What is Digestion?
To perform various functions of the body, energy is required, which is obtained from food. The process of conversion of complex food material into simple and diffusible forms by hydrolysis is termed as Digestion.
Human Digestive System
- All parts of the body are involved in the uptake and digestion of food along with the elimination of undigested material.
The alimentary canal is mainly referred to as the pathway by which food enters our body and moves out through the anus after digestion. It is a tube-like structure that starts from the mouth and ends in the anus.
- The alimentary canal plays a primary role in human digestion and is also termed the digestive tract.
The alimentary canal is divided into the following parts:
- Mouth and Buccopharyngeal Cavity
- Small Intestine
- Large Intestine
1. Mouth and Buccopharyngeal Cavity
The mouth is also called the buccal cavity or the oral cavity. In the human digestive system mouth is the upper end of the beginning of the alimentary canal, which leads to the pharynx and to the oesophagus.
In humans, both the mouth or the buccal cavity and the nasal cavity are separated by the palate. The buccal cavity mainly comprises the primary organ of the digestive system including the teeth, tongue and salivary glands.
The mouth is an opening through which the food is taken inside the body. It is bounded by lips and its inner parts comprise of the cheeks, tongue, upper jaw and lower jaw.
- The upper jaw is fixed, which forms the roof of the mouth cavity and consists of the palate, teeth, and gums surrounding the teeth. The lower jaw is movable and forms the floor of the mouth cavity, which consists of the tongue along with the teeth and gums surrounding them.
- The mouth plays an important role in speech, helps in breathing in certain cases, when the nose is blocked and during strenuous exercise. It is also the most important part of the human body, which permits us to enjoy the food we eat and also to communicate with the rest of the world.
The mouth or the oral cavity comprises of two parts:
- The oral cavity proper: It mainly comprises the tongue.
- The oral vestibule: It is the slit-like space between the teeth and the buccal cavity and between the lips and cheeks.
Different Parts of the Buccal Cavity
- Lips are the soft, muscular and movable structures, which are formed by the complex of orbicularis oris muscles.
- The reddish-pink appearance of the lips is mainly because of the underlying blood vessels which are covered by the thin and transparent epithelium tissues.
- It is a large, muscular organ, which occupies most of the oral cavity and can take up a variety of shapes and positions.
- There are 50 to 100 taste receptor cells in each taste buds, which are located in the lining of the mouth.
It is the main sense organ of human beings and is involved in:
- Sense of taste.
- A major organ of speech.
- Oral cleansing of the mouth.
- Squeezing food into the oropharynx while swallowing.
- Formation of clear and distinct sound during the speech.
(iii) Buccal mucosa
- It is the inner lining of the cheeks and the back of the lips. It provides a round shape to the cheeks.
Try yourself:This is a muscular organ in the mouth that helps position the food between the teeth and is covered in nerve endings that allow us to taste our food.
Correct Answer is Option (a)
The tongue is a muscular organ in the mouth of a typical vertebrate. It manipulates food for mastication and swallowing as part of the digestive process, and is the primary organ of taste.
- The teeth are the strongest and rigid substances in the human body. A normal adult has 32 teeth and is divided into incisor, canine, molars and premolars. There is a third molar, which is called the wisdom teeth, which appear in a person’s late teens or early twenties.
Try yourself:How many types of teeth are present in an adult?
Correct Answer is Option (c)
Most adults have 32 teeth, called permanent or secondary teeth, & they are of 4 types.
- Canines also called cuspids
- Premolars also called bicuspids
- Each tooth consists of a crown with one or more tips, a neck, and a root. The Pulp cavity is the centre part of the tooth, filled with blood vessels, nerves and connective tissues and is surrounded by an enamel, which protects the tooth against scratch, cut and from the invasion of bacteria present in the mouth.
The pharynx is the common passage of the respiratory system and digestive system located in the throat. It connects the mouth to the oesophagus and the nose to the larynx. Food and water from the oral cavity and air from both the oral and nasal cavity comes to the pharynx. The pharynx is present in vertebrates and also in invertebrates such as annelids, arthropods, etc.
- The pharynx is present behind the oral and nasal cavity. It is a part of the throat and a common passage for both the digestive and respiratory system.
- The pharynx opens into the oesophagus in the digestive tract and opens into the larynx in the respiratory tract.
- The pharynx is also present in some of the invertebrates. The shape and size vary in different organisms. It may be thick and muscular, rotated or turned outward.
Structure and Parts
The pharynx can be divided into three main regions according to its location. They are:
- Nasal pharynx – As the name suggests, it is the part of the pharynx that is present posterior to the nasal cavity.
- Oral pharynx – It is the part behind the oral cavity and continues in the throat till the hyoid bone.
- Laryngeal pharynx – It is the lowermost part of the pharynx from the epiglottis and continues to the oesophagus.
The main features of the pharynx are:
- The wall of the pharynx consists of both longitudinal and circular muscles. These muscles determine the shape of the lumen.
- Isthmus connects the oropharynx to the nasopharynx. It is useful for breathing through the mouth and inserting food into the oesophagus through the nasal tube if required.
- Pharyngeal adenoids or tonsils are located in the wall of the nasopharynx.
- The nasopharynx is lined with pseudostratified, columnar and ciliated respiratory epithelium.
- Eustachian tubes connect the middle ears to the pharynx. It helps in equalizing air pressure on the eardrum.
- Palatine tonsils are present in the lateral wall of the oropharynx.
- The wall of the oropharynx is made up of non-keratinized, stratified squamous epithelium.
- The opening of the pharynx to the larynx is controlled by a muscular flap known as epiglottis. It is present over the larynx opening and prevents food from entering the trachea.
- The wall of the laryngopharynx consists of stratified squamous epithelium.
- Laryngopharynx regulates the movement of air to the lungs and food to the oesophagus.
- Pharyngeal, tubal, palatine and lingual tonsils present in different parts of the pharynx are called Waldeyer’s ring. These are lymphoid tissues present in the nasopharynx and oropharynx. It provides defence against the invasion of microorganisms in the digestive and respiratory tract.
Try yourself:The nasopharynx extend till?
Correct Answer is Option (a)
The upper portion of the pharynx, the nasopharynx, 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 above the oral cavity.
The pharynx performs a dual function. It allows passage for both air and food.
The main functions of the pharynx are:
- It makes air warm and humidifies it before reaching the lungs.
- It helps in the movement of food to the oesophagus. Circular muscles help in pushing the food down and longitudinal muscles help in swallowing the food by lifting and widening the walls.
- The pharynx also helps in speech, it amplifies the sound produced by the larynx or soundbox.
- Lymphoid tissues present in the pharynx are the first line of defence against foreign pathogens.
Some of the diseases associated with the pharynx are:
- Pharyngitis - inflammation of the pharynx
- Tonsillitis - inflammation of tonsils
- Pharyngeal cancer
The food we swallow passes through a pipe that leads to the stomach. This pipe is known as the food pipe or the oesophagus. Food reaches the stomach from the pharynx through peristaltic contractions.
What is Food Pipe?
- “Food pipe or oesophagus is the part of the digestive system of vertebrates with a distinct path exclusively meant for passage of food.”
- A food pipe is a muscular tube that is 25 centimetres long, passing just behind the heart region connecting the mouth to the stomach. It is made up of connective tissues and is the uppermost part of the digestive system.
- The veins and arteries that pass through the oesophagus or food pipe are called oesophageal veins and oesophageal arteries respectively. The upper part of the oesophagus is behind the windpipe. Oesophagus joins the stomach at a point called the gastro-oesophageal junction. When we swallow food, the oesophagus contracts and squeezes the food which pushes the food down towards the stomach.
Try yourself:The esophagus connects _________
Correct Answer is Option (b)
The esophagus is a muscular tube connecting the throat with a stomach. It is about 8 inches long.
Layers of Food Pipe
The food pipe or oesophagus is divided into the following layers:
- Mucosa: The inner layer
- Submucosa: The layer responsible for producing secretions
- Muscularis: Food is pushed down through this layer made up of muscles
- Adventitia: Oesophagus is attached to the other parts through this outer layer
Functions of Food Pipe
The food pipe or oesophagus performs the following important functions:
- The food we eat reaches the oesophagus after passing through the larynx.
- After the buccal cavity, the oesophagus is the next point of contact for food into the gastrointestinal tract.
- Responsible in preventing the entry of food into the windpipe by controlling the movement of the epiglottis.
- Radially symmetric contractions of the oesophagus help push food downwards.
- Rhythmic contractions and relaxations of oesophageal muscles result in the swift movement of food towards the gastro-oesophageal junction.
Care needs to be taken about the health of the oesophagus or food pipe as negligence can cause severe disorders in the food pipe. One such ailment of the food pipe is Oesophageal cancer.
Disorder Of The Food Pipe – Oesophageal Cancer
It is a cancer of the food pipe where the cells multiply in an uncontrolled way. It is mostly diagnosed in people above 60 years of age. When the tumour is small, it does not show any symptoms. It is only when it enlarges, the symptoms become apparent and develop.
The commonly occurring symptoms are:
- Dysphagia – patient finds it difficult to swallow food
- Regurgitation – Food tends to come back up before even making it to the stomach
- While swallowing, the patient suffers from throat pain or pain in the middle of the chest
- Heartburn or indigestion
- Tends to develop a hoarse voice
- Weight loss
- Uneasiness or dull pain between the shoulder blades
The initial treatment of oesophageal cancer is dependant upon the stage as to how far it has grown. With an early diagnosis, endoscopic procedures can be carried out, however, if it has advanced to a higher stage, a part or most of the food pipe is surgically removed. Many other treatments are recommended such as chemotherapy, targeted therapy, radiation therapy, immunotherapy, etc.
- Teeth are ectomesodermal in origin. A major portion of teeth arises from Dermis. Part of the tooth present outside the gums only is derived from ectoderm or Epidermis (Enamel part).
- In human teeth of the upper jaw are attached to the maxilla bone. While teeth of the lower jaw are attached to Mandible bone. But in rabbit upper incisors are attached to the premaxilla. While upper premolars and molars attached to the maxilla bone. While lower teeth are attached to dentary bone.
Structure of Teeth
There are three parts of the tooth. These are:
- Crown: It is the outer part of the tooth, exposed outside gums.
- Neck: It is the middle part of the tooth which is embedded inside the gums.
- Root: It is the part of the tooth that is inserted inside the socket of the jaw bone. (Alveoli)
- The crown part of the tooth is made up of a very hard substance called Enamel. It is the hardest material in the animal kingdom.
- Enamel is ectodermal. It is secreted by Ameloblast cells of the ectoderm. It has the maximum amount of inorganic salt (96%) in it, Inorganic salt is mainly found in the form of phosphate and carbonate of Ca, Mg, Na and K. 3% of water is found in the enamel. Along the keratin & ossein protein (1%) are also found in teeth. Ossein is a protein of bones. The remaining part of the tooth develops from the mesoderm of the embryo.
- Dentine is the main part of the tooth. Approximately 69% inorganic salts are present in dentine and 65% are present in cement. (62% inorganic salts are present in bones.)
- Dentine surrounds a cavity called pulp-cavity. This cavity contains soft connective tissue, blood capillaries, nerve fibres. The pulp cavity is necessary for the nutrition and survival of the teeth. At the base of pulp-cavity, an aperture is present. Through this aperture, blood capillaries and nerve fibres enter inside the teeth. This aperture is called apical-foramen.
- A special type of cells from the lining of the pulp-cavity called the Odontoblast cells. These cells are the dentine secreting cells. The cytoplasmic process of odontoblasts is embedded into dentine in the form of the fine tubule. These processes are called canaliculi. These canaliculi secretes dentine. The teeth continue to grow till the odontoblast cells remain active. In adults, the pulp-cavity shrinks and the odontoblasts become inactive so the teeth stop growing. The cement layer is made up of the cementocytes cells. Between the root and the bones of the teeth, a periodontal membrane is present.
- In Rabbit and rat, the pulp-cavity of the incisor remains wide throughout their life, so these teeth grow continuously throughout their life span.
- If one incisor of Rabbit & rat is broken then the opposite incisor grows continuously, finally, the animal can neither close the mouth nor gnaw the food. So the animal dies due to starving.
Try yourself:Which of the following cells secrete enamel?
Correct Answer is Option (a)
- Ameloblasts are cells that secrete the enamel proteins enamelin and amelogenin which will later mineralize to form enamel, the hardest substance in the human body.
Osteoclasts are the cells that degrade bone to initiate normal bone remodelling and mediate bone loss in pathologic conditions by increasing their resorptive activity. They are derived from precursors in the myeloid/monocyte lineage that circulate in the blood after their formation in the bone marrow.
Odontoblast is a cell of neural crest origin that is part of the outer surface of the dental pulp, and whose biological function is dentinogenesis, which is the formation of dentin.
Osteoblasts are the cells that form new bone. They also come from the bone marrow and are related to structural cells. They have only one nucleus. They produce new bone called "osteoid" which is made of bone collagen and other protein.
- Tooth enamel is the hard outermost layer of the tooth forming the surface of the crown.
- Ameloblasts are like osteoblasts in the production of bone tissue.
- Dental enamel can not regenerate.
- After the first layer of dentin (predentin) has been laid by odontoblasts, the ameloblasts become fully functional.
Types of Teeth Found in Mammals
1. Incisor: These are long, chisel-like teeth for gnawing the food. They are more developed in gnawing animals.
- Example: lagomorphs, rodents, the tusk of elephant are modification of upper Incisor. Tusk is used to protecting from enemies, attack on enemies (not for feeding purpose)
2. Canines: These are sharp-pointed teeth meant for tearing the food. Canines are most developed in carnivorous animals. canines are absent in herbivorous animals.
- Example: Rabbits do not have canines. In herbivores, the space of canine in gums is empty and this empty space is called diastema.
3. Pre Molar: These teeth are meant for chewing and crushing food, they are triangular in shape.
4. Molars (Cheek teeth): These also meant for chewing & crushing food. They are rectangular in shape. Premolar and molar help in the mastication of food. In human teeth of the upper jaw are attached to the maxilla bone. While teeth of the lower jaw are attached to the mandible bone.
- In mammals, except Premolar and Last molar, all type of teeth appear twice in life. Teeth that appear during childhood are called milk teeth or temporary teeth. Due to the activity of osteoclast cells. These milk teeth are shed, off then permanent teeth appear.
- When temporary molars shed, their socket are filled by premolar and new socket are formed for a permanent molar. This occurs once in a lifetime.
- In frog, only the upper jaw has teeth.
- In Rabbit teeth of the upper jaw are attached to the premaxilla and maxilla bone, while teeth of the lower jaw are attached to the entry bone. Hippocampus, tortoise and birds do not have teeth.
Difference Between Milk Teeth and Permanent Teeth
- There will two phases of dentition in one's life. i.e., primary dentition (milk teeth or deciduous teeth) and secondary dentition (permanent teeth). The permanent teeth are visible throughout a person’s lifetime. The baby teeth continue to grow till a child is about 11 years old, after that milk teeth fall and give rise to permanent teeth.
Types Of Teeth on Different Basis
1. On the Basis of Appearance in Life
- Monophyodont: The teeth which appear only once in life.
Example: Pre Molar & Last molar of man.
- Diphyodont: The teeth which appear twice in life.
Example: Incisors, Canines, Molars of human.
- Polyphyodont: The teeth which appear more than twice in life.
Example: Fish, Amphibians.
2. On the Basis of Position From Jaw
- Thecodont: The teeth which are present in the bony socket of the jaw.
Example: Man & crocodile.
- Pleurodont: The teeth which are present on the lateral side of the jaw bone.
- Acrodont: The teeth which are present on the terminal part of Jaw bone.
Example: Fish, amphibian.
3. On the Basis of Structure and Function
- Heterodont: When the teeth are of a different type in mammals on the basis of structure and function.
- Homodont: Whether all teeth are of similar type in animal on the basis of structures and function.
Example: Fish, Amphibians.
4. On the Basis of Crown
- These are canine teeth of carnivorous animals.
- In this type of structure canine teeth become long and pointed which, is bent towards the backward direction.
(ii) Hypsodont (Smiling teeth)
- In this type of teeth, the crown part is large root is either absent or small such as incisor and canine.
- These teeth are also called smiling teeth.
(iii) Brachyodont (Cheek teeth)
- In this type of teeth, the crown part is a small root that is long such as premolar and molar.
- Wisdom teeth: These are the last molar teeth of humans which appear in the age of 18 to 25 year.
- The upper surface of the premolar & molar is the board. Some small projections are present in the upper surface of the premolar and molar. These projections are called Lophs or cusps.
5. On the Basis of Structure of Lophs
- Lophodont: In this type of teeth the lophs are large, wide and flat such as rabbit & elephant.
- Bunodont: In this type of teeth. Lophs are small and spherical in shape, such as human.
- Solenodont: In this type of teeth the lophs are large and semilunar in shape.
Example: Ruminant animals (Cow, Buffalo).
- Carnesial: in this type of teeth the lophs are long & pointed.
Example: Carnivorous Animal.
Note : In humans, premolar teeth appear in the alveoli of molar teeth while permanent molar teeth are developed in new alveoli.
Carnessialteeth are modified last premolar of upper jaw and first molar of lower jaw, for shearing and tearing of tendon.
Try yourself:When teeth are embedded in the jaw bone, it is called
Correct Answer is Option (b)
- Thecodont is a dentition in which the base of the tooth is enclosed in jaw sockets. The tooth is embedded in a socket of the jawbone. Ankylosis is absent and the roots are cylindrical. Examples include living crocodilians and mammals.
- Diphyodont is a type of dentition in which two successive sets of teeth are developed during the lifetime of the organism. The first set of teeth is deciduous and the other set is permanent. The deciduous set of teeth is replaced by permanent adult teeth.
- The animal whose teeth differ morphologically is said to be Heterodont. For example Most mammalian like man, the dog generally possess incisors, canines, premolar and molar.
The method of expressing or describing the total number of teeth in man and animals according to the arrangement is termed as the Dental Formula. This formula is expressed using letters and figures. The letters used in this formula are based on the 4 types of teeth like– Incisor, Canine, Premolar, and Molar.
The Dental Formula is expressed as:
(The number of each type of teeth in the upper jaw) / (The number of teeth on one side of the lower jaw)
Dental Formula =
(2 incisors 1 canine 2 premolar 3 molar) / (2 incisors 1 canine 2 premolar 3 molar)
Humans have two dental formulae:
The primary dentition (20 teeth):
I2/2 C1/1 M2/2 = 10
The permanent dentition (32 teeth):
I2/2C1/1 P2/2 M 3/3 = 16.
Where: I – Incisors, C-Canine and M-Molar
Among all other mammals, both humans and apes have similar sets of teeth and dental formulae.
In humans, premolar teeth appear in the alveoli of molar teeth while permanent molar teeth are developed in new alveoli.