Articles of Biology

Integumentary System
The system responsible to cover the body is known as integumentary system. It mainly consists of skin.
The skin consists of two main parts :
(i). Epidermis
        (ii). Dermis
(i). Epidermis :- The outer most covering of skin is called epidermis. It is made up of many layers. The innermost layer is called stratum germinativum or stratum malpighi. The cells of this layer divide continuously throughout life to form new layers. These layers move upwards to the surface. As they move upwards, they become more flat, scaly and finally dead. The outermost layer is called stratum corneum. Between these two layers 3 more layers are found. In this way the epidermis is 5- layered. The sequence of layers from inside to outside is as follows :-
1. Stratum germinativum or stratum malpighi
2. Stratum spinosum
3. Stratum granulosum
4. Stratum lucidum
5. Stratum corneum
(ii). Dermis :- It remains inside the epidermis and is thicker than epidermis. It is made up of fibrous connective tissue. It also contains blood vessels, lymph capillaries, nerves etc.
Hair :- Hair are elongated, thread like outgrowth of epidermis. The root of hair lies in the dermis of skin in hair follicle.  Hair follicle is a tube like depression. At the bottom of hair follicle, dermal papilla is found. Sebaceous glands open in hair follicle. The oily secretion of these glands keeps the hair oily.
Glands :- Mainly 3 types of glands are found in dermis. These are as follows :
1. Sebaceous glands open into hair follicle. These keep the hair moist by secreting an oily substance called sebum.
2. Sweat glands open at the surface of skin and secrete sweat. The sweat contains water, minerals and some excretory substances.
3. Mammary glands are found on the ventral side. These secrete milk.
Functions of Skin :- Skin performs following functions :
1. Skin protects the internal organs.
2. It prevents harmful bacteria, fungi and other worms to enter the body.
3. Some excretory substances are also removed by skin by means of sweat.
4. Skin regulates the temperature of body. It is called thermo-regulation.
5. Skin contains nerve fibres to receive stimuli of touch, pain, heat etc.

Digestive System
Alimentary canal and glands related with it form digestive system. Its main function is to digest the food.
The alimentary canal of man is about 10 metre long and tube like structure which starts from mouth and ends at anus. Alimentary canal includes :
1. Buccopharyngeal cavity
2. Oesophagus
3. Stomach
4. Small intestine
5. Large intestine
1. Buccopharyngeal cavity :- The part of alimentary canal from mouth to just before oesophagus is known as buccopharyngeal cavity or buccal cavity. Mouth is a transverse slit like aperture bounded by two soft movable lips. Buccal cavity is mainly divided into 2 parts :
a. Oral cavity b. Pharyngeal cavity
 a. Oral cavity :- The oral cavity is divided into two parts :
(i). Vestibule :- It is outer narrow space bounded externally by lips and cheeks and internally by teeth and gums.
(ii). Oral Cavity Proper :- It is the part just behind the vestibule. Its roof is made up of hard and soft palate and floor by tongue.
Tongue :- Tongue is a thick and muscular sensory organ. The anterior end of tongue is free but its posterior end is attached to the floor of buccal cavity by frenulum. Its upper surface is uneven due to presence of lingual papillae on it.
Teeth :- Teeth of man are thecodont (embedded in jaw sockets), diphyodont (grow twice in life time) and heterodont (4 types of teeth). The first set of teeth is called milk set or deciduous set. The milk teeth are 20 in number. The second set of teeth is called permanent set. The permanent teeth are 32 in number.
Structure of Teeth :- Each tooth consists of 3 parts : crown, neck and root. Crown remains outside the gum. Neck remains inside the gum and root penetrates into the jaw bone. The periodontal membrane lines the bony socket that lodges the root of the tooth. Pulp cavity occupies the central part of the tooth. It is filled with the pulp. Blood capillaries and nerve fibres are present in the pulp. A single layer of odontoblast cells line the pulp cavity. Main part of tooth is formed by a protein called dentine. Dentine surrounds pulp cavity. The crown is covered by a thick layer of enamel. Enamel is the hardest material found in the body. Remaining part of tooth is covered by a thin layer of cement.
b. Pharyngeal cavity :- Oral cavity communicates with the phayrngeal cavity through oropharyngeal isthmus. Pharynx is a muscular tube of 12 cm. The cavity of pharynx is divided into
(i). Nasopharynx
(ii). Oropharynx
(iii). Laryngopharynx
Nasopharynx lies behind the nasal chambers. It is passage for the air.
Oropharynx lies behind the buccal cavity. It is passage for the food.
Laryngopharynx is the lowest part of pharynx.
It has two apertures :-
a. Glottis :- It is slit like aperture which leads into trachea and can be closed by leaf-like epiglottis.
a. Guttet :- It is slit like aperture which leads into oesophagus.
2. Oesophagus :- It is a muscular tube that joins pharynx with stomach.
3. Stomach :- Stomach is the broadest part of alimentary canal. It lies just behind the diaphragm on the left side of abdominal cavity. It is divided into 3 parts :- Cardiac, Fundic and Pyloric. Oesophagus opens directly into cardiac. Pyloric is the last part of stomach. In between cardiac and pyloric fundic is present.  Cardiac and pyloric both the parts are provided with an orifice. These orifices are known as cardiac and pyloric orifice respectively. Each of which is guarded by a ring like sphincter.
4. Small intestine :- Pyloric part of stomach opens behind into the longest part of alimentary canal known as small intestine. It is divisible into 3 parts :-
a. Duodenum :- It is the first part of small intestine. Stomach directly opens into it. It forms a C-shaped curve. Bile-pancreatic duct opens in duodenum.
b. Jejunum :- It is small part just behind the duodenum. It opens in the caecum. This opening is guarded by an ileo-caecal valve.
c. Ileum :- It is the last part of small intestine. It opens in the first part of large intestine.
The wall of jejunum and ileum is provided with many finger like projections known as villi. These villi increase the surface area of intestine many times.
5. Large intestine :- It is the last part of alimentary canal. It is again divided into 3 parts :-
a. Caecum :- It is the first part of large intestine. Ileum directly opens into it through ileo-caecal valve. Caecum leads into a tubular vermiform appendix.
b. Colon :- It is the second part of large intestine. It is inverted U-shaped.
c. Rectum :- It is the last part of large intestine. It opens outside through anus.

Liver is the largest gland of body. It is soft and solid, reddish brown structure. It consists of two lobes, right and left. Right lobe is larger. A pear shaped gall bladder is located upon right lobe. The bile formed in liver is stored in gall bladder.
Functions of Liver :- Liver performs following functions :-
1. Liver secretes bile which helps in digestion of fats. It also prevents putrefaction of food by killing harmful bacteria.
2. It converts excess glucose into glycogen and stores it.
3. It can also convert glycogen into glucose when needed.
4. It converts harmful ammonia into less harmful urea.
5. It forms heparin which prevents clotting of blood in blood vessels.
6. In embryonic condition, it takes part in formation of red blood corpuscles (R.B.Cs.).
7. It stores vitamin A, D and B12.
8. It also destroys dead and wornout R.B.Cs.

Pancreas is the second largest gland of body. It is pinkish, elongated and irregularly branched flattened gland. It is situated between descending and ascending arms of duodenum.
It consists of 2 types of cells. One type of cells produce pancreatic juice. Another type of cells are scattered irregularly and are called islets of Langerhans. These cells secrete insulin hormone. In this way, pancreas is exocrine as well as endocrine gland.

Respiratory system
The respiratory system consists of lungs and air passages related to them. This system includes following organs :-
1. Nostrils and Nasal Chambers :- Nostrils are one pair of apertures found at the lower end of nose just above the mouth. These are the openings of nasal cavities.
Nasal chambers open behind into the nasopharynx by internal nostrils. In the beginning of nasal chambers hairs are present.
2. Nasopharynx :- Nasopharynx lies behind nasal chambers and has internal nares in its roof and oval openings of eustachian canals on the sides. A slit like aperture glottis is present in pharynx which leads into trachea.
3. Larynx :- It is organ for the production of voice. It lies in the anterior midline of the neck. It is thin walled tubular structure supported by 4 cartilages.
4. Trachea :- It is also known as wind pipe. Its upper end continues with the lower end of larynx. Its wall is supported by  C-shaped cartilagenous rings. At its lower end, it is divided into right and left primary bronchi.
5. Lungs :- These are 2 in number and situated in thoracic cavity. These are enclosed in double walled sacs called pleura.  Space between two layers is called pleural cavity. The fluid filled in this cavity is called pleural fluid. Pleural fluid lubricates the lungs for their expansion and relaxation. It also protects the lungs from shock and injury.
Lungs are soft and spongy organs. Right lung is divided into superior lobe, middle lobe and inferior lobe. Left lung is also divided into upper lobe and lower end. In each lung, primary bronchus divides into secondary bronchi which again divides into tertiary bronchi. Tertiary bronchi divide repeatedly to form terminal bronchioles which give rise to alveolar ducts. These ducts open into alveoli which are blind sacs.

Mechanism of Breathing :- Breathing is a process in which pure air is taken in and impure air is given out. It involves two processes :-
1. Inspiration :- The process of taking the air in the lungs is called inspiration.
2. Expiration :- The process of giving the air out of the lungs is called expiration.
Lung lie in the thoracic cage. It is an air tight cage bounded on dorsal side by vertebral column, on ventral side by sternum, on lateral sides by ribs and on posterior side by diaphragm.
During inspiration, intercostal muscles contract pulling the ribs and sternum out and diaphragm becomes flat so that the thoracic cage is enlarged. Due to increase in volume, the pressure inside the cage decreases. As a result, the air comes in from outside through nostrils. When this air reaches the alveoli, the oxygen in taken in the blood and CO2 is given out by the process of diffusion. After it, the muscles again return to their normal position expelling the air out of nostrils.