Breathing and Exchange of Gases - Mechanism of breathing and its regulation in humans

Breathing involves two stages: Inspiration, where atmospheric air is drawn in, and Expiration, where alveolar air is released out.

The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. Inspiration occurs when the intra-pulmonary pressure is less than the atmospheric pressure (negative pressure in the lungs).

Inspiration is initiated by the contraction of the diaphragm (increasing thoracic volume in the antero-posterior axis) and the external inter-costal muscles (lifting up the ribs and sternum, increasing volume in the dorso-ventral axis).

Expiration occurs when the diaphragm and external inter-costal muscles relax, returning the diaphragm and sternum to their normal positions and reducing the thoracic volume, thereby increasing intra-pulmonary pressure above atmospheric pressure.

Respiratory Volumes: Tidal Volume ($TV \approx 500$ mL), Inspiratory Reserve Volume ($IRV \approx 2500-3000$ mL), Expiratory Reserve Volume ($ERV \approx 1000-1100$ mL), and Residual Volume ($RV \approx 1100-1200$ mL).

Regulation of Respiration: Human beings have a significant ability to maintain and moderate the respiratory rhythm. This is primarily handled by the Respiratory Rhythm Center in the medulla region of the brain.

A Pneumotaxic Center in the pons region can moderate the functions of the respiratory rhythm center by reducing the duration of inspiration and thereby altering the respiratory rate.

A Chemosensitive area situated adjacent to the rhythm center is highly sensitive to $CO_2$ and hydrogen ions ($H^+$). Increase in these substances activates this center, which signals the rhythm center to make remedial adjustments.

Receptors associated with the aortic arch and carotid artery also recognize changes in $CO_2$ and $H^+$ concentration and send necessary signals to the rhythm center.