AIPMT-NEET Biology Aspirants, read out the next AIPMT-NEET Biology Study material/ Notes of human respiratory system. In this post we will discuss about nasal cavity, pharynx, larynx, trachea, bronchi.

Respiration is the overall exchange of gases between atmosphere, blood and body cells which occurs by the process of pulmonary ventilation, external respiration and internal respiration.

• Breathing is the process of exchange of oxygen (O₂) from atmosphere with carbon dioxide (CO₂) produced by cells. OR in simple language we can say breathing is the process of moving air in and out of the cell. This is commonly also called as Respiration.
• Respiratory system performs the following functions: 1) helps in intake of O₂ by cells and removal of CO₂  produced by cells,  2) help to regulate blood pH, 3) nose has smell receptor and filters air, 4) help in production of sound.
• Human respiratory system can be divided into 2 parts: 1) Upper respiratory system which consists of nose, nasal cavity, pharynx; and 2) Lower respiratory system which includes larynx, trachea, bronchi and lungs.

Let’s discuss these structures one by one:

Nose:  Nose which is visible outside is external portion of nose and internal portion of nose which lies inside skull is called as nasal cavity. External nose has hyaline cartilage that’s why it is somewhat flexible in nature and internally it is lined by mucous membrane. It has two openings called as external nares or nostrils. Nostrils lead to nasal cavity through nasal passage. External nose helps in warming and filtration of inhaled air.

Nasal cavity: It has external nose/nostrils in anterior part and posteriorly, it merges with pharynx with the help of 2 openings called as internal nares.

• Nasal cavity is superior to oral cavity. And it has openings of mucus and tear ducts.
• Nasal septum is the vertical partition which divides the nasal cavity into left and right parts. Anterior portion of nasal septum is also made up of hyaline cartilage.
• Portion of nasal cavity just inside the nostrils is called as nasal vestibule. Air inhaled through nostrils passes through vestibule containing skin lined by hairs which filters dust particles from air.
• Nasal cavity is lined by pseudostratified ciliated columnar epithelium with goblet cells. Also called as respiratory epithelium. Cilia helps in movement of mucus which traps dust particles and then it is pushed towards pharynx, from where it can be swallowed or spit out.

Pharynx: It is commonly called as throat. Length of pharynx is about 13 cm extending from internal nares to larynx. Pharynx is superior to larynx in position.

• Contraction of skeletal muscles present in the wall of pharynx helps in the action of swallowing (deglutition).
• Pharynx has 3 regions: (1) Nasopharynx- it is the superior portion of pharynx, and extends from internal nares of nasal cavity to soft palate; (2) Oropharynx– it is the middle portion of pharynx, and extends from soft palate to hyoid bone. (3) Laryngopharynx- it is the inferior portion of pharynx, and extends from hyoid bone to larynx anteriorly and esophagus posteriorly.
• Pharynx is common pathway for food and air.
• Tonsils are also found in the pharynx (nasopharynx and oropharynx).
• Nasopharynx exchanges air with auditory tubes to maintain equal air pressure between middle ear and atmosphere.
• Nasopharynx has pseudostratified ciliated columnar epithelium.
• Oropharynx and laryngopharynx have nonkeratinized stratified squamous epithelium due to chances of abrasion by food particles.
• Oropharynx and laryngopharynx both have common passageway for food and air.
• Opening from mouth called as Fauces is present in oropharynx.

Human respiratory system

Larynx: It is also called as voice box or sound box. It connects laryngopharynx with trachea. It is present anterior to esophagus, in the middle of the neck. It extends from C4-C6, i.e. fourth to sixth cervical vertebrae.

• Larynx is a cartilaginous box of nine pieces of cartilage.
• Out of nine cartilages, thyroid cartilage is composed of hyaline cartilage which forms a triangular shape in anterior of larynx, which is also called as Adam’s apple. It is more prominent in males.
• Arytenoid cartilage helps in the production of speech.
• Epiglottis is leaf-shaped flap of elastic cartilage. It is attached to thyroid cartilage by pointed end of flap and broader part of flap is free to move up and down. During swallowing, pharynx and larynx rise. Rise of pharynx makes more space for incoming food, while rise of larynx causes the epiglottis to move down and forming the lid over glottis to close larynx and thus prevents the entry of food into the larynx.
• As larynx closes, food is automatically routed towards esophagus. Sometimes when food or dust is passed into the larynx by chance, cough reflex helps in removal of the particles.

Trachea: It is also called as windpipe. Trachea is located anterior to esophagus. It is a straight tube of length 12 cm and 2.5 cm in diameter; and extends form larynx to fifth thoracic vertebrae (T5)/ mid-thoracic cavity. At the level of T5, trachea divides into left and right primary bronchi.

• Tracheal wall has 4 layers: Mucosa, submucosa, hyaline cartilage and adventitia. Mucosa is the innermost one and has pseudostratified ciliated columnar epithelium. Submucosa is outer to mucosa and has areolar connective tissue.
• Next to submucosa is layer of hyaline cartilage. It forms 16-20 incomplete rings (of hyaline cartilage), as letter C upon trachea. These rings are horizontally stacked upon one another. Open part of rings faces towards esophagus, i.e. posterior. These rings help to provide support to trachea so that it does not collapse.

Bronchi: At the level of T5, trachea divides into left and right primary (main) bronchi. Right main bronchus/ right primary bronchus goes into right lung, and left primary (main) bronchus goes into left lung.

• Left and right primary bronchi divide into smaller secondary bronchi or lobar bronchi, i.e. one branch for each lobe of the lung. Secondary bronchi further branch into more smaller bronchi called as tertiary bronchi or segmental bronchi.
• Tertiary bronchi then divides into bronchioles . Bronchioles further divide into branches and finally end up in very thin tubes terminal bronchioles. 
• Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
• The branching network of bronchi, bronchioles and alveoli comprise the lungs.
• Trachea, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
• Extensive branching from the trachea to the terminal bronchioles resembles an inverted tree and is commonly called as bronchial tree. Whole bronchial tree is a conducting zone of the respiratory system.
• Terminal bronchioles contain special cells called as Club cells or Clara cells. These cells help to protect bronchioles from harmful toxins and carcinogens inhaled in air.

BRANCHING OF BRONCHIAL TREE: Trachea – Primary bronchi (Main bronchi) – Secondary bronchi (Lobar bronchi) – Tertiary bronchi (Segmental bronchi) ­– Bronchioles – Terminal bronchioles.