Bronchiole
Larynx
Pharynx
Trachea
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It structurally extends from the nose, and pharynx, all the way to the alveoli of the lungs. The part starting with the external nostrils up to the terminal bronchioles constitutes the conducting part (transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies and also brings the air to body temperature), whereas the alveoli and their ducts form, the respiratory or exchange part (the site of actual diffusion of O2 and CO2 between blood and atmospheric air) of the respiratory system.
- Nose: It consists of a pair of external nostrils opening out above the upper lips, which leads to a nasal chamber through the nasal passage.
- Pharynx: The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air. The pharynx opens through the larynx region into the trachea.
- Larynx: Also called ‘sound box’ is a cartilaginous box which helps in sound production. During swallowing, the glottis (thin opening at the top of the larynx) can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx.
- Trachea: It is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the fifth thoracic vertebra into a right and left primary bronchi.
- Bronchi: Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The trachea, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
- Alveoli: Numerous vascularised bag like structures which rise from terminal bronchioles.
- Lungs: The branching network of bronchi, bronchioles and alveoli comprise the lungs. Humans have two lungs which are covered by a doublelayered pleura (with pleural fluid between them), which reduces friction on the lung surface.
- Diaphragm: It is a dome-shaped, muscular and membranous structure that separates the thoracic (chest) and abdominal cavities in humans (mammals) and is the principal muscle of respiration.
The order of the structure of respiratory structure through which air passes until it reaches the alveoli are as follow: Nose, nasal cavity, mouth/oral cavity, larynx, trachea, lungs, bronchi, bronchioles, and alveoli.
Breathing involves two stages: inspiration (atmospheric air is drawn in) and expiration (alveolar air is released out). Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure. The diaphragm and a specialised set of muscles- external and internal intercostals between the ribs help in the generation of such gradients. Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber. The contraction of external intercostal muscles lifts up the ribs and the sternum, also increasing the volume of the thoracic chamber. The overall increase in the thoracic volume causes a similar increase in pulmonary volume, decreasing the intra-pulmonary pressure below the atmospheric pressure, forcing air outside to move into the lungs, i.e., inspiration. Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions, reducing the thoracic volume and the pulmonary volume. This increases intra-pulmonary pressure slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration.
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