146 The Alimentary Canal
Mouth
The mouth receives and mechanically breaks down food, produces saliva, and is the first portion of the alimentary canal.
Learning Objectives
Describe the features of the mouth that play a role in digestion
Key Takeaways
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.
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, jaw bones, 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 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 hands 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.
Learning Objectives
Describe the pharynx’s role in digestion
Key Takeaways
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 (mesopharynx), 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 (mesopharynx), 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 tubarius 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 mesopharynx 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 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 a stratified squamous epithelium.
Esophagus
The esophagus is a muscular tube that moves food from the pharynx to the stomach via peristalsis.
Learning Objectives
Describe the role of the esophagus in digestion
Key Takeaways
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.
The word esophagus is derived from the Latin œsophagus, which derives from the Greek word oisophagos, 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 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.
