Human Anatomy

The Digestive System

Digestion is the breakdown of nutrients through mechanical and enzymatic action into small, water-soluble molecules that can be absorbed into the bloodstream.

Organs of Digestion

Also known as the alimentary system, the digestive system is one of the largest organ systems in the body. It allows us to take in food, extract the nutrients for energy, growth, and repair, and excrete waste.

The digestive system extends from the moth through the stomach and intestines to the anus. Throughout this pathway, food is mechanically and chemically processed, nutrients are absorbed, and the undigested remains are excreted. The digestive tract is made up of the mouth (oral cavity); pharynx; esophagus; stomach; small intestine, including the duodenum, jejunum, and ileum; and large intestine, including the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The accessory components of the digestive system are the teeth, tongue, salivary glands, liver, pancreas, gallbladder, and peritoneum.

The secretions of the salivary glands, liver and pancreas help extract nutrients from food. The process of digestion, however, happens in several steps.

The first step is mastication. It takes place in the mouth and refers to the teeth cutting, tearing, and grinding food into smaller chunks. The tongue then swirls these chunks with saliva to form a rounded mass that is called a bolus.

The next step is swallowing, which is when the bolus of food is pushed into the oropharynx by the tongue. From there, the bolus continues to the laryngopharynx, and the swallowing reflex is initiated at the esophagus. Peristalic waves then move the bolus to the lower esophageal sphincter.

Digestion is the next step, when complex molecules are broken down into smaller molecules that can be easily absorbed. Amylase in saliva start digesting carbohydrates, but most chemical digestion happens in the stomach and small intestine. Secretions from the digestive system’s accessory organs and glands - the liver, pancreas, and gallbladder - help the digestive process.

Absorption is the next step, when the smaller molecules as well as electrolytes and water cross the digestive epithelium into the interstitial fluid of the digestive tract. Much of this absorption happens in the small intestine.

Excretion, the next step, involves the removal of undigested food residue and the waste products of metabolism. Accessory organs like the liver excrete waste products into the digestive tract.

Compaction is when the large intestine moves the left over from the digestive process via peristalsis. As this waste descends through the colon, it is dehydrated and compacted.

Defecation occurs when the compacted waste, i.e feces, is eliminated through the anus.

The organs of the lower digestive system are efficiently arranged in the abdominal cavity: the intestines curl around themselves, the stomach and liver fit around each other, and the gallbladder and pancreas are tucked in behind.

Intestinal villi: These small, fingerlike projections - each roughly 1/64 to 1/16 inch (0.5-1.6 mm) in length - increase the surface area of the intestines, so they more readily absorb nutrients. They extend into the lumen (or hollow space) of the small intestine and contain many tiny, bristly microvilli.

Liver: creates chemicls the body needs to function normally, breaks down and detoxifies substances, and acts as storage unit.

Stomach: breaks down consumed foods and liquids before they are passed to the small intestine.

Small intestine: where most chemical digestion takes place.

Larges intestine: the final stage of the digestive tract; its primary function is absorbing water and electrolytes from feces.

Appendix: contains beneficial bacteria.

Rectum: acts as a temporary storage site for feces.

Eating and Swallowing

Eating is the simple act of taking in food through the mouth. Swallowing is a more complicated process, which requires coordinated activity of the tongue, palate, pharynx, and esophagus - more than 22 muscle groups in all, which are controlled by separate regions of the brain.

Eating is the ingestion of food through the mouth and the rest of the digestive tract. Swallowing is the act of pushing food from the mouth to the esophagus and down into the gastrointestinal tract. The human body is equipped with strict controls that help ensure food does now enter the trachea, which leads to the lungs, as it is swallowed. If food particles accidentally enter the trachea, it results in choking.

The tongue plays an important role in eating and swallowing. Firstly, it provides mechanical digestion by compressing, abrading, and contorting food. As the tongue manipulates food during mastication (chewing), it shapes it into a bolus that is suitable for swallowing. The tongue secretes mucin, which provides lubrication, and lipase, which begins to break down fat.

The tongue’s movements also pass food across the surfaces of the teeth. The buccal, labial, and lingual muscles help move food around the mouth while it is being chewed. These muscles close the jaws and slide or rock the lower jaw from side to side. This helps break down the tough connective tissues in meat and the plant fibers in fruits and vegetables. The mouth also saturates food with saliva and enzymes, including salivary amylase, which starts breaking down carbohydrates.

Once food is throughly chewed and formed into a bolus, it is squeezed through the pharynx into the esophagus via peristaltic contractions. As food is swallowed, the tongue blocks the mouth, the palate rises to close off the nasopharynx, and the larynx rises so that the epiglottis (a muscular flap) covers the opening to the respiratory passageway.

At this stage, food moves through the esophagus, and the cardiac (or lower) esophageal sphincter muscle opens to let food pass into the stomach. Weakening of this sphincter at the stomach secretions to come back up into the esophagus, which is a condition called reflux.

Frontal sinus: air-filled sacs on either side of nasal cavity that filter and clean air breathed through nose.

Conchae: increase surface area of nasal cavity.

Sphenoid sinus: the sinus cavities that are furthest back.

Epiglottis: a small movable flap that prevents food and liquids from entering the larynx.

Nose and throat: Humans are able to breathe through both the nose and the mouth, which allows them to continue taking in vital oxygen while they are eating.

Peristalsis ia a pattern of smooth muscle contractions and relaxations. In the digestive tract, they propel a bolus of food through the esophagus, stomach, and intestines. The esophagus is made up of both circular and longitudinal muscles. During a peristaltic wave, the circular muscles contract behind the bolus, pushing it along. Peristaltic waves tend to be unidirectional, so the food only goes down, not back up. Longitudinal muscles also contract, shortening adjacent segments of the esophagus, which propels the bolus further. This wave of contractions runs the entire length of the esophagus, pushing the bolus all the way to the stomach. It is believed that peristalsis is mediated by the local, intrinsic nervous system.

Teeth: There are four types of teeth, and they have specific chewing functions. Incisors clip and cut food. They are the blade-shaped teeth found at the front of the mouth. Canines tear and slash food. They are conical teeth with a sharp Ridgeline and a pointed tip and are found either side of the incisors. Premolars have cusps, and molars have flat surfaces that crush, mash, and grind food. Premolars and molars are found at the back of the mouth.

The Digestive Process

Two kinds of processes take place during digestion: mechanical and chemical. Physical processes ensure the food being eaten is properly formed and moves through the gastrointestinal tract. The chemical processes breaks down the food’s complex molecules into simpler and smaller molecules that can be easily absorbed into the bloodstream.

The digestion process begins in the mouth, with the secretion of the enzyme amylase.

The mechanical processes of digestion involve making the ingested food smaller, both to increase its surface area so the chemical processes can work efficiently and to help it move through the gastrointestinal tract. Various organs and other parts of the body are involved in mechanical digestion.

As previously describe, the teeth shred and cut up the food while it is in the mouth. This mastication breaks food down into smaller pieces and saturates it with saliva. The tongue is also involved, forming the food into a bolus. Once it forms the bolus, the tongue propels it to the pharynx, and the bolus then travels through the esophagus to the stomach.

The stomach further mechanically breaks food down by mixing and churning it. (The chemical processes that happen in the stomach are outlined below.) The stomach’s peristaltic contractions then help people the partially processed food into the small intestine.

The small intestine breaks food down into even smaller pieces. Food moves through the small intestine quite slowly, which allows it to be chemically digested and absorbed. The resulting food residues and digestion by-products are moved to the large intestine.

The large intestine compacts these residues and by-products. This process creates feces, which is propelled toward the rectum and eliminated through the anus.

The chemical processes of digestion break down the complex food molecules into their chemical building blocks, which are more easily absorbed by the body. As with the mechanical processes, different organs and parts of the body are involved in chemical digestion.

The chemical processes of digestion begin in the mouth, where salivary amylase breaks dow carbohydrates and lingual lipase breaks down fats.

Once food is in the stomach, gastric juices start digesting the proteins. Gastric juice consist of hydrochloric acid, lipase, and intrinsic factor. Hydrochloric acid converts pepsinogen into pepsin, which breaks down the protein’s tertiary and secondary structures. Gastric lipase break down short- and medium- chain fats, while intrinsic factor helps the body absorb vitamin B12.

Once the food moves from the stomach to the small in†destine, the majority of the chemical digestion processes happen. The intestinal enzymes maltase-gulcoamylase, lactase, and sucrase-isomaltase convert complex sugars into glucose, and lipase breaks down fats into fatty acids and glycerol.

The pancreas is an accessory digestive gland that secretes enzymes- such as amylase, trypsin, chymotrypsin, lipase, nucleotides, and nucleosides - into the intestines to further break down food. Pancreatic amylase converts starch into maltose. Trypsin and chymotrypsin digest proteins. Pancreatic lipase breaks down fats. Pancreatic nucleotides and nucleosides digest the DNA and RNA found in food. The liver is an accessory digestive organ. Its bile salts emulsify lipids.

As it is digested, food moves from the small intestine into the large intestine. Within the large intestine, beneficial bacteria (called gut flora) break down undigested fiber into short-chain fatty acids. Enteric bacteria are also found in the large intestine. These make vitamins, such as vitamin K and biotin. Residual water and electrolytes are absorbed in the large intestine.

Salivary glands: These exocrine glands in the mouth secrete enzymes and are classified as serous, mucous, or seromucous (mixed). Serous glands secrete alpha amylase, which breaks down starch, and mucous glands secrete mucin, which acts as a lubricant.

Digestion Enzymes: There are several digestive enzymes produced throughout the digestive tract. Amylase is produced in the mouth. Pepsin and gelatins are found in the stomach. The pancreas produces lipase and trypsin, which is used in the small intestine.

The Stomach

The stomach is the most extendable part of the gastrointestinal system. It is a muscular organ that links the esophagus and small intestine. The lower esophageal sphincter controls where food enters the stomach, and the pyloric sphincter controls where food exits the stomach, into the small intestine.

The stomach provides early-stage digestion of food. It acts as a temporary storage reservoir for food and liquids, allowing large meals to be consumed quickly and then digested over an extended period of time.

The stomach processes food both mechanically and chemically. It is where enzymatic digestion truly gets started. The principal mechanical process is carried out by the stomach’s smooth muscles, which contract to liquefy food, creating a consistency suitable for digestion in the small intestine.

The stomach also controls the release of food into the small intestine, so the intestine is capable of digesting it. The acidic pH of the stomach’s gastric juices kills most of the bacteria in ingested food and protects the remainder of the digestive tract.

The stomach is shaped like the letter J. Its four main parts are the cardia, funds, corpus, and pylorus. The cardia surrounds the opening of the esophagus and stomach, called the cardiac orifice. The funds stores gas produced during the digestive process. It can also store undigested food if the stomach’s gastric body of is very full. The corpus, also called the gastric body, is the largest part of the stomach. Together with the fundus, it forms about 80 percent of the stomach. The first three parts of the stomach are called the proximal stomach.

The last part of the stomach is the pylorus, which consists of the pyloric antrum and the pyloric canal. This is the distal part of the stomach, The antrum holds the liquified food before it is released into the small intestine. The canal controls when food passes from the stomach into the small intestine, which happens through the pyloric sphincter. This sphincter prevents food from reentering the stomach when the small intestine contracts, and hit also prevents large food particles from passing into the small intestine. The stomach wall has four layers. The serosa is the outermost layer. It consists of the visceral peritoneum, which contains loose connective tissue and elastic fibers, collagen fibers, nerves, and vessels. The next layer is the muscular coat, which consists of an inner oblique layer, middle circular layer, and longitudinal outer muscular layer. Together, these layers allow the stomach to contract. The next layer is the submucosa, which consists of a dense irregular layer of connective tissue. This layer also contains the blood vessels, lymphatics, and nerves that branch into the mucosa. It contains epithelial cells covered by a protective layer of mucus. The epithelium contains the gastric glands and gastric pits.

Anatomy of the Stomach

This muscular hollow, J-shaped organ located on the left side of the upper abdomen receives food from the esophagus through a muscular valve called the lower esophageal sphincter and releases it into the small intestine at the pyloric sphincter.

The stomach wall: This cross section shows the components that make up the stomach when the small intestine contracts, and it also prevents large food particles from passing into the small intestine.

The Small Intestine

The small intestine is the upper portion of the intestines and extends from the stomach to the large intestine. It is the longest organ in the digestive system, measuring 20 feet (6m) long and 1 inch (2.5) in diameter in adults. It is where food is mostly digested and nutrients and minerals are absorbed.

The small intestine (or small bowel) is a long, continuous tube where the food we eat gets digested and where water gets absorbed into the body. It is lined with a velvety tissue and is made up of three parts: the duodenum, jejunum, and ilium.

The duodenum is the first and shortest section of the small intestine. It extends from the pyloric phincter, wraps around the pancreas, and ends at the duodenojejunal junction (also called the duodenojejunal flexure). This is where most of the chemical digestion of food by enzymes takes place.

The jejunum is the middle section of the small intestine. It starts at the duodenojejunal junction, and its primary function is absorption of digested food.

The ilium is the final and longest section of the small intestine. It terminates at the ileocecal fold and ileocecal sphincter control the movement of what remains of the digested food from the small intestine to the large intestine. The main function of the ileum is to absorb vitamin B12, bile salts, and any by-products of digestion that were not absorbed by the jejunum.

The small intestine has a very large surface sram, which allows it to efficiently absorb food. Three kinds of structures help increase its abortive area: mucosal folds, villi, and microvilli. The inner surface of the small intestine has circular mucosal folds that not only increase its surface area but also help blend food as it is digested. The small intestine also contains villi, which are fingerlike projections that extend into the lumen, which is the name for the intestine’s central cavity. Capillaries on the villi take the amino acids and glucose that are by-products of digestion to the hepatic portal vein and liver. fatty acids and glycerol, the by-products small lymph vessels called lacteals. Microvilli are projections on each villus that increase the surface area available for absorption.

Histologically, the small intestine has four tissue layers, like the stomach. The innermost layer is the mucosa, which is covered by villi and secretes digestive juices. The next layer is the submucosa, which consists of dense, irregular, or loose connective tissues. The muscular is responsible for peristalsis. It usually has two distinct layers of smooth muscle: circular and longitudinal. The outermost layer is the serosa. It has a thin layer of cells that connective tissue. serous fluid reduces friction from the movement of the muscular.

The location of the small intestine: The 20-foot (6m) long small intestine is nestled below the stomach and within the large intestine.

Intestinal flora: Lactobacilli produce antibiotic like proteins called bacteriocins that may restrict the growth of some intestinal pathogens.

Functions of the small intestine: This is where most of the final stage of absorption of nutrients from food takes place. This organ receives bile and pancreatic juice through the pancreatic duct, which facilitate digestion.

Small intestine: The gastrointestinal wall contains the muscular coat, which is a layer that contains both longitudinal and circular smooth muscles. Through peristalsis, these muscles move digested material into, along, and out of the intestines.

The pancreas and duodenum: The wide (or head) end of the oblong-shaped pancreas sits in a loop of the duodenum, which is the upper portion of the small intestine and where most enzymatic digestion takes place. The main body of the pancreas lies behind the stomach, and its tapering tail sits above the left kidney, below the spleen. Almost all essential digestive enzymes enter the small intestine from the pancreas. The pancreatic duct joins the common bile duct, forming what is called the ampulla of Vater, also known as the papilla of Vater. This structure opens into the duodenum via the major duodenal papilla. The sphincter of Oddi is a muscular valve that controls the release of secretions into the duodenum.

The Liver and Pancreas

The liver is an organ of the digestive system that helps detoxify the body and produces proteins and chemicals that help digest food. The pancreas is a large gland located behind the stomach that secretes digestive enzymes into the duodenum. The pancreas contains the islets of Langerhans, which secrete the hormones insulin and glucagon into the blood.

The liver is the largest solid organ in the human body. It is located in the right side of the abdominal cavity, beneath the diaphragm. It has four anatomical lobes. The right lobe is the largest, and the left lobe is smaller and flatter. The lower and posterior surfaces of the right lobe are separated into two more lobes by the fossa (a shallow indentation) for the portal vein, the fossa for the gallbladder, and the fosse for the inferior vena cava. These two smaller liver lobes are the quadrate lobe and caudate lobe.

Each lobe is divided into hexagonal hepatic lobules arranged by hepatocytes that radiate from a terminal branch of the hepatic vein - the central vein. Each hepatic lobule is surrounded by an average of six portal tracts. Nutrient-rich blood arrives from the intestines via the hepatic portal system and filters through the lobules.

Hepatocytes are cuboidal epithelial cells that line the liver’s sinusoids, which provide passages for blood. Hepatocytes make up the majority of the livers cells. They perform most of the liver’s functions: metabolism, storage, digestion, and bile production. The function of bile is to emulsify fats, which increases their surface area and thereby facilitates the action of pancreatic lipase. The normal adult pancreas is an elongated, tapered gland located across the back of the abdomen, behind the stomach. It has a digestive function and a hormonal function. The organization of the pancreas reflects this dual role. The exocrine (or digestive) component of the pancreas produces enzymes necessary for the digestion of food. The endocrine (or hormonal) component produces insulin and regulates blood glucose levels.

About 95 percent of the pancreas consists of exocrine tissue. Pancreatic acinar cells are the main unit of the gland’s exocrine function. They synthesize, store, and secrete digestive enzymes. These large pyramidal-shaped cells release their secretions into the main pancreatic duct (also called the duct of Wiring) and the accessory pancreatic duct (also called the duct of Santorini). The pancreatic duct ends at the major duodenal papilla (also called the papilla of Vateri), and the accessory pancreatic duct ends at the minor duodenal papilla.

The endocrine components of the pancreas are called islets of Langerhans, and they make up only 1 to 2 percent of the cell mass in an adult pancreas. These cells are irregularly shaped and, comprise five types of cells: beta cells secrete insulin, alpha cells secrete glucagon, pancreatic polypeptide, delta cells secret somatostatin, and epsilon cells secrete somatostatin, and epsilon cells secrete ghrelin.

Liver position: This lobular organ is located in the upper right part of the belly, beneath the diaphragm, and above the stomach, right kidney, and intestines.

Biliary Tree: The biliary tree is a term used to describe the gallbladder, liver, and bile ducts, which together produce, store, and release bile into the small intestine. Bile produced by hepatocytes in the liver is collected in small grooves called canaliculi. The canaliculi of two adjacent hepatocytes form bile canaliculi. The canaliculi of two adjacent hepatocytes form bile canaliculi. These connect to a network of larger canals that merge into bile ducts in portal tracts, where blood flows into the liver. From there, the bile is either stored in the gallbladder or released into the small intestine. This diagnostic image below shows the biliary tree and the main pancreatic duct.

The gallbladder is a small pear-shaped organ that lies in the upper right side of the abdomen. It can store up to 2 1/2 ounces (75ml) of digestive bile, which is thick liquid that can be green, brown, or yellow and facilitates the digestion of fats.

Insulin: This hormone, made by the pancreas, allows the body to use sugar (glucose) from carbohydrates in food for energy and to store glucose for future use. Insulin helps keep blood sugar levels from getting too high or too low.

The Large Intestine and Rectum

The large intestine is made up of the cecum, colon, rectum, and anus. These parts of the body reabsorb water, electrolytes, and vitamins and compact food waste into feces. The colon and rectum are held in place inside the abdomen by folds of tissue called mesenteries.

The large intestine is part of the final stage of the digestive system. It is connected to the small intestine at one end and the anus at the other. It is roughly 5 feet (1.5 m) long and 3 inches (7.6 cm) in diameter. It is divided into four parts: the cecum, colon, rectum, and anal canal.

The cecum is a short, pouch like region of the large intestine between the colon and vermiform appendix. It connects the colon to the ileum through the ielocecal valve. The colon is the longest section of the large intestine (and the large intestine is often commonly called the colon). It is divided into four parts: the ascending colon, transverse colon, descending colon, and sigmoid colon. The main function of the colon is to absorb fluid, electrolytes, and vitamins.

The rectum is the lower part of the intestine. It connects the sigmoid colon to the anal canal and functions as a reservoir for feces. It facilitates the reabsorption of water and electrolytes from feces. As feces enter the rectum, nerves create the urge to defecate.

The anus is the opening at the lower end of the rectum, through which feces is passed from the body. The anal canal is collapsed by the internal and external anal sphincters to prevent the unintentional passage of fecal material.

The mesocolon is a mesentery that attaches the colon to the wall of the abdomen. Mesenteries are fatty connective tissues that contain blood vessels, nerves, lymph nodes, and lymph vessels. The rectum is surrounded by a mesentery called the mesorectum.

Thee location of the colon: The large intestine is shaped like an upside down U and wraps around the small intestine, actually bending twice up under the rib cage.

A healthy gut: There are many ways to keep the colon healthy: eating lots of fresh fruits and vegetables, limiting meats and processed foods; exercising regularly; limiting alcohol; and quitting smoking.

The rectum is a continuation of the sigmoid colon; it ends in an expanded section called the rectal ampulla, which is where feces are stored before they are released via the anal canal.

The appendix is a small, wormlike sac of tissue that is located in the cecum, in the lower-right abdomen. Its lymphatic tissue serves an immune function, and it also harbors beneficial bacteria.

Defecation is the act of eliminating feces from the digestive tract. There are two main anal sphincters: internal and external. The internal anal sphincter is controlled by parasympathetic fibers that relax involuntarily. The external anal sphincter is skeletal muscle that is controlled by somatic nerves, which allow a person to consciously control defecation.