The alimentary canal is a muscular hollow continuous tubular organ that starts at the mouth and terminates at the anus and is responsible for the digestion and absorption of the ingested food and liquids. The alimentary canal or alimentary tract is part of the digestive (gastrointestinal) system.
Anatomy of the Alimentary Canal (or gastrointestinal tract)
The anatomy of the alimentary canal differs extensively in different organisms; however, it is only seen in the organism with bilateral symmetry. In humans, the alimentary canal is highly complex and is divided into different organs and tissues that specialize in a particular function of the digestive system. In humans, the length of the alimentary canal is around 9m ~ 30 feet and is open at both ends, at one end is the mouth and at the other is the anus. Essentially, in humans, the alimentary canal is made up of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
The mouth, pharynx, esophagus, stomach, and duodenum comprise the upper gastrointestinal tract whereas the small intestine and large intestine are parts of the lower gastrointestinal tract. For the different GI tract organs, refer to the GI tract diagram (Figure 1).
In general, the digestion pathway of the food in the alimentary canal is as follows-
The food is ingested in the oral cavity where it is chewed upon (this process is known as mastication of food) and is moistened with saliva. Further, the food bolus from the oral cavity is transferred to the stomach via the esophagus. In the stomach, food bolus is converted to chyme with the action of enzymatic secretions. This chyme is then transferred to the small intestine where it is further treated upon by various enzymes and the majority of the nutrients are absorbed in this part of the alimentary canal. From the small intestine, the residual food is transferred to the large intestine for the reabsorption of water and electrolytes. Once water, residual nutrients, and electrolytes are reabsorbed the solid waste is expelled from the body via the anal canal.
Mouth
The mouth receives and mechanically breaks down food, produces saliva, and is the first portion of the alimentary canal.
Key Points
The mouth is also known as the oral cavity. Its purpose is to mechanically break down food, moisten it with saliva, and swallow the food into the esophagus and the stomach.
While vocal sounds are primarily produced in the throat, the tongue, lips, and jaw are also needed to produce the range of sounds included in human language.
Saliva is produced by three main pairs of salivary glands: the parotid, the submandibular, and sublingual. When food is chewed and mixed with this saliva, the resulting wad is known as a bolus.
Key Terms
- mastication: The process of physical and mechanical breakdown of food; chewing.
- mucous membrane: A membrane that secretes mucus. It forms the lining of various body passages that communicate with the air, such as the respiratory, genitourinary, and alimentary tracts.
- mouth: The opening of a organism through which food is ingested.
- saliva: A clear, slightly alkaline liquid secreted into the mouth by the salivary glands and mucous glands that consists of water, mucin, protein, and enzymes. It moistens the mouth, lubricates ingested food, and begins the breakdown of starches.
- uvula: A soft, punching-bag-like piece of tissue that hangs at the back of the mouth and functions in closing the air passages during swallowing, in conjunction with the epiglottis of the trachea.
- hard palate: The bony roof of the mouth, located ventrally to the soft palate.
- alimentary canal: The organs of a human or a non-human animal through which food passes.
- alveolar arch: The part of the upper or lower jawbones in which the teeth are set.
Oral cavity
The oral cavity is the part of the alimentary canal wherein food is ingested. The tongue, teeth, and palate form the components of the oral cavity. Further, the food is broken down, by the action of the teeth, into smaller fragments. The food is softened by the saliva in the oral cavity. The saliva is secreted by the salivary gland and contains enzymes like amylase and lysozyme. The movement of the jaws and tongue facilitates the movement of the food throughout the oral cavity. Once the food bolus is moistened and fragmented, it is then transferred by deglutition (swallowing) to the esophagus.
Transport passage
The transport passages simply facilitate the movement of the food bolus via peristalsis, from one part of the alimentary canal to the other without inducing any metabolic changes in it. Essentially, these transport passages are muscular tubes lined internally by stratified squamous epithelium and some mucous glands which provide lubricating mucus.
EXAMPLES
The mouth has a variety of roles in human anatomy and sociology. While its primary function is to begin the process of mechanically and chemically digesting food, the mouth is also the beginning of the alimentary canal—a larger digestive tube. Without the human mouth, expressions of the lips and language of the tongue and throat would be impossible.
The mouth is the first portion of the alimentary canal. It receives food and moistens the food with saliva, while the food is mechanically processed (mastication) by the teeth. The mouth is also known as the oral cavity, and within the oral cavity sits the tongue, the soft and hard palate, the uvula, and numerous salivary glands.
The oral mucosa is the mucous membrane epithelial tissue that lines the inside of the mouth. This membrane maintains a moist and lubricated environment within the mouth to prepare the digestive system for the entry of food.
The Mouth as a Communication and Breathing Tool
Inside of the mouth: An illustration of the inside of a human mouth. The cheeks have been omitted in the drawing and the lips pulled back for an unobstructed view of the teeth, tongue, jawbones, uvula, and alimentary canal.
In addition to its primary function as the beginning of the digestive system, the mouth also plays a significant role in human communication and breathing. The primary features of the human voice are produced in the throat, but the tongue, lips, and jaw also work together to produce the range of sounds we see in human language.
Air is drawn in through the mouth to the trachea and lungs, and the lips and tongue form words. The lips mark the transition from the mucous membrane to the outer epithelial skin that covers most of the body. Lips are remarkably sensitive and often serve as an infant’s second hand with which to explore the world.
Mechanical Food Breakdown by Teeth
In the digestive process, the mouth’s purpose is to prepare food for further digestion in the stomach and the small intestine. This process begins with the mechanical breakdown of food by the teeth, which fit into the alveolar arches. The front teeth (incisors and canines) are used to cut and tear food, while the teeth further back (bicuspids and molars) crush and grind.
Food Lubrication and Chemical Digestion By Saliva
Saliva is projected from three main pairs of salivary glands: the large parotid glands near the cheeks, the submandibular glands beneath the mandible, and the sublingual glands beneath the tongue.
Saliva keeps the mouth moist and lubricates the food, helping the tongue form the food into a soft wad, called a bolus. The fluid of saliva also contains several enzymes, notably lysozyme—an antibacterial agent—and amylase, which catalyzes large starch molecules into simpler sugars via hydrolysis.
Cross-section of the head and neck: A cross-section of the head and neck in mid-sagittal view, showing the structures of the mouth and throat.
Once properly chewed and lubricated, food and drink are swallowed into the esophagus, the tube that leads to the stomach.
The Structures of the Lips and External Mouth
Infant humans are born with an instinctual sucking reflex, by which they know how to gain nourishment using their lips and jaw. The philtrum, or bow of the lip, is the vertical groove or dip just below the nose.
The nasolabial folds are the deep creases of tissue that extend from the nose to the sides of the mouth. One of the first signs of age on the human face is the increase in prominence of the nasolabial folds.
Pharynx
The pharynx is part of the digestive and respiratory systems and consists of three main parts: the nasopharynx, oropharynx, and laryngopharynx.
Key Points
The human pharynx (plural: pharynges) is the part of the throat situated immediately inferior to (below) the mouth and nasal cavity, and superior to the esophagus and larynx.
The human pharynx is conventionally divided into three sections: the nasopharynx (epipharynx), the oropharynx (nasopharynx), and the laryngopharynx (hypopharynx).
The nasopharynx extends from the base of the skull to the upper surface of the soft palate. Adenoids are lymphoid tissue structures located in the posterior wall of the nasopharynx. The nasopharynx communicates with the middle ear, nasal cavities, and auditory tube.
The oropharynx lies behind the oral cavity and extends from the uvula to the level of the hyoid bone. A flap of connective tissue called the epiglottis closes over the glottis when food is swallowed to prevent aspiration.
The laryngopharynx is the caudal part of the pharynx; it is the part of the throat that connects to the esophagus. It includes three major sites: the pyriform sinus, the postcricoid area, and the posterior pharyngeal wall.
Key Terms
- adenoid: One of the two folds of lymphatic tissue covered by ciliated epithelium. They are found in the roof and posterior wall of the nasopharynx at the back of the throat behind the uvula. They may obstruct normal breathing and make speech difficult when swollen—a condition often called adenitis.
- epiglottis: A cartilaginous organ in the throat of terrestrial vertebrates that covers the glottis when swallowing to prevent food and liquid from entering the trachea. In Homo sapiens it is also a speech organ.
- uvula: The fleshy appendage that hangs from the back of the palate and closes the nasopharynx during swallowing.
- pharynx: The part of the alimentary canal that extends from the mouth and nasal cavities to the larynx, where it becomes continuous with the esophagus.
The human pharynx (plural: pharynges) is the part of the throat situated immediately inferior to (below) the mouth and nasal cavity, and superior to the esophagus and larynx.
Head and neck overview: The human pharynx is situated immediately below the mouth and nasal cavity, and above the esophagus and larynx.
The human pharynx is conventionally divided into three sections: the nasopharynx (epipharynx), the oropharynx (nasopharynx), and the laryngopharynx (hypopharynx). The pharynx is part of the digestive system and also the respiratory system, as well as an important part in vocalization.
Nasopharynx
The nasopharynx is the most cephalad (toward the head) portion of the pharynx. It extends from the base of the skull to the upper surface of the soft palate. It includes the space between the internal nares and the soft palate and lies superior to the oral cavity.
The pharyngeal tonsils, more commonly referred to as the adenoids, are lymphoid tissue structures located in the posterior wall of the nasopharynx. Polyps or mucus can obstruct the nasopharynx, as can congestion due to an upper respiratory infection.
The eustachian tubes connect the middle ear to the pharynx and open into the nasopharynx. The opening and closing of the eustachian tubes serves to equalize the barometric pressure in the middle ear with that of the ambient atmosphere.
The anterior portion of the nasopharynx connects with the nasal cavities through openings known as choanae. The nasopharynx and its associated nasal tissues are lined with ciliated pseudostratified columnar epithelium, which is excellent for sweeping debris from the nasal passages.
The Pharyngeal Ostia
On the lateral walls of the nasopharynx are the pharyngeal Ostia of the auditory tube—triangle-shaped openings bound from behind by a firm prominence called the torus tuberous or cushion.
This binding is formed by a cartilaginous tube-like opening. Two folds arise from the cartilaginous opening:
- The salpingopharyngeal fold, a vertical fold of mucous membrane that extends from the inferior part of the torus.
- The salpingopalatine fold, a smaller fold that extends from the superior part of the torus to the palate.
Behind the Ostia of the auditory tube is a deep recess known as the pharyngeal recess (or fossa of Rosenmüller).
The posterior wall of the nasopharynx holds the pharyngeal tonsils, which can be especially marked in childhood. Superior to the pharyngeal tonsil, in the midline, an irregular flask-shaped depression of the mucous membrane sometimes extends upward; it is known as the pharyngeal bursa.
Oropharynx
The oropharynx or nasopharynx lies behind the oral cavity and extends from the uvula to the level of the hyoid bone. It opens anteriorly, through the isthmus faucium, into the mouth, and contains the palatine tonsil—another grouping of adenoid tissue.
The anterior wall consists of the base of the tongue and the epiglottis tissue. The lateral walls are made up of the tonsil and associated tonsilar tissues. The superior wall consists of the inferior surface of the soft palate and the uvula.
Because both food and air pass through the pharynx, a flap of connective tissue called the epiglottis closes over the glottis (tracheal opening) when food is swallowed to prevent accidental inhalation. The oropharynx is lined by non-keratinized stratified squamous epithelium.
Laryngopharynx
The hypopharynx or laryngopharynx is the caudal (most inferior) part of the pharynx; it is the part of the throat that connects to the esophagus. It lies inferior to the epiglottis and extends to the location where this common pathway diverges into the respiratory (larynx) and digestive (esophagus) pathways.
At that point, the laryngopharynx is continuous with the esophagus posteriorly. The esophagus conducts food and fluids to the stomach; air enters the larynx anteriorly. During swallowing, food has the right of way and air passage temporarily stops.
The laryngopharynx includes three major sites:
- The pyriform sinus.
- The postcricoid area.
- The posterior pharyngeal wall.
Like the oropharynx above it, the laryngopharynx serves as a passageway for food and air and is lined with stratified squamous epithelium.
The major transport passages of the alimentary canal include
- Pharynx: It is located at the back of the mouth and its function is to transfer the partially digested food bolus from the oral cavity to the upper part of the esophagus. This opening is known as Oropharynx. Pharynx also serves as a bridge that connects the air chambers of the nasal cavity and upper part of the trachea. This nasal opening of the pharynx is known as the nasopharynx. Nasopharynx also posses a collection of lymphatic tissue known as the tonsil. Epiglottis is the cartilage that prevents the entry of food particles into the air passage.
Esophagus
The esophagus is a muscular tube that moves food from the pharynx to the stomach via peristalsis.
Key Points
The esophagus is the muscular tube that moves food material from the pharynx to the stomach via waves of muscle movement known as peristalsis. The junction between the esophagus and the stomach is known as the gastroesophageal junction or GE junction.
The entry to the esophagus opens only when swallowing or vomiting due to specialized muscles that control the opening.
Key Terms
- esophagus: The esophagus is an organ in vertebrates that consists of a muscular tube through which food passes from the pharynx to the stomach.
- peristalsis: The rhythmic, wave-like contraction of both longitudinal and circular smooth muscle fibers within the digestive tract that forces food through it.
- mucus: A slippery secretion from the lining of the mucous membranes.
EXAMPLES
Swallowing is a voluntary act that utilizes the muscles of the mouth and tongue to push food into the esophagus. Once food material is pushed into the throat or pharynx, the trachea (windpipe) is blocked by a flap of tissue known as the epiglottis to prevent the aspiration of food. Food then moves down the esophageal tube through waves of muscle movement, or peristalsis, until it reaches the stomach.
The esophagus is an organ in vertebrates that consists of a muscular tube through which food passes from the pharynx to the stomach. It is a major component of the upper digestive system.
Esophagus: The location of the esophagus within the greater digestive system in humans.
The word esophagus is derived from the Latin œsophagus, which derives from the Greek word esophagus, meaning entrance for eating. It is lined with mucus to aid in the passage of food.
Length and Location
In humans, the esophagus is continuous with the laryngeal part of the pharynx within the neck, and it passes through the thorax diaphragm and into the abdomen to reach the cardiac orifice of the stomach. It is usually about 10–50 cm long depending on an individual’s height. Due to the inferior pharyngeal constrictor muscle, the entry to the esophagus opens only when swallowing or vomiting.
Layers of Tissue
The esophageal tube in humans is comprised of two main layers of smooth muscle, though striated muscle comprises the tube near the pharynx. This combination of muscle tissue allows peristalsis to push food downward, and aids in regurgitation at the pharynx.
The innermost layer of smooth muscle is arranged in a series of concentric rings, while the outermost layer is arranged longitudinally.
In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called a bolus) from the pharynx to the stomach.
The Gastroesophageal Junction
The junction between the esophagus and the stomach (the gastroesophageal junction or GE junction) is not actually considered a valve in the anatomical sense, although it is sometimes called the cardiac sphincter.
Primary Function of the Alimentary Canal
The primary function of the alimentary tract or alimentary canal is to ingest food material and divide it into small fractions. What happens in the alimentary canal? A series of secretions, mainly enzymes act upon these smaller fractions and convert them into smaller molecules. These small molecules are thereby absorbed into the blood and lymph circulation. These small molecules are chiefly, amino acids, small peptides, sugars, and fatty acids, which are the building blocks in the synthesis of essential proteins, carbohydrates, and lipids.
Additional functions of the alimentary canal or gastrointestinal tract
The primary function of the alimentary canal is to carry out the process of digestion of food and absorption of nutrients from it. Apart from these primary functions, there are other supplemental yet essential roles of the alimentary canal. They are as follows:
- The alimentary canal acts as an immune barrier to various harmful microbes. This function is carried out by gut-associated lymphoid tissue (GALT) and variable pH conditions that are present throughout the alimentary canal.
- The colonic bacteria also help in maintaining immune homeostasis.
- The colonic bacterial colony also prevents the growth of harmful bacteria in our alimentary canal.
- Drug metabolism also occurs in the alimentary canal wherein the drug molecule is metabolized into smaller fractions and eventually eliminated from the body. Metabolism of antigens also occurs in the alimentary canal thereby detoxifying the body of the antigens.
The Main Lower Parts of the Human Alimentary Canal
Digestive tract
A. Stomach
The stomach forms the first part of the digestive tract. Food bolus from the esophagus enters the stomach. The stomach is the dilated portion of the digestive tract where fragmented food that has been received from the oral cavity via the esophagus, is retained, macerated, and partially digested. The pyloric sphincter present at the lower end of the stomach prevents the passage of food until it is converted into a thick semiliquid paste or pulp (better known as chyme). The pH of the stomach is generally acidic, although the presence or absence and nature of the food determines the pH of the stomach (Table 1).
Table 1: pH in different parts of the alimentary canal
| Parts of the alimentary canal | pH |
|---|---|
| Oral cavity | 6.5-7.5 |
| Stomach | 1–2 |
| Small intestine | |
| Duodenum | 6-6.5 |
| Ileum | 7.4-7.5 |
| Large intestine | |
| Cecum | 5.7 |
| Ascending colon | 5.7 |
| Transverse colon | 6.6 |
| Descending colon | 7.0 |
The stomach is divided into five basic parts:
- Cardia: The foremost part of the stomach which is nearest to the esophagus
- Fundus: cardia is followed by the fundus part of the stomach and forms the upper part of the stomach
- Body (corpus): The body is the main part of the stomach, present between the upper and lower part of the stomach
- Antrum: The distal or the lower part of the stomach, which is near to the intestine. In this part of the stomach, mixing the food with gastric juices occurs.
- Pylorus: The last part of the stomach that acts as a regulator to control the emptying of the stomach contents into the small intestine
The stomach muscles are seldom inactive. The amount of food in the stomach determines the movement of the stomach muscles viz., expansion, or contraction. As soon as the food enters the stomach, the stomach muscle relaxes momentarily, thereafter the stomach muscle starts contracting.
Periodic stomach contractions result in churning and kneading of the food into a semisolid mixture called chyme. The rhythmic and periodic peristaltic movement of the stomach results in the movement of the food toward the pylorus and small intestine.
These cyclic and rhythmic movements of the stomach muscles are known as Gastric MMC(Migrating motor complex). Various secretions of the stomach are also released during this cycle and help in converting food bolus into chyme. These secretions include:
- Dilute acidic solution of hydrochloric acid (approximately 0.16 N)
- Proteolytic enzymes mainly, pepsin and minor quantities of other enzymes like rennin and gastric lipase
- Mucins
B. Small intestine
The small intestine is the longest part of the alimentary canal and the most important part of the alimentary canal wherein the majority of the digestive function is performed. The primary function of the small intestine is the digestion of the food followed by the absorption of the nutrients from the food.
The small intestine in adults measures approximately 6 m ~ 20-25 ft in length and have a surface area of approximately 250 m2. To increase the surface area, the small intestine exhibits significant architectural modifications in its mucosa and submucosa, i.e., folding in the mucosa and submucosa to form multiple finger-like projections or folds in the epithelium which are known as villi.
The large surface area ensures that the majority of the nutrients are absorbed during the passage of food through it. The small intestine is the main site for the absorption of amino acids, sugars, fats, and some larger molecules produced by digestion. The chyme from the stomach is transferred to the small intestine via the sphincter pylorus.
The small intestine is basically divided into three main parts. They are (1) duodenum, (2) jejunum, and (3) ileum. For the small intestine’s diagram.
The duodenum is the proximal 20–25 cm of the small intestine and is like a C-shape structure. The bile duct, liver, and pancreas open in the duodenum. Secretion from these glands neutralizes the acidic content of the chyme from the stomach and further promotes the digestion of proteins, fats, and carbohydrates.
The duodenum is characteristically retroperitoneal and the end which emerges from the peritoneum marks the beginning of the jejunum. The jejunum, which is almost 2/5th of the whole small intestine, extends till the ileum which terminates at the ileocaecal valve.
The small intestine also has a characteristic large collection of lymphoid tissue that is known as Peyer patches. The jejunum is the main absorptive site of the digestive tract and has a complex villous system.
The ileum is characterized by the presence of GALT (i.e. the gut-associated lymphoid tissue). The lymphoid cells further combine to form large nodules i.e. Peyer’s patches. The ileum is present in the lowermost part of the abdomen and has a limited blood supply.
Peristaltic movement in the small intestine forces the food material to move through different segments of the small intestine and eventually moves to the large intestine.
The ileum is involved in the absorption of vitamin B12, bile salts, and all the unabsorbed digestion products from the duodenum and jejunum.
Water and electrolytes are absorbed throughout the small intestine. The pH of the intestine is alkaline (Table 1).
C. Large Intestine
The large intestine or large bowel or colon is the last part of the human alimentary canal. The primary function of the large intestine is to absorb water from the indigestible residue of food before it is eliminated from the body as feces.
The large intestine is the place in the alimentary canal wherein the liquid content received from the small intestine is converted into solid indigestible waste material, feces.
The large intestine specializes in the absorption of water, salts, ionic contents from the residual food. Extensive mucus production occurs in the large intestine. This is to facilitate the bowel movement to eliminate the feces from the body.
As compared to the small intestine, the large intestine is wider and shorter. It is approximately 1.5 meters, or 5 feet, in length and has smooth inner walls.
As for the pH, the level varies depending on the part of the human intestines. The overall pH of the large intestine is alkaline (in the range of 6.5 to 7.5).
Good bacteria or probiotic bacteria reside in the colon (more than 700 species of bacterial flora exist in the colon). These probiotic bacteria are essential for a healthy human body. These bacteria are also involved in the synthesis of Vitamins such as vitamin K. Vitamin K is essential for the normal clotting of the blood. The large intestine is further divided into the following parts: (1) caecum, (2) ascending, transverse and descending colons, (3) sigmoid colon, and (4) rectum.
Partly digested food enters into the caecum from the small intestine and further moves through to enter the colon, wherein the residual water, nutrients, and electrolytes are reabsorbed. The residual solid waste further moves through the colon and is stored in the rectum for some time, and is eventually expelled from the body through the anal canal and anus. A small appendage or blind-ending tubular diverticulum that arises from the caecum is known as an appendix. The appendix is almost 5-10cm in length and has a diameter of approximately 0.8cm. Interestingly, the diameter of the appendix reduces with the increase in the age of human beings. The appendix, which is part of the alimentary canal (as opposed to the appendix of the testis, vermiform appendix, etc.), is a vestigial organ. Its specific function in the human body is yet to be established.
Common Disorders of the Alimentary Canal
- Vomiting: Forceful egestion of the food from the stomach. It can voluntary or involuntary action.
- Diarrhea: Overactivation or stimulation of the colon results in frequent and watery stools. Generally, this is induced by any harmful microbe ingested through food or poison.
- Constipation: Fewer and dry bowel movements or hard stool that is difficult to pass out.
- Bloody stool: Stools accompanied by the presence of blood.
- Heartburn or acidity: Acidic content refluxed into the esophagus.
- Stomach ulcers: Sore in the stomach lining. They are also known as peptic ulcers.
- Gastritis: Inflammation of the stomach lining
- Inflammatory bowel diseases (IBD): Chronic inflammation of the digestive tract particularly the large intestine. Crohn’s disease and ulcerative colitis are the two most common IBDs.
- Hemorrhoids: Swollen and painful blood vessels of the anal canal.
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