● We have a pair of `color{violet}("external nostrils")` opening out above the upper lips.

● It leads to a `color{violet}("nasal chamber")` through the `color{violet}("nasal passage.")`

● The `color{violet}("nasal chamber")` opens into `color{brown}("nasopharynx,")` which is a portion of `color{violet}("pharynx,")` the common passage for `color{violet}("food and air.")`

● `color{violet}("Nasopharynx")` opens through glottis of the `color{violet}("larynx region")` into the `color{brown}("trachea.")`

● `color{brown}("Larynx")` is a cartilaginous box which helps in `color{violet}("sound production")` and hence called the `color{brown}("sound box.")`

● During swallowing glottis can be covered by a thin `color{violet}("elastic cartilaginous")` flap called `color{brown}("epiglottis")` to prevent the entry of `color{violet}("food")` into the `color{violet}("larynx.")`

● `color{brown}("Trachea")` is a straight tube extending up to the `color{violet}("mid-thoracic cavity,")` which divides at the level of 5th thoracic vertebra into a right and left primary `color{brown}("bronchi.")`

● Each `color{violet}("bronchi")` undergoes repeated divisions to form the `color{violet}("secondary and tertiary bronchi")` and `color{violet}("bronchioles")` ending up in very thin terminal `color{brown}("bronchioles.")`

● The `color{violet}("tracheae, primary, secondary and tertiary bronchi,")` and initial bronchioles are supported by `color{violet}("incomplete cartilaginous rings.")`

● Each `color{violet}("terminal bronchiole")` gives rise to a number of very thin, irregular walled and vascularised bag-like structures called `color{brown}("alveoli.")`

● The `color{violet}("branching")` network of `color{violet}("bronchi, bronchioles")` and alveoli comprise the `color{brown}("lungs.")`

● We have two `color{violet}("lungs")` which are covered by a `color{brown}("double layered pleura,")` with `color{brown}("pleural fluid")` between them.

● It reduces friction on the `color{violet}("lung surface.")`

● The outer `color{violet}("pleural membrane")` is in close contact with the `color{violet}("thoracic lining")` whereas the inner pleural membrane is in contact with the `color{violet}("lung surface.")`

● The part starting with the `color{violet}("external nostrils")` up to the `color{violet}("terminal bronchioles")` constitute the conducting part whereas the `color{violet}("alveoli")` and their ducts form the `color{violet}("respiratory or exchange part")` of the respiratory system.

● The conducting part transports the atmospheric air to the `color{violet}("alveoli,")` clears it from foreign particles, humidifies and also brings the air to `color{violet}("body temperature.")`

● Exchange part is the site of actual diffusion of `O_2` and `CO_2` between `color{violet}("blood")` and `color{violet}("atmospheric air.")`

● The `color{violet}("lungs")` are situated in the `color{violet}("thoracic chamber")` which is anatomically an air-tight chamber.

● The `color{violet}("thoracic chamber")` is formed dorsally by the `color{violet}("vertebral column,")` ventrally by the sternum, laterally by the ribs and on the lower side by the `color{violet}("dome-shaped diaphragm.")`

● The anatomical setup of `color{violet}("lungs in thorax")` is such that any change in the volume of the `color{violet}("thoracic cavity")` will be reflected in the `color{violet}("lung (pulmonary) cavity.")`

● Such an arrangement is essential for `color{violet}("breathing,")` as we cannot directly alter the `color{violet}("pulmonary volume.")`

● `color{violet}("Respiration involves")` the following steps:

(i) `color{brown}("Breathing or pulmonary ventilation")` by which atmospheric air is drawn in and `CO_2` rich alveolar air is released out.

(ii) `color{brown}"Diffusion of gases"` (`O_2` and `CO_2`) across `color{violet}("alveolar membrane.")`

(iii) `color{brown}"Transport of gases"` by the `color{violet}("blood.")`

(iv) `color{brown}"Diffusion"` of `O_2` and `CO_2` between `color{violet}("blood")` and `color{violet}("tissues")`.

(v) `color{brown}"Utilisation"` of `O_2` by the `color{violet}("cells")` for `color{violet}("catabolic reactions")` and resultant release of `CO_2 .`