Gastrointestinal tract, also called digestive tract or alimentary canal, pathway by which food enters the body and solid wastes are expelled. The gastrointestinal tract includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. See digestion.

The Oral Cavity or Mouth:

The oral cavity, or mouth, is the point of entry of food into the digestive system. The cheeks, tongue, and palate frame the mouth, which is also called the oral cavity (or buccal cavity). The arched shape of the roof of your mouth is called the palate. If you run your tongue along the roof of your mouth, the front part is the hard palate, and at the back, the roof becomes fleshier. This part of the palate, known as the soft palate. A fleshy bead of tissue called the uvula drops down from the center of the posterior edge of the soft palate. Although some have suggested that the uvula is a vestigial organ, it serves an important purpose. When you swallow, the soft palate and uvula move upward, helping to keep foods and liquid from entering the nasal cavity. Unfortunately, it can also contribute to the sound produced by snoring. Two muscular folds extend downward from the soft palate, on either side of the uvula. Between these two arches are the palatine tonsils, clusters of lymphoid tissue that protect the pharynx. The lingual tonsils are located at the base of the tongue. These glands make antibodies that help fight infection. Tonsillitis means that your tonsils are inflamed. Both bacteria and viruses can cause tonsillitis.

The Tongue

The tongue is a workhorse, facilitating ingestion, mechanical digestion, chemical digestion (lingual lipase), sensation (of taste, texture, and temperature of food), swallowing, and vocalization. The tongue perform three important digestive functions in the mouth:

• Position food for optimal chewing
• Gather food into a bolus (rounded mass)
• Position food so it can be swallowed

A fold of mucous membrane on the underside of the tongue, the lingual frenulum, tethers the tongue to the floor of the mouth. People with the congenital anomaly ankyloglossia, also known by the non-medical term “tongue tie,” have a lingual frenulum that is too short or otherwise malformed. Severe ankyloglossia can impair speech and must be corrected with surgery.

The Teeth

The teeth, or dentes (singular = dens), are organs similar to bones that you use to tear, grind, and otherwise mechanically break down food. During the course of your lifetime, you have two sets of teeth (one set of teeth is a dentition). Your 20 deciduous teeth, or baby teeth, first begin to appear at about 6 months of age. Between approximately age 6 and 12, these teeth are replaced by 32 permanent teeth. Moving from the center of the mouth toward the side, these are as follows:

• The eight incisors, four top and four bottom, are the sharp front teeth you use for biting into food.
• The four cuspids (or canines) flank the incisors and have a pointed edge (cusp) to tear up food. These fang-like teeth are superb for piercing tough or fleshy foods.
• Posterior to the cuspids are the eight premolars (or bicuspids), which have an overall flatter shape with two rounded cusps useful for mashing foods.
• The most posterior and largest are the 12 molars, which have several pointed cusps used to crush food so it is ready for swallowing. The third members of each set of three molars, top and bottom, are commonly referred to as the wisdom teeth, because their eruption is commonly delayed until early adulthood. It is not uncommon for wisdom teeth to fail to erupt; that is, they remain impacted. In these cases, the teeth are typically removed by orthodontic surgery.

Anatomy of a Tooth: The teeth are secured in the alveolar processes (sockets) of the maxilla and the mandible.  Gingivae (commonly called the gums) are soft tissues that line the alveolar processes and surround the necks of the teeth.

The two main parts of a tooth are the crown, which is the portion projecting above the gum line, and the root, which is embedded within the maxilla and mandible. Both parts contain an inner pulp cavity, containing loose connective tissue through which run nerves and blood vessels. The region of the pulp cavity that runs through the root of the tooth is called the root canal. Surrounding the pulp cavity is dentin, a bone-like tissue. In the root of each tooth, the dentin is covered by an even harder bone-like layer called cementum. In the crown of each tooth, the dentin is covered by an outer layer of enamel, the hardest substance in the body.

Although enamel protects the underlying dentin and pulp cavity, it is still nonetheless susceptible to mechanical and chemical erosion, or what is known as tooth decay. The most common form, dental caries (cavities) develops when colonies of bacteria feeding on sugars in the mouth release acids that cause soft tissue inflammation and degradation of the calcium crystals of the enamel.

The Salivary Glands

Many small salivary glands are housed within the mucous membranes of the mouth and tongue. These minor exocrine glands are constantly secreting saliva, either directly into the oral cavity or indirectly through ducts, even while you sleep.

The Major Salivary Glands

• The submandibular glands, which are in the floor of the mouth, secrete saliva into the mouth through the submandibular ducts.

 

• The sublingual glands, which lie below the tongue, use the lesser sublingual ducts to secrete saliva into the oral cavity.

 

• The parotid glands lie between the skin and the masseter muscle, near the ears

Saliva: The extensive chemical process of digestion begins in the mouth. As food is being chewed, saliva, produced by the salivary glands, mixes with the food. Saliva is a watery substance produced in the mouths of many animals.  Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains  lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by the cells in the tongue. Lipases are a class of enzymes that can break down triglycerides-  fats. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx.

The pharynx:

The pharynx (throat) is involved in both digestion and respiration. It receives food and air from the mouth, and air from the nasal cavities.  The pharynx located in throat, connects mouth to the esophagus. Pharynx has three subdivisions. The most superior, the nasopharynx, is involved only in breathing and speech. The other two subdivisions, the oropharynx and the laryngopharynx, are used for both breathing and digestion.

Pharynx opens to two passageways:

• the trachea, which leads to the lungs,      ( air way)     and
• the esophagus, which leads to the stomach. 

The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This arrangement allows food to be kept out of the trachea.  When the food “goes down the wrong way,” it goes into the trachea. When food enters the trachea, the reaction is to cough, which usually forces the food up and out of the trachea, and back into the pharynx.

Esophagus

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the food toward the stomach. The peristalsis wave is unidirectional—it moves food from the mouth to the stomach.

A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus.  When the lower esophageal sphincter does not completely close, the stomach’s contents can reflux (that is, back up into the esophagus), causing heartburn or gastroesophageal reflux disease (GERD)..

 

The Stomach

The stomach is a J-shaped saclike organ in the upper abdomen.  The stomach is much like a bag which  store food and secretes gastric digestive juices. The wall of the stomach is made of the same four layers as most of the rest of the alimentary canal. In addition to the typical circular and longitudinal smooth muscle layers, the muscularis has an inner oblique smooth muscle layer. As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.

The pH in the stomach is between 1.5 and 2.5. This highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients.A large part of digestion occurs in the stomach.

The stomach wall is adapted for the functions of the stomach. In the epithelium, gastric pits lead to gastric glands that secrete gastric juice. The gastric glands  contain different types of cells that secrete a variety of enzymes, including hydrochloride acid (HCl), which activates the protein-digesting enzyme pepsin.

• Parietal cells— produce both hydrochloric acid (HCl) and intrinsic factor. HCl is responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents and is needed to activate the protein-digesting enzyme, pepsin. The acidity also kills much of the bacteria you ingest with food and helps to denature proteins, making them more available for enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of vitamin B₁₂ in the small intestine.  The parietal cells are capable of creating a very acidic pH of 2.0 within the stomach.
• Chief cells—Located primarily in the basal regions of gastric glands are chief cells, which secrete pepsinogen, the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin.
• Mucous neck cells—secrete thin, acidic mucus that is much different from the mucus secreted by the goblet cells of the surface epithelium. The role of this mucus is not currently known.
• Enteroendocrine cells— secrete various hormones – include gastrin,

The combination of all of the stomach secretions is known as gastric juice. Food enters the stomach and combines with gastric juice to form a pasty substance called chyme. Chyme then leaves the stomach by way of the pyloric sphincter and enters the duodenum.

The Mucosal Barrier: The mucosa of the stomach is exposed to the highly corrosive acidity of gastric juice. Gastric enzymes that can digest protein can also digest the stomach itself. The stomach is protected from self-digestion by the mucosal barrier. This barrier has several components. First, the stomach wall is covered by a thick coating of bicarbonate-rich mucus. This mucus forms a physical barrier, and its bicarbonate ions neutralize acid. Second, the epithelial cells of the stomach’s mucosa meet at tight junctions, which block gastric juice from penetrating the underlying tissue layers. Finally, stem cells located where gastric glands join the gastric pits quickly replace damaged epithelial mucosal cells, when the epithelial cells are shed. In fact, the surface epithelium of the stomach is completely replaced every 3 to 6 days.

Small Intestine

The small intestine is a long muscular tube of about 20 feet long and about an inch in diameter. Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed and  absorb most of the nutrients from what we eat and drink.

Small Intestine – Structure

The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the Duodenum,  Jejunum, and Ileum.  

• The shortest region is the 25.4-cm (10-in) duodenum, which begins at the pyloric sphincter and makes a C-shaped curve and join the jejunum.. Digestive juices from the pancreas, liver, and gallbladder  enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. Some chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum.

• The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the duodenum to the ileum.  Here, hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.
• The ileum is the longest part of the small intestine, measuring about 1.8 meters (6 feet) in length. The ileum joins the cecum, the first portion of the large intestine, at the ileocecal sphincter (or valve). The undigested food is sent to the colon from the ileum via peristaltic movements of the muscle. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.

Histology

The wall of the small intestine is composed of the same four layers typically present in the alimentary system. The small intestine has a highly folded inner surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli.  Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border. These structures are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the blood in hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens.

Large Intestine

The large intestine, illustrated in Figure 13, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter.  The colon can be divided into four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes.

Bacterial Flora

Most bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria live within the large intestine and are referred to as the bacterial flora. Most of the more than 700 species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K. Fecal transplantation (or bacteriotherapy) is the transfer of stool from a healthy donor into the gastrointestinal tract for the purpose of treating recurrent C. difficile colitis.  When antibiotics kill off too many “good” bacteria in the digestive tract, fecal transplants can help replenish bacterial balance.

 

Rectum and Anus

The rectum is the terminal end of the large intestine, as shown in Figure 13. The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material.

The small intestine absorbs about 90 percent of the water you ingest (either as liquid or within solid food). The large intestine absorbs most of the remaining water, a process that converts the liquid chyme residue into semisolid feces (“stool”). Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body.

Feces are eliminated through contractions of the rectal muscles. You help this process by a voluntary procedure called Valsalva’s maneuver, in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis.

Accessory Organs

Each accessory digestive organ aids in the breakdown of food. Within the mouth, the teeth and tongue begin mechanical digestion. Other accessory organs  add secretions (enzymes) that catabolize food into nutrients.

Accessory organs include salivary glands, the liver,  the pancreas, and  the gallbladder.

• 1. Salivary glands: There are three major salivary glands that secrete saliva into the oral cavity —the parotid, the submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food.

• 2. The Liver: The liver is the largest gland in the body, weighing about three pounds in an adult.  It is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver lies inferior to the diaphragm in the right upper quadrant of the abdominal cavity and receives protection from the surrounding ribs. The liver is divided into two primary lobes: a large right lobe and a much smaller left lobe. The lobes are further divided into lobules by blood vessels and connective tissue. The liver performs many functions and is considered a vital organ.

Liver functions include:

Detoxifying the blood, producing bile, (a digestive juice that is required for the breakdown of fatty components of the food in the duodenum.), metabolism of carbohydrates, fats and proteins, storing iron, blood and vitamins, recycling red blood cells and producing plasma proteins.

Bile : composition and function.

Bile is a mixture secreted by the liver to accomplish the emulsification of lipids in the small intestine. Recall that lipids are hydrophobic, that is, they do not dissolve in water. Thus, before they can be digested in the watery environment of the small intestine, large lipid globules must be broken down into smaller lipid globules, a process called emulsification.

Hepatocytes (cells of liver) secrete about one liter of bile each day. A yellow-brown or yellow-green alkaline solution (pH 7.6 to 8.6), bile is a mixture of water, bile salts, bile pigments, phospholipids (such as lecithin), electrolytes, cholesterol, and triglycerides. The components most critical to emulsification are bile salts and phospholipids, which have a nonpolar (hydrophobic) region as well as a polar (hydrophilic) region. The hydrophobic region interacts with the large lipid molecules, whereas the hydrophilic region interacts with the watery chyme in the intestine. This results in the large lipid globules being pulled apart into many tiny lipid fragments of about 1 µm in diameter. This change dramatically increases the surface area available for lipid-digesting enzyme activity. This is the same way dish soap works on fats mixed with water. Bile salts act as emulsifying agents, so they are also important for the absorption of digested lipids.

Bilirubin, the main bile pigment, is a waste product produced when the spleen removes old or damaged red blood cells from the circulation. Bilirubin is eventually transformed by intestinal bacteria into stercobilin, a brown pigment that gives your stool its characteristic color!

Gallbladder stores and releases bile: The gallbladder is 8–10 cm (~3–4 in) long and is nested in  the right lobe of the liver. The liver secretes bile which is stored in the gallbladder. This muscular sac stores, concentrates, and, when stimulated, propels the bile into the duodenum via the common bile duct.  When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.

Pancreas

The pancreas is another important gland that secretes digestive juices. The pancreas produces over a liter of pancreatic juice each day. Unlike bile, it is clear and composed mostly of water along with some salts, sodium bicarbonate, and several digestive enzymes.

• Sodium bicarbonate is responsible for the slight alkalinity of pancreatic juice (pH 7.1 to 8.2), which serves to buffer the acidic gastric juice in chyme, inactivate pepsin from the stomach, and create an optimal environment for the activity of pH-sensitive digestive enzymes in the small intestine.
• Pancreatic enzymes are active in the digestion of sugars, proteins, and fats.
  • Pancreatic amylase digesting starch
  • Pancreatic lipase digesting  fat
  • Pancreatic trypsin digesting proteins
  • Pancreatic nuclease digesting nucleic acids

Pancreas also perform endocrine  functions releasing hormones into the blood.

 

 

Table 1. Functions of the Digestive Organs (summary)
Organ	Major functions	Other functions
Mouth	Ingests food Chews and mixes food Begins chemical breakdown of carbohydrates Moves food into the pharynx Begins breakdown of lipids via lingual lipase	Moistens and dissolves food, allowing you to taste it Cleans and lubricates the teeth and oral cavity Has some antimicrobial activity
Pharynx	Propels food from the oral cavity to the esophagus	Lubricates food and passageways
Esophagus	Propels food to the stomach	Lubricates food and passageways
Stomach	Mixes and churns food with gastric juices to form chyme Begins chemical breakdown of proteins Releases food into the duodenum as chyme Absorbs some fat-soluble substances (for example, alcohol, aspirin) Possesses antimicrobial functions	Stimulates protein-digesting enzymes Secretes intrinsic factor required for vitamin B12absorption in small intestine
Small intestine	Mixes chyme with digestive juices Propels food at a rate slow enough for digestion and absorption Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water Performs physical digestion via segmentation	Provides optimal medium for enzymatic activity
Accessory organs	Liver: produces bile salts, which emulsify lipids, aiding their digestion and absorption Gallbladder: stores, concentrates, and releases bile Pancreas: produces digestive enzymes and bicarbonate	Bicarbonate-rich pancreatic juices help neutralize acidic chyme and provide optimal environment for enzymatic activity
Large intestine	Further breaks down food residues Absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria Propels feces toward rectum Eliminates feces	Food residue is concentrated and temporarily stored prior to defecation Mucus eases passage of feces through colon
Rectum	receive stool from the colon, to let the person know that there is stool to be evacuated, and to hold the stool until evacuation happens.
Anus	the opening at the end of the alimentary canal through which solid waste matter leaves the body.