Alimentary Canal in Digestive System of Human Beings | HSEB Notes Class 12 Zoology

Biology | Zoology
HSEB Notes on Digestive System of Human Beings
Lesson :  Digestion
Class : 12

Human beings are heterotrophic. It means they can not manufacture their food and depend upon plants or other animals. They have holozoic type of nutrition. The nutrition is animal like. Human beings are omnivorous in nature. The digestive system consists of alimentary canal and digestive glands. Alimentary canal is complete and has well defined regions. It is concerned with ingestion, digestion, absorption and egestion of waste material.

Alimentary canal

It is a long coiled tube of about 8 to 10 meters long. It is of various diameter at various parts. It starts from mouth and ends in anus. Different parts are:

• Mouth
• Vestibule
• Buccal cavity
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large intestine
• Anus

Mouth 

It is a transverse slit like opening bounded by movable upper and lower lip. It opens into a small gap called vestibule. Vestibule is a space between lips and jaw. It opens not buccal cavity.

Buccal cavity

It is known as oral cavity or mouth cavity also. It is bounded by upper and lower jaws. The upper jaw is fixed and lower jaw is only movable up and down and sidewise. The jaws are provided with teeth in a row. At the floor, there is tongue. The roof of buccal cavity is made by palate. 

Teeth 

Human beings and most of other mammals are heterodont. They have different types of teeth with different functions.

1. incisors -help to cut large piece into small pieces
2. canines - help in tearing flesh
3. premolars – help in mastication
4. molars – help in mastication

They are diphyodont. They have two sets of teeth, milk teeth and permanent teeth. 

Dental formula – it is expression of number and type of teeth on right or left side (one side) of jaw.

Dental formula of milk teeth i 2/2 c 1/1 pm 0/0 m2/2 5x2/5x2 = 20

Dental formula of permanent teeth i 2/2 c1/1 pm 2/2 m 3/3 8x2/8x2 = 32

i stands for incisor

c stands for canine

pm stands for premolar

m stands for molar

the numerator is for number of teeth in upper jaw

the denominator is for number of teeth in lower jaw

milk teeth start to drop out at about age of 5 or 6. the 3rd molar teeth are also known as wisdom teeth and appear at about the age of 17 to 21. teeth in females appear earlier than in males.

Structure of tooth 

It has 3 regions

• Crown – part which project above gum
• Neck - part surrounded by gum
• Root - part embedded in bone, the incisor and canine and lower premolar have one root, upper premolar and lower molar have 2 roots and upper molar have 3 roots.

Tooth consists of enamel which is the hardest part of human body. It covers the dentine of crown. Dentine has many canaliculi that pass radially from the pulp cavity. Cement covers root of tooth. Periodontal membrane covers cement and fixes tooth in socket(thecodont).

Inside tooth, there is pulp cavity containing mass of cells, blood vessels and nerve constitute pulp. It is for growth of tooth. Dentine forming odontoblast and enamel forming ameloblast cells are also present.

Tongue 

It is highly muscular organ attached at the floor of buccal cavity by a fold called frenulum. The upper surface is provided with numerous papillae containing taste buds. The taste buds are sensitive to taste of food. Types of papillae

• Filliform- smallest, most numerous, conical, mostly found at center of tongue, white in color.
• Fungiform - less in no. red and rounded, found at tip and margin of tongue.
• Vallate papillae- large in size, about 5 to 12 in no. arranged in inverted v shape at the base of tongue.
• Foliate – leaf like, not developed in man, found at sides of tongue.

Tip of tongue – sweet

Sides of tongue – sour

Posterior end of tongue – bitter.

Functions of tongue

• Detects taste
• Helps in chewing, mix saliva
• Aids in swallowing
• Cleans teeth and gum
• Plays role in speech

Palate 

The roof of buccal cavity is called palate. Anterior part is called hard palate. It bears transverse ridges called rugae. The posterior part is smooth and called as soft palate. The hinder part freely hangs down as a small flap called uvula. Buccal cavity receives saliva from salivary gland.

Pharynx

It is wide opining at back of mouth cavity. It leads to two openings : gullet and glottis. There is a muscular flap called epiglottis which closes glottis when food is swallowed. There are 2 openings of internal nares above and two openings of Eustachian tubes at the sides. It is the only part common to digestive and respiratory system.

Oesophagus

It is a long narrow muscular tube which connect mouth to stomach. It is about 25 cm long. It pierces diaphragm to open into stomach. It undergoes peristalsis to carry down food and water or fluid.

Stomach

It is a large muscular elastic bag situated below diaphragm on left side. It has four parts

• Cardiac – it is so called because it lies near heart. In between oesophagus and cardiac part of stomach there is cardiac sphincter. 
• Fundus - it extends superiorly from the cardiac part. It is usually filled with air.
• Body - it is main part of stomach. 
• Pyloric part - it is distal part of stomach. it opens into duodenum. It opens and closes several times. At the time of opening, a small amount of partially digested food(chime) is passed into duodenum.

Gastric gland secretes gastric juice.

Small intestine

It is divisible into 3 parts.

• Duodenum – it is c shaped and about 25 cm long. It receives bile juice and pancreatic juice through common bile duct.
• Jejunum - it is about 2.5 meter in length. It is coiled part.
• Ileum – it is about 3.5 meter long. It is highly coiled part. Both jejunum and ileum are suspended by mesenteries. The inner wall of ileum has number of folds called villi. It is mainly for digestion and absorption.

Large instestine

It is about 1.5 meter long and divisible into

• Caecum – it is pouch like structure about 6 cm long. There is ileocaecal valve preventing back flow. Attached to caecum is a slender vermiform appendix of about 10 cm long. It is vestigial in man but functional in herbivores. The inflammation of appendix is called appendicitis at the time of infection.
• Colon – it is inverted U shaped and divisible into 

Ascending colon - it is the first part on right side. It moves upward from caecum.

Transverse colon - it is horizontal part.

Descending colon – it moves down at left side. It descends down

Pelvic colon – it is s shaped or sigmoid. Undigested material can remain in colon for about 36 hours.

Colon is for absorption of water mainly.

Rectum - small muscular region, straight, about 15 cm long. It opens to the exterior through anus. Undigested material remains here for a short time. The anus is guarded by 2 sphincters Through out the alimentary canal, mucous glands secrete mucus.

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