Anatomy and Physiology – Digestive System

Anatomy and Physiology – Digestive System

DIGESTIVE SYSTEM

• The digestive system is a group of organs working together to convert food
• into energy and basic nutrients to feed the entire body. Food passes through
• a long tube inside the body known as the alimentary canal or the
• gastrointestinal tract (GI tract).
• The mouth is the beginning of the digestive tract. In fact, digestionstarts
• here as soon as you take the first bite of a meal. Chewing breaks the food
• into pieces that are more easily digested, while saliva mixes with food to
• begin the process of breaking it down into a form your body can absorb and
• use. The hollow organs that make up the GI tract are the mouth,
• esophagus,stomach, small intestine, large intestine—which includes
• the rectum—and anus. Food enters the mouth and passes to the anus
• through the hollow organs of the GI tract. The liver, pancreas,
• and gallbladder are the solid organs of the digestive.
• Digestion is important for breaking down food into nutrients, which the
• body uses for energy, growth, and cell repair. Food and drink must be
• changed into smaller molecules of nutrients before the blood absorbs them
• and carries them to cells throughout the body. The digestive tract, also
• known as the gastrointestinal (GI) tract, starts at the mouth, continues to the
• esophagus, stomach, small intestine, large intestine (commonly referred to as
• the colon) and rectum, and ends at the anus. The entire system — from
• mouth to anus — is about 30 feet (9 meters) long.
• The human digestive system consists of the gastrointestinal tract plus
• the accessory organs of digestion (the tongue, salivary, pancreas, liver,
• and gallbladder). In this system, the process of digestion has many stages,
• the first of which starts in the mouth (oral cavity). Digestion involves the
• breakdown of food into smaller and smaller components which can be
• absorbed and assimilated into the body. The secretion of saliva helps to
• produce a bolus which can be swallowed to pass down the esophagus and
• into the stomach.
• Saliva also contains a catalytic enzyme called amylase which starts to act on
• food in the mouth. Another digestive enzyme called lingual is secreted by
• some of the lingual papillae on the tongue and also from serous glands in the
• main salivary glands. Digestion is helped by the mastication of food by
• the teeth and also by the muscular actions of peristalsis and segmentation
• contractions. Gastric juice in the stomach is essential for the continuation of
• digestion as is the production of mucus in the stomach.
• Peristalsis is the rhythmic contraction of muscles that begins in the
• esophagus and continues along the wall of the stomach and the rest of
• the gastrointestinal tract. This initially results in the production
• of chyme which when fully broken down in the small intestine is absorbed
• as chyle into the lymphatic system. Most of the digestion of food takes place
• in the small intestine. Water and some minerals are reabsorbed back into the
• blood, in the colon of the large intestine. The waste products of digestion are
• defecated from the anus via the rectum.
• There are several organs and other components involved in the digestion of
• food. The organs known as the accessory digestive glands are the liver, gall
• bladder and pancreas. Other components include
• the mouth, teeth and epiglottis.
• The largest structure of the digestive system is the gastrointestinal tract (GI
• tract). This starts at the mouth and ends at the anus, covering a distance of
• about nine (9) metres. The largest part of the GI tract is the colon or large
• intestine. Water is absorbed here and remaining waste matter is stored prior
• to defecation. Most of the digestion of food takes place in the small
• intestine. A major digestive organ is the stomach. Within its mucosa are
• millions of embedded gastric glands. Their secretions are vital to the
• functioning of the organ. There are many specialized cells of the GI tract.
• These include the various cells of the gastric glands, taste cells, pancreatic
• duct cells,enterocytes and microfold cells.
• Mouth
• The mouth is the first part of the gastrointestinal tract and is equipped with
• several structures that begin the first processes of digestion. These include
• salivary glands, teeth and the tongue. The mouth consists of two regions, the
• vestibule and the oral cavity proper. The vestibule is the area between the
• teeth, lips and cheeks, and the rest is the oral cavity proper. Most of the oral
• cavity is lined with oral mucosa, a mucous membrane that produces a
• lubricating mucus, of which only a small amount is needed. Mucous
• membranes vary in structure in the different regions of the body but they all
• produce a lubricating mucus, which is either secreted by surface cells or
• more usually by underlying glands. The mucous membrane in the mouth
• continues as the thin mucosa which lines the bases of the teeth. The main
• component of mucus is a glycoprotein called mucin and the type secreted
• varies according to the region involved. Mucin is viscous, clear, and
• clinging. Underlying the mucous membrane in the mouth is a thin layer
• of smooth muscle tissue and the loose connection to the membrane gives it
• its great elasticity. It covers the cheeks, inner surfaces of the lips, and floor
• of the mouth.
• The roof of the mouth is termed the palate and it separates the oral cavity
• from the nasal cavity. The palate is hard at the front of the mouth since the
• overlying mucosa is covering a plate of bone; it is softer and more pliable at
• the back being made of muscle and connective tissue, and it can move to
• swallow food and liquids. The soft palateends at the uvula. The surface of
• the hard palate allows for the pressure needed in eating food, to leave the
• nasal passage clear. The lips are the mouth's front boundary and
• the fauces (the passageway between the tonsils, also called the throat), mark
• its posterior boundary. At either side of the soft palate are the palatoglossus
• muscles which also reach into regions of the tongue. These muscles raise the
• back of the tongue and also close both sides of the fauces to enable food to
• be swallowed. Mucus helps in the mastication of food in its ability to soften
• and collect the food in the formation of the bolus.
• Salivary glands
• There are three pairs of main salivary glands and between 800 and 1,000
• minor salivary glands, all of which mainly serve the digestive process, and
• also play an important role in the maintenance of dental health and general
• mouth lubrication, without which speech would be impossible. The main
• glands are all exocrine glands, secreting via ducts. All of these glands
• terminate in the mouth. The largest of these are the parotid glands – their
• secretion is mainly serous. The next pair are underneath the jaw,
• the submandibular glands, these produce both serous fluid and mucus. The
• serous fluid is produced by serous glands in these salivary glands which also
• produce lingual lipase. They produce about 70% of the oral cavity saliva.
• The third pair are the sublingual glands located underneath the tongue and
• their secretion is mainly mucous with a small percentage of saliva.
• Within the oral mucosa (a mucous membrane) lining the mouth and also on
• the tongue and palates and mouth floor, are the minor salivary glands; their
• secretions are mainly mucous and are innervated by the facial nerve (the
• seventh cranial nerve). The glands also secreteamylase a first stage in the
• breakdown of food acting on the carbohydrate in the food to transform the
• starch content into maltose. There are other glands on the surface of the
• tongue that encircle taste buds on the back part of the tongue and these also
• produce lingual lipase.Lipase is a digestive enzyme that catalyses
• the hydrolysis of lipids (fats). These glands are termed Von Ebner's
• glands which have also been shown to have another function in the secretion
• of histatins which offer an early defense (outside of the immune system)
• against microbes in food, when it makes contact with these glands on the
• tongue tissue. Sensory information can stimulate the secretion of saliva
• providing the necessary fluid for the tongue to work with and also to ease
• swallowing of the food.
• Saliva
• Saliva functions initially in the digestive system to moisten and soften food
• into the formation of a bolus. The bolus is further helped
• provided by the saliva in its passage from the mouth into the oesophagus.
• Also of importance is the presence in saliva of the digestive
• enzymes amylase and lipase. Amylase starts to work on
• thestarch in carbohydrates, breaking it down into the
• simple sugars of maltose and dextrose that can be further broken down in the
• small intestine. Saliva in the mouth can account for 30% of this initial starch
• digestion. Lipase starts to work on breaking down fats. Lipase is further
• produced in the pancreas where it is released to continue this digestion of
• fats. The presence of salivary lipase is of prime importance in young babies
• whose pancreatic lipase has yet to be developed
• As well as its role in supplying digestive enzymes, saliva has a cleansing
• action for the teeth and mouth. It also has an immunological role in
• supplying antibodies to the system, such as immunoglobulin A. This is seen
• to be key in preventing infections of the salivary glands, importantly that
• of parotitis.
• Saliva also contains a glycoprotein called haptocorrin which is a binding
• protein to vitamin B12. It binds with the vitamin in order to carry it safely
• through the acidic content of the stomach. When it reaches the duodenum,
• pancreatic enzymes break down the glycoprotein and free the vitamin which
• then binds with intrinsic factor
• Tongue
• Food enters the mouth where the first stage in the digestive process takes
• place, with the action of the tongue and the secretion of saliva. The tongue is
• a fleshy and muscular sensory organ, and the very first sensory information
• is received via the taste buds on its surface. If the taste is agreeable the
• tongue will go into action, manipulating the food in the mouth which
• stimulates the secretion of saliva from the salivary glands. The liquid quality
• of the saliva will help in the softening of the food and its enzyme content
• will start to break down the food whilst it is still in the mouth. The first part
• of the food to be broken down is the starch of carbohydrates. The tongue is
• attached to the floor of the mouth by a ligamentous band called
• the frenum and this gives it great mobility for the manipulation of food
• (and speech); the range of manipulation is optimally controlled
• of several muscles and limited in its external range by the stretch of the
• frenum. The tongue's two sets of muscles, are four intrinsic muscles that
• originate in the tongue and are involved with its shaping, and four extrinsic
• muscles originating in bone that are involved with its movement.
• Taste
• Taste is a form of chemoreception that takes place in the specialised
• receptors of taste cells, contained in structures called taste budsin the mouth.
• Taste buds are mainly on the upper surface (dorsum) of the tongue. Taste
• perception is vital to help prevent harmful or rotten foods from being
• consumed. This is a function of the gustatory system where the taste buds
• are at the forefront. There are taste buds elsewhere in the mouth not just on
• the surface of the tongue. The taste buds are innervated by a branch of the
• facial nerve thechorda tympani, and the glossopharyngeal nerve. Taste
• messages are sent via these cranial nerves to the brain. The brain can
• distinguish between the chemical qualities of the food. The five basic tastes
• are referred to as those of saltiness, sourness, bitterness and sweetness, and
• the most recent addition of a certain savouriness termed umami. The
• detection of saltiness and sourness enables the control of salt and acid
• balance. The detection of bitterness warns of poisons – many of a plant's

defences are of poisonous compounds that are bitter. Sweetness guides to those foods that will supply energy; the initial breakdown of the energygiving carbohydrates by salivary amylase creates the taste of sweetness since simple sugars are the first result. The taste of umami is thought to signal protein-rich food. Sour tastes are acidic which is often found in bad food.

• The brain has to decide very quickly whether to eat the food or not. It was
• the findings in 1991, describing the first olfactory receptors that helped to
• prompt the research into taste. The olfactory receptors are located on cell
• surfaces in the nose which bind to chemicals enabling the detection of
• smells. It is assumed that signals from taste receptors work together with the
• signals from those in the nose, to form an idea of complex food flavours.
• Teeth
• Teeth are complex structures made of materials specific to them. They are
• made of a bone-like material called dentin, which is covered
• tissue in the body—enamel. Teeth have different shapes to deal with
• different aspects of mastication employed in tearing and chewing pieces of
• food into smaller and smaller pieces. This results in a much larger surface
• area for the action of digestive enzymes. The teeth are named after their
• particular roles in the process of mastication—incisors are used for cutting
• or biting off pieces of food; canines, are used for
• tearing, premolars and molars are used for chewing and grinding.
• Mastication of the food with the help of saliva and mucus results in the
• formation of a soft bolus which can then be swallowed to make its way
• down the upper gastrointestinal tract to the stomach. The digestive enzymes
• in saliva also help in keeping the teeth clean by breaking down any lodged
• food particles.
• Epiglottis
• The epiglottis is a flap that is made of elastic cartilage and attached to the
• entrance of the larynx. It is covered with a mucous membrane and there are
• taste buds on its lingual surface which faces into the mouth. Its laryngeal
• surface faces into the larynx. The epiglottis functions to guard the entrance
• of the glottis, the opening between the vocal folds. It is normally pointed
• upward during breathing with its underside functioning as part of the
• pharynx, but during swallowing, the epiglottis folds down to a more
• horizontal position, with its upper side functioning as part of the pharynx. In
• this manner it prevents food from going into the trachea and instead directs it
• to the oesophagus, which is posterior. During swallowing, the backward
• motion of the tongue forces the epiglottis over the glottis' opening to prevent
• any food that is being swallowed from entering the larynx which leads to the
• lungs; the larynx is also pulled upwards to assist this process. Stimulation of
• the larynx by ingested matter produces a strong cough reflex in order to
• protect the lungs.
• Pharynx
• The pharynx is a part of the conducting zone of the respiratory system and
• also a part of the digestive system. It is the part of the throat immediately
• behind the nasal cavity at the back of the mouth and above the oesophagus
• and larynx.The pharynx is made up of three parts. The lower two parts–
• the oropharynx and the laryngopharynx are involved in the digestive system.
• The laryngo pharynx connects to the esophagus and it serves as a
• passageway for both air and food. Air enters the larynx anteriorly but
• anything swallowed has priority and the passage of air is temporarily
• blocked. The pharynx is innervated by the pharyngeal plexus of the vagus
• nerve Muscles in the pharynx push the food into the esophagus. The pharynx
• joins the esophagus at the esophageal inlet which is located behind
• the cricoids cartilage.
• Esophagus
• The esophagus is a muscular tube connecting the pharynx to the stomach
• that is part of the upper gastrointestinal tract. It carries swallowed masses
• of chewed food along its length. At the inferior end of the esophagus is a
• muscular ring called the lower esophageal sphincter or cardiac sphincter.
• The function of this sphincter is to close of the end of the esophagus and trap
• food in the stomach.
• Stomach
• The stomach is a muscular sac that is located on the left side of the
• abdominal cavity, just inferior to the diaphragm. In an average person, the
• stomach is about the size of their two fists placed next to each other. This
• major organ acts as a storage tank for food so that the body has time to
• digest large meals properly. The stomach also contains hydrochloric acid
• and digestive enzymes that continue the digestion of food that began in the

mouth.

Small Intestine

• The small intestine is a long, thin tube about 1 inch in diameter and about
• 10 feet long that is part of the lower gastrointestinal tract. It is located just
• inferior to the stomach and takes up most of the space in the abdominal
• cavity. The entire small intestine is coiled like a hose and the inside surface
• is full of many ridges and folds. These folds are used to maximize the
• digestion of food and absorption of nutrients. By the time food leaves the
• small intestine, around 90% of all nutrients have been extracted from the
• food that entered it.
• Liver and Gallbladder
• The liver is a roughly triangular accessory organ of the digestive system
• located to the right of the stomach, just inferior to the diaphragm and
• superior to the small intestine. The liver weighs about 3 pounds and is the
• second largest organ in the body. The liver has many different functions in
• the body, but the main function of the liver in digestion is the production of
• bile and its secretion into the small intestine. The gallbladder is a small,
• pear-shaped organ located just posterior to the liver. The gallbladder is used
• to store and recycle excess bile from the small intestine so that it can be
• reused for the digestion of subsequent meals.
• Pancreas
• The pancreas is a large gland located just inferior and posterior to the
• stomach. It is about 6 inches long and shaped like short, lumpy snake with
• its ―head‖ connected to the duodenum and its ―tail‖ pointing to the left wall
• of the abdominal cavity. The pancreas secretes digestive enzymes into the

small intestine to complete the chemical digestion of foods.

Large Intestine

• The large intestine is a long, thick tube about 2 ½ inches in diameter and
• about 5 feet long. It is located just inferior to the stomach and wraps around
• the superior and lateral border of the small intestine. The large intestine
• absorbs water and contains many symbiotic bacteria that aid in the breaking
• down of wastes to extract some small amounts of nutrients. Feces in the
• large intestine exit the body through the anal canal.
• The digestive system is responsible for taking whole foods and turning them
• into energy and nutrients to allow the body to function, grow, and repair
• itself. The six primary processes of the digestive system include:
• Ingestion of food
• Secretion of fluids and digestive enzymes
• Mixing and movement of food and wastes through the body
• Digestion of food into smaller pieces
• Absorption of nutrients
• Excretion of wastes
• Ingestion
• The first function of the digestive system is ingestion, or the intake of food.
• The mouth is responsible for this function, as it is the orifice through which
• all food enters the body. The mouth and stomach are also responsible for the
• storage of food as it is waiting to be digested. This storage capacity allows
• the body to eat only a few times each day and to ingest more food than it can
• process at one time.
• Secretion
• In the course of a day, the digestive system secretes around 7 liters of fluids.
• These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile.
• Saliva moistens dry food and contains salivary amylase, a digestive enzyme
• that begins the digestion of carbohydrates. Mucus serves as a protective
• barrier and lubricant inside of the GI tract. Hydrochloric acid helps to digest
• food chemically and protects the body by killing bacteria present in our
• food. Enzymes are like tiny biochemical machines that disassemble large
• macromolecules like proteins, carbohydrates, and lipids into their smaller
• components. Finally, bile is used to emulsify large masses of lipids into tiny
• globules for easy digestion.
• Mixing and Movement
• The digestive system uses 3 main processes to move and mix food:
• Swallowing. Swallowing is the process of using smooth and skeletal muscles
• in the mouth, tongue, and pharynx to push food out of the mouth, through
• the pharynx, and into the esophagus.
• Peristalsis. Peristalsis is a muscular wave that travels the length of the GI
• tract, moving partially digested food a short distance down the tract. It takes
• many waves of peristalsis for food to travel from the esophagus, through the
• stomach and intestines, and reach the end of the GI tract.
• Segmentation. Segmentation occurs only in the small intestine as short
• segments of intestine contract like hands squeezing a toothpaste tube.
• Segmentation helps to increase the absorption of nutrients
• and increasing its contact with the walls of the intestine.
• Digestion
• Digestion is the process of turning large pieces of food into its component
• chemicals. Mechanical digestion is the physical breakdown of large pieces
• of food into smaller pieces. This mode of digestion begins with the chewing
• of food by the teeth and is continued through the muscular mixing of food
• by the stomach and intestines. Bile produced by the liver is also used to
• mechanically break fats into smaller globules. While food is being
• mechanically digested it is also being chemically digested as larger and more
• complex molecules are being broken down into smaller molecules that are
• easier to absorb. Chemical digestion begins in the mouth with salivary
• amylase in saliva splitting complex carbohydrates into simple carbohydrates.
• The enzymes and acid in the stomach continue chemical digestion, but the
• bulk of chemical digestion takes place in the small intestine thanks to the
• action of the pancreas. The pancreas secretes an incredibly strong digestive
• cocktail known as pancreatic juice, which is capable of digesting lipids,
• carbohydrates, proteins and nucleic acids. By the time food has left
• the duodenum, it has been reduced to its chemical building blocks—fatty
• acids, amino acids, monosaccharides, and nucleotides.
• Absorption
• Once food has been reduced to its building blocks, it is ready for the body to
• absorb. Absorption begins in the stomach with simple molecules like water
• and alcohol being absorbed directly into the bloodstream. Most absorption
• takes place in the walls of the small intestine, which are densely folded to
• maximize the surface area in contact with digested food. Small blood and
• lymphatic vessels in the intestinal wall pick up the molecules and carry them
• to the rest of the body. The large intestine is also involved in the absorption
• of water and vitamins B and K before feces leave the body.
• Excretion
• The final function of the digestive system is the excretion of waste in a
• process known as defecation. Defecation removes indigestible substances
• from the body so that they do not accumulate inside the gut. The timing of
• defecation is controlled voluntarily by the conscious part of the brain, but
• must be accomplished on a regular basis to prevent a backup of indigestible
• materials.
• The digestive system consists of the digestive tract and its associated organs.
• The digestive tract is a tube about 7 m (24 ft) long through which food
• passes while it is broken down. The tract consists of the mouth and pharynx
• (throat), esophagus, stomach, the small and large intestines, and the anus.
• The associated digestive organs include three pairs of salivary glands, the
• liver, the pancreas, and the gallbladder.
• The peritoneum
• This folded membrane covers and secures the digestive organs and also
• covers the inside of the abdominal wall.
• One-way valve
• The pyloric sphincter, which joins the stomach to the small intestine, is one
• of several one-way valves in the digestive tract.
• Digestive organs, Metabolism
• Metabolism
• The chemical processes that occur within a living organism in order to
• maintain life.

"The metabolism of fatty acids in the kidney"

How Human Body Metabolism Works

• To understand the digestive system and body metabolism, knowing
• the organs involved and what roles they play is the first step, they we
• should learn how metabolism works. The process of metabolism can
• be split up in to two types, anabolism, and catabolism:
• Anabolism – Also known as constructive metabolism supports the
• storage of energy for use at a later date, as well as cell growth and
• repair. During the process of anabolism, small molecules are
• transformed into larger molecules of fat, protein, and/or
• carbohydrates.
• Catabolism – Also known as destructive metabolism, catabolism
• is the process of breaking down large fat, protein, and/or
• carbohydrate molecules to use for energy.
• A gland called the pancreas secretes hormones to help your body to
• determine whether its main metabolic activity will be catabolic or
• anabolic.
• The speed in which your body metabolized the food that you consume
• varies depending on the individual. An individual‘s rate of
• metabolism is dependent upon the volume of thyroxine that is
• produced within the thyroid; thyroxine being a hormone that regulates
• metabolism.
• Calories and Basal Metabolic Rate
• The chemical process of metabolism is very complex, which has lead
• many people to look at it in the simplest way possible; it affects how
• easily you gain or loose weight. As we eat, we store calories (a calorie
• is a unit of measurement used to determine the quantity of energy
• given to the body by a particular food), the calories we store are later
• metabolized and utilized as energy. But, it is possible to overload on
• calories and end up with an excess amount, which is stored as body
• fat.
• The amount of calories a person can burn in a day is dependent upon
• their basal metabolic rate (BMR), this is the measurement of the rate
• at which a person ―burns‖ calories as energy. The higher a person‘s
• BMR, the faster they can burn calories meaning less are stored as
• body fat. This means that they will be able to eat more and store less.
• Although it is thought that a person‘s BMR is inherited, there are
• steps you can take to increase yours. Exercise is the best method, as
• this will not only help you lose calories and stay fit, it is often the case
• that low body fat levels equals low BMR, and vice versa.
• Your Digestive System and How it Works
• Digestion is the breakdown of large insoluble food molecules into small
• water-soluble food molecules so that they can be absorbed into the watery
• blood plasma. In certain organisms, these smaller substances are absorbed
• through the small intestine into the blood stream
• The digestive system is a series of hollow organs joined in a long, twisting
• tube from the mouth to the anus (see Figure 1). Inside this tube is a lining
• called the mucosa? In the mouth, stomach, and small intestine, the mucosa
• contains tiny glands that produce juices to help digest food.
• Two solid organs, the liver and the pancreas, produce digestive juices that
• reach the intestine through small tubes. In addition, parts of other organ
• systems (for instance, nerves and blood) play a major role in the digestive
• system.
• Why is digestion important
• Digestion involves the mixing of food, its movement through the digestive
• tract, and the chemical breakdown of the large molecules of food into
• smaller molecules. Digestion begins in the mouth, when we chew and
• swallow, and is completed in the small intestine. The chemical process
• varies somewhat for different kinds of food.
• Movement of Food Through the System
• The large, hollow organs of the digestive system contain muscle that enables
• their walls to move. The movement of organ walls can propel food and
• liquid and also can mix the contents within each organ.
• Typical movement of the esophagus, stomach, and intestine is called
• peristalsis. The action of peristalsis looks like an ocean wave moving
• through the muscle.
• The muscle of the organ produces a narrowing and then propels the
• narrowed portion slowly down the length of the organ. These waves of
• narrowing push the food and fluid in front of them through each hollow
• organ.
• The first major muscle movement occurs when food or liquid is swallowed.
• Although we are able to start swallowing by choice, once the swallow
• begins, it becomes involuntary and proceeds under the control of the nerves.
• The esophagus is the organ into which the swallowed food is pushed. It
• connects the throat above with the stomach below. At the junction of the
• esophagus and stomach, there is a ringlike valve closing the passage
• between the two organs. However, as the food approaches the closed ring,
• the surrounding muscles relax and allow the food to pass.
• The food then enters the stomach, which has three mechanical tasks to do.
• First, the stomach must store the swallowed food and liquid. This requires
• the muscle of the upper part of the stomach to relax and accept large
• volumes of swallowed material.
• The second job is to mix up the food, liquid, and digestive juice produced by
• the stomach. The lower part of the stomach mixes these materials by its
• muscle action. (The mixture is referred to as chyme.)
• The third task of the stomach is to empty its contents slowly into the small
• intestine.
• Several factors affect emptying of the stomach, including the nature of the
• food (mainly its fat and protein content) and the degree of muscle action of
• the emptying stomach and the next organ to receive the contents (the small
• intestine).
• As the food is digested in the small intestine and dissolved into the juices
• from the pancreas, liver, and intestine, the contents of the intestine are mixed
• and pushed forward to allow further digestion.
• Finally, all of the digested nutrients are absorbed through the intestinal
• walls. The waste products of this process include undigested parts of the
• food, known as fiber, and older cells that have been shed from the mucosa.
• These materials are propelled into the colon, where they remain, usually for

a day or two, until the feces are expelled by a bowel movement.

Small Intestine/Bowel

• The mixture of food, liquid, and digestive juice (chyme) that passes out of
• the stomach, in a regulated controlled manner, enters into the small
• intestine/bowel. The average total length of the normal small bowel in adults
• is about 7 meters/22 feet. The small intestine has 3 segments:
• the duodenum,
• the jejunum, and
• the ileum.
• Each part or section performs an important role in nutrient absorption.
• Duodenum – The chyme first enters into the duodenum where it is exposed
• to secretions that aid digestion. The secretions include bile salts, enzymes,
• and bicarbonate. The bile salts from the liver help digest fats and fat soluble
• vitamins (Vitamin A, D, E, and K). Pancreatic enzymes help digest
• carbohydrates and fats. Bicarbonate from the pancreas neutralizes the acid
• from the stomach.
• Jejunum – The chyme is then further transited down into the second or
• middle part of the small intestine, the jejunum. Mainly in the first half of the
• jejunum, the majority (about 90%) of nutrient absorption occurs involving
• proteins, carbohydrates, vitamins, and minerals.
• Ileum – The ileum is the last section of the small intestine and leads to the
• large intestine or colon. The ileum mainly absorbs water, bile salts, and
• vitamin B12.
• The ileocecal valve is a one-way valve located between the ileum and the
• cecum, which is the first portion of the colon. This valve helps control the
• passage of contents into the colon and increases the contact time of nutrients
• and electrolytes (essential minerals) with the small intestine. It also prevents
• back-flow (reflux) from the colon up into the ileum, and helps minimize the
• movement of bacteria from the large intestine up into the small bowel.
• Large Intestine or Colon
• The primary function of the large intestine or colon is to absorb fluids and
• electrolytes, particularly sodium and potassium, and to convert remaining
• luminal contents into more solid stool. The colon absorbs on average 1–1.5
• liters (about 1–1.5 quarts) of fluid every day and has a capacity to adapt its
• fluid absorption to as much as 5 liters/quarts per day if needed.
• Another function of the colon is to break down (ferment) dietary fiber to
• produce short chain fatty acids – substances that can be absorbed and
• provide added nutrition.
• The first portion of the colon, the cecum, is shaped like a pouch, and is the
• area of storage for the contents arriving from the ileum. The second portion
• is the ascending colon, where fluids are absorbed and where some stool
• formation begins.
• Production of Digestive Juices
• The glands that act first are in the mouth – the salivary glands. Saliva
• produced by these glands contains an enzyme that begins to digest the starch
• from food into smaller molecules.
• The next set of digestive glands is in the stomach lining. They produce
• stomach acid and an enzyme that digests protein. One of the unsolved
• puzzles of the digestive system is why the acid juice of the stomach does not
• dissolve the tissue of the stomach itself. In most people, the stomach mucosa
• is able to resist the juice, although food and other tissues of the body cannot.
• After the stomach empties the food and juice mixture into the small
• intestine, the juices of two other digestive organs mix with the food to
• continue the process of digestion.
• One of these organs is the pancreas. It produces a juice that contains a wide
• array of enzymes to break down the carbohydrate, fat, and protein in food.
• Other enzymes that are active in the process come from glands in the wall of
• the intestine or even a part of that wall.
• The liver produces yet another digestive juice – bile. The bile is stored
• between meals in the gallbladder. At mealtime, it is squeezed out of the
• gallbladder into the bile ducts to reach the intestine and mix with the fat in
• our food. The bile acids dissolve the fat into the watery contents of the
• intestine, much like detergents that dissolve grease from a frying pan. After
• the fat is dissolved, it is digested by enzymes from the pancreas and the
• lining of the intestine.
• Absorption and Transport of Nutrients
• Digested molecules of food, as well as water and minerals from the diet, are
• absorbed from the cavity of the upper small intestine. Most absorbed
• materials cross the mucosa into the blood and are carried off in the
• bloodstream to other parts of the body for storage or further chemical
• change. As already noted, this part of the process varies with different types
• of nutrients.
• Carbohydrates. It is recommended that about 55 to 60 percent of total daily
• calories be from carbohydrates. Some of our most common foods contain
• mostly carbohydrates. Examples are bread, potatoes, legumes, rice,
• spaghetti, fruits, and vegetables. Many of these foods contain both starch
• and fiber.
• The digestible carbohydrates are broken into simpler molecules by enzymes
• in the saliva, in juice produced by the pancreas, and in the lining of the small
• intestine.
• Starch is digested in two steps: First, an enzyme in the saliva and pancreatic
• juice breaks the starch into molecules called maltose; then an enzyme in the
• lining of the small intestine (maltase) splits the maltose into glucose
• molecules that can be absorbed into the blood.
• Glucose is carried through the bloodstream to the liver, where it is stored or
• used to provide energy for the work of the body.
• Table sugar is another carbohydrate that must be digested to be useful. An
• enzyme in the lining of the small intestine digests table sugar into glucose
• and fructose, each of which can be absorbed from the intestinal cavity into
• the blood. Milk contains yet another type of sugar, lactose, which is changed
• into absorbable molecules by an enzyme called lactase, also found in the
• intestinal lining.
• Protein. Foods such as meat, eggs, and beans consist of giant molecules of
• protein that must be digested by enzymes before they can be used to build
• and repair body tissues. An enzyme in the juice of the stomach starts the
• digestion of swallowed protein.
• Further digestion of the protein is completed in the small intestine. Here,
• several enzymes from the pancreatic juice and the lining of the intestine
• carry out the breakdown of huge protein molecules into small molecules
• called amino acids. These small molecules can be absorbed from the hollow
• of the small intestine into the blood and then be carried to all parts of the
• body to build the walls and other parts of cells.
• Fats. Fat molecules are a rich source of energy for the body. The first step in
• digestion of a fat such as butter is to dissolve it into the watery content of the
• intestinal cavity.
• The bile acids produced by the liver act as natural detergents to dissolve fat
• in water and allow the enzymes to break the large fat molecules into smaller
• molecules, some of which are fatty acids and cholesterol. The bile acids
• combine with the fatty acids and cholesterol and help these molecules to
• move into the cells of the mucosa.
• In these cells the small molecules are formed back into large molecules,
• most of which pass into vessels (called lymphatics) near the intestine. These
• small vessels carry the reformed fat to the veins of the chest, and the blood
• carries the fat to storage depots in different parts of the body.
• Vitamins. Another vital part of our food that is absorbed from the small

intestine is the class of chemicals we call vitamins. The two different types of vitamins are classified by the fluid in which they can be dissolved: watersoluble vitamins (all the B vitamins and vitamin C) and fat-soluble vitamins (vitamins A, D, E, and K).

• Water and salt. Most of the material absorbed from the cavity of the small
• intestine is water in which salt is dissolved. The salt and water come from
• the food and liquid we swallow and the juices secreted
• digestive glands.

How is the digestive process controlled

Hormone Regulators

• A fascinating feature of the digestive system is that it contains its own
• regulators. The major hormones that control the functions of the digestive
• system are produced and released by cells in the mucosa of the stomach and
• small intestine. These hormones are released into the blood of the digestive
• tract, travel back to the heart and through the arteries, and return to the
• digestive system, where they stimulate digestive juices and cause organ
• movement.
• The hormones that control digestion are gastrin, secretin, and
• cholecystokinin (CCK):
• Gastrin causes the stomach to produce an acid for dissolving and
• digesting some foods. It is also necessary for the normal growth of the
• lining of the stomach, small intestine, and colon.
• Secretin causes the pancreas to send out a digestive juice that is rich
• in bicarbonate. It stimulates the stomach to produce pepsin, an
• enzyme that digests protein, and it also stimulates the liver to produce
• bile.
• CCK causes the pancreas to grow and to produce the enzymes of
• pancreatic juice, and it causes the gallbladder to empty.
• Additional hormones in the digestive system regulate appetite:
• Ghrelin is produced in the stomach and upper intestine in the absence
• of food in the digestive system and stimulates appetite.
• Peptide YY is produced in the GI tract in response to a meal in the
• system and inhibits appetite.
• Both of these hormones work on the brain to help regulate the intake of food

for energy.

Nerve Regulators

• Two types of nerves help to control the action of the digestive system.
• Extrinsic (outside) nerves come to the digestive organs from the unconscious
• part of the brain or from the spinal cord. They release a chemical called
• acetylcholine and another called adrenaline. Acetylcholine causes the muscle
• of the digestive organs to squeeze with more force and increase the "push"
• of food and juice through the digestive tract. Acetylcholine also causes the
• stomach and pancreas to produce more digestive juice. Adrenaline relaxes
• the muscle of the stomach and intestine and decreases the flow of blood to
• these organs.
• Even more important, though, are the intrinsic (inside) nerves, which make
• up a very dense network embedded in the walls of the esophagus, stomach,
• small intestine, and colon. The intrinsic nerves are triggered to act when the
• walls of the hollow organs are stretched by food. They release many
• different substances that speed up or delay the movement of food and the

production of juices by the digestive organs.