palate, hard because bone (maxillae and palatine) makes up this portion of it.
Further back in the mouth, the soft palate consists of muscle and lacks any bone support. A conical muscular projection, the uvula, is suspended from the rear of the soft palate.
- Saliva consists of water (99.5 percent), digestive enzymes, lysozyme (an enzyme that kills bacteria), proteins, antibodies (IgA), and various ions.
- Saliva lubricates the mouth, moistens food during chewing, protects the mouth against pathogens, and begins the chemical digestion of food.
- Chemical digestion is carried out by the digestive enzyme salivary amylase, which breaks down polysaccharides (starch) into short chains of glucose, especially the disaccharide maltose (which consists of two glucose molecules).
- There are three pairs of salivary glands: the parotid (located near the masseter muscle), submandibular (located deep in the mandible), and sublingual (located under the tongue). They deliver their secretions to the mouth via ducts.
Apart from salivary glands, mouth also consists of Buccal glands which are located in the mucosa that lines the mouth. As, mouth is the beginning of digestive system, hence it is very important to keep your mouth healthy.
The pharynx, or throat, receives the food from the mouth during swallowing. From the mouth, it moves back and down into the oropharynx. Then it descends into the laryngopharynx. In proceed, the food then passes into the esophagus.
It is a 25‐cm (10‐inch) long tube. The esophagus begins at the laryngopharynx and descends behind the trachea through the mediastinum (cavity between the lungs).
The esophagus then passes through the diaphragm at an opening called the esophageal hiatus. It then turns just a bit to the left and connects to the stomach.
Peristalsis: Food is forced through the esophagus toward the stomach by peristalsis. Two sphincter muscles, the upper esophageal sphincter at the top of the esophagus and the lower esophageal sphincter at the bottom of the esophagus, control the movement of food into and out of the esophagus.
The stomach is a J‐shaped, baglike organ that expands to store food. In the stomach,
- The cells in mucosa secrete mucus, which protects the mucosa from the action of acid and digestive enzymes.
- The parietal cells secrete hydrochloric acid necessary for the absorption of vitamin B 12 in the small intestine.
- Zymogenic cells secrete pepsin that breaks down proteins.
- Enteroendocrine cells secrete various hormones that diffuse into nearby blood vessels. One important hormone, gastrin, stimulates other glands in the stomach to increase their output.
- There is secretion of gastric juice which enter the stomach and mix with food
The stomach serves a variety of functions:
- Storage. The wall of the stomach can expand to store two to four liters of material. Temporary storage is important because we eat considerably faster than you can digest food and absorb its nutrients.
- Mixing. The stomach mixes the food with water and gastric juice to produce a creamy medium called chyme.
- Physical breakdown. Three layers of smooth muscles (rather than the usual two) in the muscularis externa churn the contents of the stomach, physically breaking food down into smaller particles. In addition, HCl denatures (or unfolds) proteins and loosens the cementing substances between cells (of the food). The HCl also kills most bacteria that may accompany the food.
- Chemical breakdown. Proteins are chemically broken down by the enzyme pepsin. Chief cells, as well as other stomach cells, are protected from self‐digestion because chief cells produce and secrete an inactive form of pepsin, pepsinogen. Pepsinogen is converted to pepsin by the HCl produced by the parietal cells. Only after pepsinogen is secreted into the stomach cavity can protein digestion begin. Once protein digestion begins, the stomach is protected by the layer of mucus secreted by the mucous cells.
- Controlled release. Movement of chyme into the small intestine is regulated by a sphincter at the end of the stomach, the pyloric sphincter.
The Small Intestine
The small intestine (small in diameter compared to the large intestine) is divided into three sections
- The duodenum, about 25 cm (10 inches) long, receives chyme from the stomach through the pyloric sphincter. Ducts that empty into the duodenum deliver pancreatic juice and bile from the pancreas and liver, respectively.
- The jejunum, about 2.5 m (8 feet) long, is the middle section of the small intestine.
- The ileum, about 3.6 m (12 feet) long, is the last section of the small intestine. It ends with the ileocecal valve (sphincter), which regulates the movement of chyme into the large intestine and prevents backward movement of material from the large intestine.
The functions of the small intestine include the following:
- Mechanical digestion. Segmentation mixes the chyme with enzymes from the small intestine and pancreas. Bile from the liver separates fat into smaller fat globules. Peristalsis moves the chyme through the small intestine.
- Chemical digestion. Enzymes from the small intestine and pancreas break down all four groups of molecules found in food (polysaccharides, proteins, fats, and nucleic acids) into their component molecules.
- Absorption. The small intestine is the primary location in the GI tract for absorption of nutrients.
- The components of carbohydrates, proteins, nucleic acids, and water‐soluble vitamins are absorbed by facilitated diffusion or active transport. They are then passed to blood capillaries.
- Vitamin B 12: Vitamin B 12 also passed to the blood capillaries.
- Lipids and fat‐soluble vitamins: Because fat‐soluble vitamins and the components of lipids are insoluble in water, they are packaged and delivered to cells within water‐soluble clusters of bile salts called micelles. They are then absorbed by simple diffusion and, once inside the cells, mix with cholesterol and protein to form chylomicrons. The chylomicrons are then passed to the lymphatic capillaries. When the lymph eventually empties into the blood, the chylomicrons are broken down by lipoprotein lipase, and the breakdown products, fatty acids and glycerol, pass through blood capillary walls to be absorbed by various cells.
- Water and electrolytes: About 90 percent of the water in chyme is absorbed, as well as various electrolytes (ions), including Na +, K +, Cl –, nitrates, calcium, and iron.
Mucosa of small intestine plays several functions like;
- The circular folds are permanent ridges in the mucosa that encircle the inside of the small intestine. The ridges force the food to spiral forward. The spiral motion helps mix the chyme with the digestive juices.
- Villi (singular, villus) are fingerlike projections that cover the surface of the mucosa increase the surface area over which absorption and digestion occur. It also consists of intestinal glands which secretes intestinal juices.
- Microvilli are microscopic extensions of the outer surface villus. Because of their brushlike appearance, they also increase the surface area over which digestion and absorption.
The submucosa that underlies the mucosa of the small intestine consists of
- Duodenal glands, found only in the submucosa of the duodenum, secrete an alkaline mucus that neutralizes the gastric acid in the incoming chyme.
- Peyer’s patches (aggregated lymphatic nodules), found mostly in the submucosa of the ileum, are clusters of lymphatic nodules that provide a defensive barrier against bacteria.
The large intestine is about 1.5 m (5 feet) long and is characterized by the following components:
- The cecum is a dead‐end pouch at the beginning of the large intestine, just below the ileocecal valve.
- The appendix (vermiform appendix) is an 8‐cm (3‐inch) long, fingerlike attachment to the cecum that contains lymphoid tissue and serves immunity functions.
- The colon, representing the greater part of the large intestine, consists of four sections: the ascending, transverse, descending, and sigmoid colons. At regular distances along the colon, the smooth muscle of the muscularis layer causes the intestinal wall to gather, producing a series of pouches called haustra. The epithelium facing the lumen of the colon is covered with openings of tubular intestinal glands that penetrate deep into the thick mucosa. The glands consist of absorptive cells and goblet cells. The absorptive cells absorb water and the goblet cells secrete mucus. The mucus lubricates the walls of the large intestine to smooth the passage of feces.
- The rectum is the last 20 cm (8 inches) of the large intestine. The mucosa in the rectum forms longitudinal folds called anal columns.
- The anal canal, the last 3 cm (1 inch) of the rectum, opens to the exterior at the anus. An involuntary (smooth) muscle, the interior anal sphincter, and a voluntary (skeletal) muscle, the external anal sphincter, control the release of the feces through the anus.
The functions of the large intestine include the following:
- Peristalsis: Rhythmic contractions of the large intestine produce a form of segmentation called haustral contractions in which food residues are mixed and forced to move from one haustrum to the next. Peristaltic contractions produce mass movements of larger amounts of material.
- Bacterial digestion: Bacteria that colonize the large intestine digest waste products. They break down indigestible material by fermentation, releasing various gases. Vitamin K and certain B vitamins are also produced by bacterial activity.
- Absorption: Vitamins B and K, some electrolytes (Na + and Cl –), and most of the remaining water is absorbed by the large intestine.
- Defecation: Mass movement of feces into the rectum stimulates a defecation reflex that opens the internal anal sphincter. Unless the external sphincter is voluntarily closed, feces will be evacuated through the anus.
Why they say All diseases begin in the gut
The secretions of the pancreas, called pancreatic juice, include various enzymes, including pancreatic amylase (digestion of starch), trypsin, carboxypepiydase, and chymotrypsin (proteases), as well as pancreatic lipase (digestion of fats).
Sodium bicarbonate is also produced, making the pancreatic juice alkaline. This alkaline solution stabilizes the pH in the duodenum, thus providing an optimal environment for the action of these enzymes.
Pancreatic juice is produced in clusters of exocrine cells called acini. The remaining cells in the pancreas (about 1 percent of the total) also form clusters (pancreatic islets). These are the endocrine cells that produce the hormones insulin, glucagon, somatostatin, and pancreatic polypeptide.
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Pancreatic juice collects in small ducts that merge to form two large ducts. The main pancreatic duct exits the pancreas and merges with the common bile duct from the liver and gallbladder. This combined duct is called hepatopancreatic ampulla.
This combined duct then enters the duodenum by passing through the hepatopancreatic sphincter. A smaller, second duct that exits the pancreas, the accessory pancreatic duct, joins the duodenum directly.
The digestive function of the liver is to produce bile, which is then delivered to the duodenum to emulsify fats. Emulsification is the breaking up of fat globules into smaller fat droplets, increasing the surface area upon which fat‐digesting enzymes (lipases) can operate.
Because bile is not involved in breaking any chemical bonds, it is not an enzyme. It is an emulsifier. Bile is also alkaline, serving to help neutralize the HCl in the chyme.
Bile consists of bile salts, bile pigments, phospholipids (including lecithin), cholesterol, and various ions. The primary bile pigment, bilirubin, is an end product of the breakdown of hemoglobin from expended red blood cells.
The bile that is lost via the feces consists of bilirubin. This is the body’s natural way of getting rid of bilirubin. Bilirubin gives the feces a brown color.
The liver performs numerous metabolic functions. Some of the most important follow:
- Bile is produced.
- Blood glucose is regulated. When blood glucose is high, the liver converts glucose to glycogen ( glycogenesis) and stores the glycogen. When blood glucose is low, glycogen is broken down ( glycogenolysis), and glucose is released into the blood.
- Proteins (including plasma proteins) and certain amino acids are synthesized.
- Ammonia (which is toxic) is converted to urea (less toxic) for elimination by the kidneys.
- Bacteria and expended red and white blood cells are broken down. From the red blood cells, iron and globin are recycled, and bilirubin is secreted in the bile.
- Vitamins (A, D, and B 12) and minerals (including iron from expended red blood cells) are stored.
- Toxic substances (drugs, poisons) and hormones are broken down.
The liver is composed of numerous functional units called lobules. Within each lobule, epithelial cells called hepatocytes are arranged in layers that radiate out from a central vein. Hepatic sinusoids are spaces that lie between groups of layers, while smaller channels called bile canaliculi separate other layers.
Each of (usually) six corners of the lobule are occupied by three vessels: one bile duct and two blood vessels (a portal triad). The blood vessels are branches from the hepatic artery (carrying oxygenated blood) and from the hepatic portal vein (carrying deoxygenated but nutrient‐rich blood from the small intestine).
Blood enters the liver through the hepatic artery and hepatic portal vein and is distributed to lobules. Blood flows into each lobule by passing through the hepatic sinusoid and collecting in the central vein. The central veins of all the lobules merge and exit the liver through the hepatic vein (not the hepatic portal vein).
Within the sinusoids, phagocytes called Kupffer cells (stellate reticuloendothelial cells) destroy bacteria and break down expended red and white blood cells and other debris. Hepatocytes that border the sinusoids also screen the incoming blood.
They remove various substances from the blood, including oxygen, nutrients, toxins, and waste materials. From these substances they produce bile, which they secrete into the bile canaliculi, which empty into bile ducts. Bile ducts from the various lobules merge and exit the liver as a single common hepatic duct.
The common hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct, which in turn merges with the pancreatic duct to form the hepatopancreatic ampulla. This last duct delivers the bile to the duodenum.
The gallbladder stores excess bile. When food is in the duodenum, bile flows readily from the liver and gallbladder into the duodenum.
When the duodenum is empty, a sphincter muscle (hepatopancreatic sphincter) closes the hepatopancreatic ampulla, and bile backs up and fills the gallbladder.
The process of swallowing is divided into three phases;
- The buccal phase occurs voluntarily in the mouth when the tongue forces the bolus of food toward the pharynx.
- The pharyngeal phase occurs involuntarily when food enters the pharynx, as follows:
- The soft palate and uvula fold upward and cover the nasopharynx to prevent the passage of food up and into the nasal cavity.
- The epiglottis, a flexible cartilaginous flap at the top of the larynx, folds down as the larynx rises. As a result, the opening to the trachea is covered, and food can pass only into the esophagus.
- The esophageal phase occurs involuntarily in the esophagus. The esophageal sphincter, normally closed, opens to allow food to pass when the larynx rises during swallowing. When food reaches the lower end of the esophagus, the lower esophageal sphincter opens to allow the food to enter the stomach.
Digestive Tract Wall
The digestive tract elongates from esophagus to anus. It is characterized by a wall with four layers. The layers from the inside lining of the tract to the outside lining are described below;
The mucosa – It is a mucous membrane that lines the inside of the digestive tract from mouth to anus. Depending on the section of the digestive tract, it protects the digestive tract wall, secretes substances, and absorbs the end products of digestion. It is composed of three layers:
The epithelium is the innermost layer of the mucosa. It is composed of simple columnar epithelium or stratified squamous epithelium. Also present are goblet cells and endocrine cells. Goblet cells secrete mucus that protects the epithelium from digestion, and endocrine cells secrete hormones into the blood.
The lamina propria lies outside the epithelium. It is composed of areolar connective tissue. Blood vessels and lymphatic vessels present in this layer provide nutrients to the epithelial layer, distribute hormones produced in the epithelium, and absorb end products of digestion from the lumen.
The lamina propria also contains the mucosa‐associated lymphoid tissue (MALT), nodules of lymphatic tissue bearing lymphocytes and macrophages that protect the GI tract wall from bacteria and other pathogens that may be mixed with food.
The muscularis mucosae, the outer layer of the mucosa, are a thin layer of smooth muscle responsible for generating local movements. In the stomach and small intestine, the smooth muscle generates folds that increase the absorptive surface area of the mucosa.
- The submucosa lies outside the mucosa. It consists of areolar connective tissue containing blood vessels, lymphatic vessels, and nerve fibers.
- The muscularis (muscularis externa) is a layer of muscle. In the mouth and pharynx, it consists of skeletal muscle that aids in swallowing. In the rest of the digestive tract, it consists of smooth muscle (three layers in the stomach, two layers in the small and large intestines) and associated nerve fibers. The smooth muscle is responsible for movement of food by peristalsis and mechanical digestion by segmentation. In some regions, the circular layer of smooth muscle enlarges to form sphincters, circular muscles that control the opening and closing of the lumen (such as between the stomach and small intestine).
- The serosa is a serous membrane that covers the muscularis externa of the digestive tract in the peritoneal cavity.
The Digestive enzymes
We eat food. The food needs to be digested. Hence, there is the role of digestive enzymes come in picture. They are the substances produced by our body which helps in digesting food.
These enzymes are the result of secretion by digestive system organs. During digestion, the carbohydrates, proteins, amino acids, lipids, and nucleic acids are broken down into its molecular components and turn them into nutrients required for our body.
The various digestive enzymes are mentioned below;
- Complex carbohydrates, or polysaccharides (such as starches), are broken down into oligosaccharides (consisting of two to ten linked monosaccharides), disaccharides (such as maltose), or individual monosaccharides (such as glucose or fructose). Enzymes called amylases break down starch.
- Proteins are broken down into short chains of amino acids (peptides) or individual
- Amino acids by enzymes called proteases.
- Lipids are broken down into glycerol and fatty acids by enzymes called lipases.
- Nucleic acids are broken down into nucleotides by enzymes called nucleases.
|salivary amylase||starches||maltose, oligosaccharides|
|pancreatic lipase||fats||fatty acids. Monosaccarides|
|nucleosidases||dipeptides||nitrogen bases, ribose, deoxyribose, phosphates|
|phosphatases||nucleotides||nitrogen bases, ribose, deoxyribose, phosphates|
Benefits of the healthy digestive system
It is famous saying that for a healthy body, you need healthy digestion. Here are the benefits of healthy digestion;
- You do not experience bloating or digestive pain
- Manage appetite and so the body weight
- Manage cognitive health
- Proper stool
- Improves immune system
- Help you to live a longer life
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