● `color{violet}("Breathing")` involves two stages :

● `color{brown}("Inspiration")` during which `color{violet}("atmospheric air")` is drawn in and `color{brown}("Expiration")` by which the alveolar air is released out.

● The movement of air into and out of the `color{violet}("lungs")` is carried out by creating a pressure gradient between the `color{violet}("lungs")` and the `color{violet}("atmosphere.")`

● `color{violet}("Inspiration")` can occur if the pressure within the `color{violet}("lungs (intra-pulmonary pressure)")` is less than the `color{violet}("atmospheric pressure,")` i.e., there is a negative pressure in the `color{violet}("lungs")` with respect to `color{violet}("atmospheric pressure.")`

● Similarly, expiration takes place when the `color{violet}("intra-pulmonary pressure")` is higher than the `color{violet}("atmospheric pressure.")`

● The `color{brown}("diaphragm")` and a specialised set of muscles – `color{brown}("external and internal intercostals")` between the ribs, help in generation of such `color{violet}("gradients.")`

● `color{violet}("Inspiration")` is initiated by the `color{brown}("contraction")` of `color{violet}("diaphragm")` which increases the volume of `color{violet}("thoracic chamber")` in the antero-posterior axis.

● The `color{violet}("contraction")` of external inter-costal muscles lifts up the `color{violet}("ribs")` and the `color{violet}("sternum causing")` an increase in the volume of the `color{violet}("thoracic chamber")` in the `color{violet}("dorso-ventral axis.")`

● The overall increase in the `color{violet}("thoracic volume")` causes a similar increase in `color{violet}("pulmonary volume.")`

● An increase in `color{violet}("pulmonary volume decreases")` the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the `color{violet}("lungs,i.e., inspiration.")`

● Relaxation of the `color{violet}("diaphragm")` and the `color{violet}("inter-costal muscles")` returns the `color{violet}("diaphragm")` and `color{violet}("sternum")` to their normal positions and reduce the `color{violet}("thoracic volume")` and thereby the `color{violet}("pulmonary volume.")`

● This leads to an increase in `color{violet}("intra-pulmonary pressure")` to slightly above the `color{violet}("atmospheric pressure causing")` the expulsion of air from the `color{violet}("lungs,")` i.e., `color{brown}("expiration.")`

● We have the ability to increase the strength of `color{violet}("inspiration and expiration")` with the help of additional `color{violet}("muscles")` in the `color{violet}("abdomen.")`

● On an average, a healthy human breathes `color{brown}("12-16 times/minute.")`

● The volume of air involved in `color{violet}("breathing movements")` can be estimated by using a `color{violet}("spirometer")` which helps in clinical assessment of `color{violet}("pulmonary functions.")`

● `color{brown}("Tidal Volume (TV):")` Volume of air inspired or expired during a `color{violet}("normal respiration")`. It is approx. `500 mL`., i.e., a healthy man can inspire or expire approximately `6000` to `8000 mL` of air per minute.

● `color{brown}("Inspiratory Reserve Volume (IRV):")` `color{violet}("Additional volume")` of air, a person can inspire by a forcible inspiration. This averages `2500 mL` to `3000 mL`.

● `color{brown}("Expiratory Reserve Volume (ERV):")` Additional volume of air, a person can expire by a `color{violet}("forcible expiration.")` This averages `1000 mL` to `1100 mL`.

● `color{brown}("Residual Volume (RV):")` Volume of air remaining in the `color{violet}("lungs")` even after a `color{violet}("forcible expiration")`. This averages `1100 mL` to `1200 mL`. By adding up a few respiratory volumes described above, one can derive various `color{violet}("pulmonary capacities")`, which can be used in `color{violet}("clinical diagnosis.")`

● `color{brown}("Inspiratory Capacity (IC):")` Total volume of air a person can inspire after a `color{violet}("normal expiration.")` This includes tidal volume and inspiratory reserve volume `( TV+IRV).`

● `color{brown}("Expiratory Capacity (EC):")` Total volume of air a person can expire after a `color{violet}("normal inspiration")`. This includes tidal volume and expiratory reserve volume `(TV+ERV)`.

● `color{violet}("Functional Residual Capacity (FRC):")` Volume of air that will remain in the `color{violet}("lungs")` after a `color{violet}("normal expiration.")` This includes `ERV+RV.`

● `color{brown}("Vital Capacity (VC):")` The maximum volume of air a person can breathe in after a `color{violet}("forced expiration.")` This includes `ERV, TV` and `IRV` or the maximum volume of air a person can `color{violet}("breathe")` out after a `color{violet}("forced inspiration.")`

● `color{brown}("Total Lung Capacity:")` Total volume of air accommodated in the `color{violet}("lungs")` at the end of a forced inspiration. This includes `RV, ERV, TV` and `IRV `or `color{violet}("vital capacity + residual volume.")`