Anatomy of the Digestive System Introduction Digestion is the process by which food is broken down into smaller pieces so that the bo dy can use them to build and nourish cells and to provide energy. Digestion involves the mixing of food, its movement through the digestive tract (als o known as t he alimentary canal ), and the chemical breakdown of larger molecules into smaller molecules. Every piece of food we eat has to be broken down into s mall er nutrients that t he body can absorb, which is why it takes hours to fully digest food. The digestive system is made up of the digestive tract. This consists of a long tube o f organs that runs from the mouth to the anus and includes the esophagus, stomach, small intestine, and large intestine, together with the liver, gallbladder, andpancreas , which produce important secretions for digestion that drain into the small intestine. The digestive tract in an adu lt is about 30 feet long. Physiology
Digestion is the process by which food is broken down into smaller pieces so that the body can
use them to build and nourish cells and to provide energy. Digestion involves the mixing of food,its movement through the digestive tract (also known as the alimentary canal), and the chemical
breakdown of larger molecules into smaller molecules. Every piece of food we eat has to be broken down into smaller nutrients that the body can absorb, which is why it takes hours to fullydigest food.
The digestive system is made up of the digestive tract. This consists of a long tube of organs that
runs from the mouth to the anus and includes the esophagus, stomach, small intestine, and largeintestine, together with the liver , gall bladder , and pancreas, which produce important secretions
for digestion that drain into the small intestine. The digestive tract in an adult is about 30 feetlong.
The body wall of the abdominal cavity and the abdominal organs are covered with serousmembranes. The serous membrane that covers the organs is the visceral peritoneum, or serosa.The serous membrane that lines the wall of the abdominal cavity is the parietal peritoneum.
Many of the organs of the abdominal cavity are held in place by connective tissue sheets calledmesenteries. The mesenteries consist of two layers of serous membranes with a thin layer of
loose connective tissue between them. Specific mesenteries are given names. The mesenteryconnecting the lesser curvature of the stomach to the liver and diaphragm is the lesser omentum,
and the mesentery connecting the greater curvature of the stomach to the transverse colon and posterior body wall is the greater omentum. The greater omentum is unusual in that it is a long,
double fold of mesentery that extends inferiorly from the stomach before looping back to the
transverse colon to create a cavity or pocket, called the omental bursa. Fat accumulates in thegreater omentum, giving it the appearance of a fat-filled apron that covers the anterior surface of the abdominal viscera. Mesentery is a general term referring to the serous membrane attached to
the abdominal organs. The term is also used specifically to refer to the mesentery that attachesthe small intestine to the posterior abdominal wall. This mesentery is also called the mesentery
proper.
Other abdominal organs lie against the abdominal wall, have no mesenteries, and are describedas retroperitoneal. The retroperitoneal organs include the duodenum, pancreas, ascending colon,
descending colon, rectum, kidneys, adrenal glands and urinary bladder.
Oral Cavity
The oral cavity, or mouth, is the first part of the digestive tract. It is bounded by the lips and
cheeks and contains the teeth and tongue.
The lips are muscular structures, formed mostly by the orbicularis oris muscle. The outer
surfaces of the lips are covered by skin. The keratinized stratified epithelium of the skin becomesthin at the margin of the lips. The color from the underlying blood vessels can be seen through
the thin, transparent epithelium, giving the lips a reddish-pink appearance. At the internal marginof the lips, the epithelium is continuous with the moist stratified squamous epithelium of the
mucosa in the oral cavity. The cheeks form the lateral walls of the oral cavity.
The buccinators muscles are located within the cheeks and flatten the cheeks against teeth. Thelips and cheeks are important in the process of mastication, or chewing. They help manipulate
the food within the mouth and hold the food in place while the teeth crush or tear it. Mastication begins the process of mechanical digestion, in which large food particles are broken down into
smaller ones. The cheeks also help form words during the speech process.
The tongue is a large, muscular organ that occupies most of the oral cavity. The major attachment of the tongue is in the posterior part of the oral cavity. The anterior part of the tongue
is relatively free. There is an anterior attachment to the floor of the mouth by a thin fold of tissuecalled the frenulum.
The tongue moves food in the mouth and, in cooperation with the lips and cheeks, holds the foodin place during mastication. It also plays a major role in the process of swallowing. The tongue isa major sensory organ for taste, as well as being one of the major organs of speech.
Teeth
There are 32 teeth in the normal adult mouth, located in the mandible and maxillae. The teeth
can be divided into quadrantsright upper, left upper, right lower, and left lower. In adults, eachquadrant contains one central and one lateral incisor; one canine; first and second premolars; and
first, second, and third molars. The third molars are called wisdom teeth because they usuallyappear in a persons late teens or early twenties, when the person is old enough to have acquired
some degree of wisdom.
The teeth of adults are permanent, or secondary, teeth. Most of them are replacements of the 2
primary, or deciduous, teeth.
Each tooth consists of a crown with one or more cusps, a neck and a root. The center of the toothis a pulp cavity, which is filled with blood vessels, nerves and connective tissue, called pulp. The
pulp cavity is surrounded by a living, cellular, bonelike tissue called dentin. The dentin of thetooth crown is covered by an extremely hard, acellular substance called enamel, which protects
the tooth against abrasion and acids produced by bacteria in the mouth. The surface of the dentinin the root is covered with cementum, which helps anchor the tooth in the jaw.
The teeth are rooted within alveoli along the alveolar processes of the mandible and maxillae.
The alveolar processes are covered by dense fibrous connective tissue and moist stratifiedsquamous epithelium, referred to as the gingival, or gums. The teeth are held in place by
periodontal ligaments, which are connective tissue fibers that extend from the alveolar walls andare embedded into the cementum.
Palate and Tonsils
The palate, or roof of the oral cavity, consists of two parts. The anterior part contains bone and is
called the hard palate, whereas the posterior portion consists of skeletal muscle and connective
tissue and is called the soft palate. The uvula is a posterior extension of the soft palate. The palate separates the oral cavity from the nasal cavity and prevents food from passing into thenasal cavity during chewing and swallowing.
The tonsils are located in the lateral posterior walls of the oral cavity, in the nasopharynx, and in
There are three pairs of salivary glands the parotid, submandibular, and sublingual glands. They produce saliva, which is a mixture of serous and mucous fluids. Saliva helps keep the oral cavity
moist and contains enzymes that begin the process of chemical digestion. The salivary glands arecompound alveolar glands. They have branching ducts with clusters of alveoli, resembling
grapes, at the ends of the ducts.
The largest of the salivary glands, the parotid glands, are serous glands located just anterior toeach ear. Parotid ducts enter the oral cavity adjacent to the second upper molars.
The submandibular glands produce more serous than mucous secretions. Each gland can be felt
as a soft lump along the inferior border of the mandible. The submandibular ducts open into theoral cavity on each side of the frenulum of the tongue. In certain people, if the mouth is opened
and the tip of the tongue is elevated, saliva can squirt out of the mouth from the ducts of theseglands.
The sublingual glands, the smallest of the three paired salivary glands, produce primarily mucous
secretions. They lie immediately below the mucous membrane in the floor of the oral cavity.Each sublingual gland has 10-12 small ducts opening onto the floor of the oral cavity.
Pharynx
The pharynx, or throat, which connects the mouth with the esophagus, consists of three parts the
nasopharynx, oropharynx, and laryngopharynx. Normally only the oropharynx andlaryngopharynx transmit food. The posterior walls of the oropharynx and laryngopharynx are
formed by the superior, middle, and inferior pharyngeal constrictor muscles.
EsophagusThe esophagus is a muscular tube, lined with moist stratified squamous epithelium that extends
from the pharynx to the stomach. It is about 25 centimeters (cm) long and lies anterior to thevertebrae and posterior to the trachea within the mediastenum. It passes through the diaphragm
and ends at the stomach. Upper and lower esophageal sphincters, located at the upper and lower ends of the esophagus, respectively, regulate the movement of food into and out of the
esophagus. The lower esophageal sphincter is sometimes called the cardiac sphincter. Numerousmucous glands produce thick, lubricating mucus that coats the inner surface of the esophagus.
Stomach
The stomach is an enlarged segment of the digestive tract in the left superior part of theabdomen. The opening from the esophagus into the stomach is called the cardiac opening
because it is near the heart. The region of the stomach around the cardiac opening is called thecardiac region. The most superior part of the stomach is the fundus. The largest part of the
stomach is the body, which turns to the right, forming a greater curvature on the left, and a lesser curvature on the right. The opening from the stomach into the small intestine is the pyloric
opening, which is surrounded by a relatively thick ring of smooth muscle called the pyloricsphincter. The region of the stomach near the pyloric opening is the pyloric region.
The muscular layer of the stomach is different from other regions of the digestive tract in that it
consists of three layers an outer longitudinal layer, a middle circular layer, and an inner oblique
layer. These muscular layers produce a churning action in the stomach, important in the digestive process. The sub mucosa and mucosa of the stomach are thrown into large folds called rugaewhen the stomach is empty. These folds allow the mucosa and sub mucosa to stretch, and the
folds disappear as the stomach is filled.
The stomach is lined with simple columnar epithelium. The mucosal surface forms numerous,tube-like gastric pits, which are the openings for the gastric glands. The epithelial cells of the
stomach can be divided into five groups. The first group consists of surface mucous cells on theinner surface of the stomach and lining the gastric pits. Those cells produce mucus which coats
and protect the stomach lining. They are mucous neck cells, which produce mucous; parietalcells, which produce hydrochloric acids and intrinsic factors; endocrine cells, which produce
regulatory hormones; and chief cells, which produce pepsinogen, a precursor of the protein-digesting enzyme pepsin.
Small Intestines
The small intestine is about 6 meters long and consists of three partsthe duodenum, jejunum, and
ileum. The duodenum is about 25 centimeter (the term duodenum means 12, suggesting that it is12 inches long). The jejunum is about 2.5 meter long and makes up two-fifths of the total length
of the small intestine. The ileum is about 3.5 meter long and makes up three-fifths of the smallintestine.
The duodenum nearly completes a nearly an 18degree arc as it curves within the abdominalcavity. Part of the pancreas lies within this arc. The common bile duct from the liver and the pancreatic duct from the pancreas join each other and empty into the duodenum.
The small intestine is the major site of digestion and absorption of food, which are accomplished
by the presence of a large surface area. The surface of the small intestine has three modificationsthat increase surface area about 600-foldcircular folds, villi, and microvilli. The mucosa and sub
mucosa form a series of circular folds that run perpendicular to the long axis of the digestivetract. Tiny finger like projections of the mucosa forms numerous villi, which are 0.5-1.5 mm
long. Most of the cells composing the surface of the villi have numerous cytoplasmic extensions,called microvilli. Each villus is covered by simple columnar epithelium. Within the loose
connective tissue core of each villus is a blood capillary called lacteal. The blood capillarynetwork and the lacteal are very important in transporting absorbed nutrients.
The mucosa of the small intestine is simple columnar epithelium with four major cell types:Absorptive cells, which have microvilli, produce digestive enzymes, and absorb digested food
Goblet cells, which produce a protective mucus Granular cells, (Paneths cells), which may help protect the intestinal epithelium from bacteria; Endocrine cells, which produce regulatory
hormones. The epithelial cells are produce within tubular glands of the mucosa, called intestinal
glands, at the base of the villi. Granular and endocrine cells are located in the bottom of theglands. The sub mucosa of the duodenum contains mucous glands, called duodenal glands,
which open into the base of the intestinal glands.
The duodenum, jejunum, and ileum are similar in structure except that there is a granular
decrease in the diameter of the small intestine, in the thickness of the intestinal wall, in thenumber of circular folds, and in the number of villi as one progress through the small intestine.Lymph nodules are common along the entire length of the digestive tract. Clusters of lymph
nodules, called Peyers patches, are numerous in the ileum. These lymphatic tissues in theintestine help protect the intestinal tract from harmful micro organisms.
The junction between the ileum and the large intestine is the ileocecal junction. It has a ring of
smooth muscle, the ileocecal sphincter, and an ileocecal valve, which allows material containedin the intestine to move from the ileum to the large intestine, but not in the opposite direction.
Secretions of the Small Intestines
Secretions from the mucosa of the small intestine mainly contain mucus, ions and water.Intestinal secretions lubricate and protect the intestinal wall from the acidic chime and the action
of the digestive enzymes. They also keep the chime in the small intestine in a liquid form tofacilitate the digestive process. Most of the secretions entering the small intestine are produced
by the intestinal mucosa, but the secretions of the liver and the pancreas also enter the smallintestine and play important roles in the process of digestion.
The epithelial cells in the walls of the small intestine have enzymes bound to their free surfaces
that play a significant role in the final steps of digestion. Peptidases break the peptide bonds in proteins to form amino acids. Disaccharidases break down dissacharides, such as maltose and
isomaltose, into monosaccharide. The amino acids and monosaccharides can be absorbed by theintestinal epithelium.
Mucus is produced by duodenal glands and by goblet cells, which are dispersed throughout the
epithelial lining of the entire small intestine and within intestinal glands. Hormones releasedfrom the intestinal mucosa stimulate liver and pancreatic secretions. Secretion by duodenal
glands is stimulated by the vagus nerve, secretin release, and chemical or tactile irritation of theduodenal mucosa.
Movement of Small Intestines
Mixing and propulsion of chime are the primary mechanical events that occur in the smallintestine. Peristaltic contractions proceed along the length of the intestine for variable distances
and cause the chime to move along the small intestine. Segmental contractions are propagatedfor only short distances and function to mix intestinal contents.
The ileocecal sphincter at the juncture of the ileum and the large intestine remains mildly
contracted most of the time, but peristaltic contractions reaching the ileocecal sphincter from thesmall intestine cause the sphincter to relax and allow movement of chime from the small
intestine into the cecum. The ileocecal valve allows chime to move from the ileum into the largeintestine, but tends to prevent movement from the large intestine back into the ileum.
Absorption in the Small Intestines
A major function of the small intestine is the absorption of nutrients. Most absorption occurs inthe duodenum and jejunum, although some absorption also occurs in the ileum.
Liver
The liver weighs about 1.36 kilograms and is located in the right upper quadrant of the abdomen,
tucked against the inferior surface of the diaphragm. The posterior surface of the liver is incontact with the right ribs 5-12. it is divided into two major lobes, the right and left lobes,
separated by a connective tissue septum, the falciform ligament. Two smaller lobes, the caudateand quadrate, can be seen from an inferior view. Also seen from the inferior view is the porta,
which is the gate through which blood vessels, ducts and nerves enter or exit the liver.
The liver receives blood from two sources. The hepatic artery brings oxygen-rich blood to theliver, which supplies liver cells with oxygen. The hepatic portal vein carries blood that is
oxygen-poor but rich in absorbed nutrients and other substances from the digestive tract to theliver. Liver cells process nutrients and detoxify harmful substance from the blood. Blood exits
the liver through hepatic veins, which empty into the inferior vena cava.
Many delicate connective tissue septa divide the liver into lobules with portal triads at thecorners of the lobules. The portal triads contain three structuresthe hepatic artery, hepatic portal
vein, and hepatic duct. Hepatic cords, formed by platelike groups of cells called hepatocytes, arelocated between the center and the margins of each lobule. The hepatic cords are separated from
one another by blood channels called hepatic sinusoids. The sinusoid epithelium contains phagocytic cells that help remove foreign particles from the blood. Blood from the hepatic portal
vein and the hepatic artery flows into the sinusoids and becomes mixed. The mixed blood flowstowards the center of each lobule into a central vein. The central veins from all the lobes unite to
form the hepatic veins, which carry blood out of the liver to the inferior vena cava.
A cleft-like lumen, the bile canaliculus, is between the cells of each hepatic cord. Bile, produced
by the hepatocytes, flows through the bile canaliculi to the hepatic ducts in the portal triads. Thehepatic ducts converge and empty into the right and left hepatic ducts, which transport bile out of
the liver. The right and left hepatic ducts unite to form a single common hepatic duct. Thecommon hepatic ducts is joined by the cystic duct from the gallbladder is a small sac on the
inferior surface of the liver that stores and concentrates bile. The common bile duct joins the pancreatic duct and opens into the duodenum at the duodenal papilia. The opening into the
duodenum is regulated by a sphincter.
The liver performs important digestive and excretory functions, store and processes nutrients,synthesizes new molecules, and detoxifies harmful chemicals.
The liver secretes about 70mL of bile each day. Bile contains no digestive enzymes, but it playsan important role in digestion by diluting and neutralizing stomach acid and by dramatically
increasing the efficiency of fat digestion and absorption. Digestive enzyme cannot act efficienton large fat globules. Bile salts emulsify fats, breaking the fat globules into smaller droplets,
much like the action of detergent in dish-water. The small droplets are more easily digested by
the digestive enzymes. Bile also contains excretory products such as bile pigments, cholesteroland fats. Bilirubin is a bile pigment that results from the breakdown of hemoglobin.
Bile excretion by the liver is stimulated by secretin, which is released from the duodenum.Cholecystokinin stimulates the gall bladder to contract and release bile into the duodenum.
Parasympathetic stimulation through the vagus nerve also stimulates bile secretion and release.
Most bile salts are reabsorbed in the ileum, and the blood carries them back to the liver, wherethey stimulate additional bile salts secretion and are once again secreted into the bile. The loss of
bile salts in the feces is reduced by this recycling process.
The liver can remove sugar from the blood and store it in the form of glycogen. It can also storefat, vitamins, copper and iron. This storage function is usually short term.
The liver transforms some nutrients into more readily usable substances. Many ingestedsubstances are harmful to the cells of the body. In addition, the body itself produces many by-
products of metabolism that, if accumulated, are toxic. The liver is an important line of defenseagainst many of those harmful substances. It detoxifies them by altering their structure, making
their excretion easier. The liver can also produce its own unique new compounds. Many of the blood proteins, such as albumin, fibrinogen, globulins, and clotting factors, are synthesized in the
liver and released into the circulation.
Pancreas
Pancreas is a fish-shaped spongy grayish-pink organ about 6 inches (15 cm) long that stretches
across the back of the abdomen, behind the stomach. The head of the pancreas is on the right sideof the abdomen and is connected to the duodenum (the first section of the small intestine). The
narrow end of the pancreas, called the tail, extends to the left side of the body.
The pancreas makes pancreatic juices and hormones, including insulin. The pancreatic juices are
enzymes that help digest food in the small intestine. Insulin controls the amount of sugar in the blood.
As pancreatic juices are made, they flow into the main pancreatic duct. This duct joins thecommon bile duct, which connects the pancreas to the liver and the gallbladder. The common bile duct, which carries bile (a fluid that helps digest fat) connects to the small intestine near the
stomach.
The pancreas is thus a compound gland. It is compoundin the sense that it is composed of bothexocrine and endocrine tissues. The exocrine function of the pancreas involves the synthesis and
secretion of pancreatic juices. The endocrine function resides in the million or so cellular islands
(the islets of Langerhans) embedded between the exocrine units of the pancreas. Beta cells of theislands secrete insulin, which helps control carbohydrate metabolism. Alpha cells of the islets
secrete glucagon that counters the action of insulin.
Large Intestines
The colon is made up of 6 parts all working collectively for a single purpose. Their purpose is
ridding the body of toxins that have entered the body from food sources, environmental poisons,or toxins produced within the body. The colons role is to transfer nutrients into the bloodstream
through the absorbent walls of the large intestine while pushing waste out of the body. In this process, digestive enzymes are released, water is absorbed by the stool, and a host of muscle
groups and beneficial microorganisms work to maintain the digestive system.
The colon is approximately 4.5 feet long, 2.5 inches wide, and is a muscular tube composed of lymphatic tissue, blood vessels, connective tissue, and specialized muscles for carrying out the
tasks of water absorption and waste removal. The tough outer covering of the colon protects the
inner layer of the colon with circular muscles for propelling waste out of the body in an actioncalled peristalsis. Under the outer muscular layer is a sub-mucous coat containing the lymphatictissue, blood vessels, and connective tissue. The innermost lining is highly moist and sensitive,
and contains the villi- or tiny structures providing blood to the colon.
The colon is actually just another name for the large intestine. The shorter of the two intestinal
groups, the large intestine, consists of parts with various responsibilities. The names of these parts are the transverse colon, ascending colon, appendix, descending colon, sigmoid colon, and
the rectum and anus.
Transverse Colon
The transverse, ascending, and descending colons are named for their physical locations withinthe digestive tract, and corresponding to the direction food takes as it encounters those sections.
Within these parts of the colon, contractions from smooth muscle groups work food material back and forth to move waste through the colon and eventually, out of the body. The intestinal
walls secrete alkaline mucus for lubricating the colon walls to ensure continued movement of thewaste.
The ascending colon travels up along the right side of the body. Due to waste being forced
upwards, the muscular contractions working against gravity are essential to keep the systemrunning smoothly. The next section of the colon is termed the transverse colon due to it running
across the body horizontally. Then, the descending colon turns downward and becomes thesigmoid colon, followed by the rectum and anus.
Ileocecal and Cecum Valves
The ileocecal valve is located where the small and large intestines meet. This valve is an opening between the small intestine and large intestine allowing contents to be transferred to the colon.
The cecum follows this valve and is an opening to the large intestine.
The rectum is about eight inches long and serves, basically, as a warehouse for poop. It hooks up
with the sigmoid colon to the north and with the anal canal to the south.
The rectum has little shelves in it called transverse folds. These folds help keep stool in placeuntil you¶re ready to go to the bathroom. When you¶re ready, stool enters the lower rectum,moves into the anal canal, and then passes through the anus on its way out.
The rectum intestinum acts as a temporary storage facility for feces. As the rectal walls expand
due to the materials filling it from within, stretch receptors from the nervous system located inthe rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the
rectum is often returned to the colon where more water is absorbed. If defecation is delayed for a prolonged period of time constipation and hardened feces results.
When the rectum becomes full, the increase in intrarectal pressure forces the walls of the anal
canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forcedinto the anal canal and peristaltic waves propel the feces out of the rectum. The internal andexternal sphincter allows the feces to be passed by muscles pulling the anus up over the exiting
