• The DigestiveSystem has two main components: The Gastrointestinal (G.I.) Tract and various Accessory
Structures and Organs.
• The G.I. Tract is also called the alimentary (‘nourishment’) canal and consists of long muscular tube
starting from the oral cavity, where food enters the mouth, continuing through the pharynx, esophagus,
stomach, small and large intestines, to the rectum and anus, where waste is expelled as fecal matter.
• The Accessory Structures and organs include:
The salivary glands; mucous glands; tongue; teeth; the liver; gallbladder and pancreas.
All of these have important functions in the digestive system. Food is propelled along the length of the G.I.
tract by peristaltic movements provided by the muscular layers in the walls of the G.I. tract. Many accessory
structures assist the tract by secreting enzymes or substances to help transform, digest, absorb or transport
food as it travels along the tract.
• The primary purpose of the gastrointestinal tract is to break down the large nutrients (polymers) from
the food ingested into smaller units (monomers). Once the nutrients have been broken down into their
smallest unit, they can then be absorbed across the epithelial lining into the body and these nutrients
and materials can be used in a number of ways, including providing energy for the body.
3.
The 6 BasicDigestive Processes
• 1. Ingestion – taking food or drink into the mouth or oral cavity.
• 2. Propulsion – movement through alimentary canal. This includes movement of tongue and cheeks and
involves contraction of the muscles of swallowing, in addition to the peristaltic movement generated by the
muscular layers of the tract and the cavities created by the canal.
• 3. Mechanical Digestion – the Physical breakdown of food (chewing, churning), mechanical processing and
moistening of foods. This is most often required prior to chemical digestion.
• 4. Chemical Digestion – the Enzymatic breakdown of food (from complex to simple building blocks) by the
use of enzymes made by the body. This involves the breaking of chemical bonds.
• 5. Absorption – the transportation of digested products from lumen of G.I. tract across an epithelial lining
and into the blood and lymph vessels, which are considered to be inside the body.
• 6. Defecation – the Elimination of indigestible material and waste products from body (feces).
4.
The Alimentary Canalis a hollow, compartmentalized and specialized tube starting at the oral cavity and
ending at the anus.
5.
The Oral Cavity
•The mouth or the oral cavity is the hollow space formed by the area between the lips, cheeks, tongue, hard,
soft palates and the throat. It allows food, liquid and air to enter the body. The mouth contains many other
organs, such as the teeth, tongue and the ducts of the extrinsic salivary glands. These structures work
together to aid in the ingestion and digestion of food. The mouth also plays a major role in the production of
speech through the movements of the tongue, lips and cheeks.
• The lips form the anterior border of the external opening of the mouth and are very flexible and elastic
structures. They contain collagen, elastin fibers and adipose tissue covered by a thin layer of keratinized
stratified squamous epithelium. The epidermis of the lips is so thin that the color of the blood supply
immediately deep to the surface can be seen, hence the lips are usually red in color. The exterior of the lips is
continuous with the skin (covered by keratinized epithelium) while the inner surface is continuous with the
mucous membrane of the mouth and is covered by non-keratinized stratified squamous epithelium.
6.
Tongue
• While manypeople think of the tongue a muscle, it is actually an organ made of epithelium, several skeletal
muscles, nerves, and connective tissues. The tongue forms the inferior portion of the mouth, but often
moves throughout the mouth to occupy almost any region of the hollow cavity.
• The apex is the tip of the tongue, the most distal part of the tongue. The body of the tongue (the corpus
linguae) is the portion of the tongue between the tip and base, and is about four inches long. The body is
separated into two halves by the median sulcus. The body is separated from the base of the tongue by a V-
shaped furrow, or groove, called the terminal sulcus.
7.
• The tonguecontains many small ridges known as papillae that help it to grip and move
food around the mouth. Taste buds are hidden in valleys around some of the papillae
which contain a type of chemoreceptor called taste buds and contribute to gustation
(the sense of taste) by detecting chemicals found in food. The tongue also helps to
produce speech by altering or stopping the flow of air through the mouth and teeth to
produce the sounds of many consonants.
• The hard and soft palates form the roof of the mouth. On the anterior end of the mouth,
the hard palate is formed by the inferior surface of the maxillae and palatine bones.
These bones are covered with a thin layer of connective tissues and mucous
membranes, which form small ridges you can feel with your tongue. The roof of the
mouth continues posteriorly as the soft palate, a flexible fleshy mass of tissues that ends
in the uvula. The hard and soft palates work together to separate the mouth from the
nasal cavity. This allows humans to breathe and chew food at the same time. The big and
relatively flat molars are specialized for extensive chewing time! The soft palate moves
superiorly during swallowing to cover the nasopharynx of the throat, preventing food
from entering the nasal cavity.
8.
Teeth
• Teeth area group of hard organs found in the
oral cavity specialized for the biting and
grinding of food, known as mastication, or
chewing, convert food into tiny pieces, which is
the main function of the teeth. Every bite
forces food into the interface of the teeth to be
chopped, while lateral motion of the jaw is
used to grind food in the premolars and molars.
• Teeth also provide shape to the mouth and face
and are important components in producing
speech. Teeth form deep roots into the bones
of the maxillae and the mandible, but grow out
through the gums of the mouth to form biting
surfaces. The gums, or gingiva, are soft mucous
membranes surrounding the teeth, protecting
the roots from decay and helping to hold the
teeth in place.
10.
• A toothcan be divided into two main parts: the crown and root. The crown is the enlarged region of the tooth
above the gum line, involved in chewing. Like an actual crown, the crown of a tooth has many ridges on its top
surface to aid in the chewing of food. The root is the tapered region below the gum line of the tooth, which
anchors the tooth into a bony socket known as an alveolus. Each tooth may have between one to three roots.
The exterior surface of the root is covered in a bone-like mixture of calcium and collagen fibers known as
cementum. Cementum provides grip for the periodontal ligaments that anchor the root to the surrounding
alveolus.
• Each tooth is an organ consisting of three layers: the pulp, dentin, and enamel. Typically, a total of 32 permanent
adult teeth form and erupt. The adult teeth are arranged with 8 teeth in 4 quadrants (upper and lower, right and
left).
11.
Teeth are classifiedinto 4 major groups and there are
8 teeth in one of four quadrants as follows:
2 incisors (1 central and 1 lateral) - chisel-shaped
teeth found in the front of the mouth.
1 canine (cuspid) - sharply pointed, cone-shaped
teeth on both sides of incisors.
2 premolars (bicuspid) - large, flat-surfaced teeth
found in the back of the mouth.
3 molars (tricuspids) – the 3rd molars are the ‘wisdom’
teeth.
12.
Salivary Glands
• Salivais composed mostly of water and mucous, but it also contains lingual amylase, an enzyme that breaks
down starch, and lysozyme, an antibacterial agent in saliva that protects the body.
• Intrinsic salivary glands – there are many small salivary glands that are located inside the oral cavity embedded
in the lips and cheeks.
• Extrinsic salivary glands – there are 3 sets of larger salivary glands that are located outside of the oral cavity but
have ducts delivering secretion to the oral cavity.
Each salivary gland is a complex, the secretory portions of these glans are rounded and called acini (for acinar or
rounded). The acini secrete their contents into specialized ducts. Each gland is divided into smaller segments
called lobes. Salivation occurs in response to the taste, smell or even appearance of food.
13.
Parotid gland isthe largest of the three major pairs of salivary
glands. It is a serous gland and located anteriorly and inferiorly
to the ear between the skin and the masseter muscle of
chewing. The parotids produce about 25% of saliva and it is a
serous (watery) secretion which is also rich in proteins.
Immunoglobins are secreted help to fight microorganisms and
a-amylase proteins start to breakdown complex carbohydrates.
Submandibular gland is a serous gland, located inferiorly to the
mandible and it has a muscular covering and empties its
contents by way of the submandibular duct into the floor of the
mouth on both sides. This gland secretes 70% of the saliva in
the mouth.
Sublingual gland is the smallest of the three, is a mucus gland
and as its name implies, lies under the floor of the mouth and
on either side of the tongue. Covered by a thin layer of tissue at
the floor of the mouth, they produce approximately 5% of the
saliva and their secretions are very sticky due to the large
concentration of mucin.
