Pulmonary Ventilation and Respiratory Pressure

• Pulmonary ventilation consists of both inspiration and expiration.
• Inspiration involves air flowing into the lungs.
• Expiration involves air flowing out of the lungs.
• Respiratory pressure is always relative to atmospheric pressure, equal to 760 mmHg at sea level

Intrapulmonary Pressure

• Pressure within the alveoli.
• It rises and falls in breathing phases, always equalizing with atmospheric pressure.

Intrapleural Pressure

• Pressure within the pleural cavity.
• It is always 4 mmHg less than the intrapulmonary pressure.

Boyle's Law

• Volume changes cause pressure changes leading to gas flow that equalizes pressure
• The relationship between pressure and volume, under constant temperature, is P1V1=P2V2
• P is the pressure of the gas in mmHg
• V is the volume in mm³
• When volume is large, gas molecules are far apart, resulting in low pressure.
• When volume is small, gas molecules are close together, resulting in high pressure.

Inspiration

• Diaphragm and external intercostal muscles activate
• The diaphragm contracts and moves inferiorly, flattening and increasing the thoracic cage's vertical diameter.
• External intercostals contract, moving the sternum forward and ribs outward, expanding the thorax diameter.
• Intrapulmonary pressure decreases relative to atmospheric pressure; air rushes into the lungs until the two pressures are equal.

Expiration

• Passive process that depends on the natural elasticity of the lungs
• Relaxation of the diaphragm and intercostals causes lungs to recoil dropping lung volume
• Alveoli compress, raising intrapulmonary pressure above atmospheric pressure
• Gas flows out of the lungs to match the change in pressure.

Lung Compliance

• How easily lungs expand (distensibility).
• Diminished by the reduction of the lungs' natural resilience, fibrosis, bronchi blockage or respiratory pathway issue, or impaired thoracic cage flexibility.
• Assessed by measuring the increase in intrapulmonary pressure; the greater the volume increase, the greater the compliance.

Respiratory Lung Volume

• Tidal Volume (TV): The air that flows into and out of the lungs during normal breathing (500 ml)
• Inspiratory Reserve Volume (IRV): volume of air that can be inspired forcibly beyond the tidal volume (2100 - 3200 ml)
• Expiratory Reserve Volume (ERV): volume of air that can be evacuated from the lungs beyond tidal volume (1000 - 1200 ml)
• Residual Volume (RV): amount of air that remains in the lungs preventing collapse (1200ml)

Respiratory Capacities

• Inspiratory Capacity (IC): Total amount of air that can be inspired after a tidal expiration (TV + IRV)
• Functional Residual Capacity (FRC): Amount of air remaining in the lungs after tidal expiration (RV + ERV)
• Vital Capacity (VC) - total amount of exchangeable air (TV + IRV + ERV)
• Total Lung Capacity (TLC): Lung capacity around 6000 ml (IRV + TV + ERV + Rv)

Dead Space

• Anatomical Dead Space: Inspired air in the conducting respiratory passageway that never contributes to gas exchange. (150ml)
• Alveolar Dead Space: Alveoli become nonfunctional and cease to act in gas exchange.
• Total Dead Space: Anatomical Dead Space + Alveolar Dead Space

Pulmonary Function Tests

• Minute Respiratory Volume (MRV) - total amount of gas flow into and out of the respiratory tract in 1 min. (Avg. 6 L/min or 12 breaths per minute)
• Spirometers are common tools used to measure expiration and inspiration to help identify restrictive vs obstructive respiratory disorders.

Nonrespiratory Air Movements

• Cough - Deep inspiration, glottis closes, and air forced superiorly from lungs against glottis for sudden opening and dislodgement from respiratory track
• Sneeze - Same as cough but air forced through nasal cavities
• Cry/Laugh - Inspiration followed by the release of air in short bursts, emotionally induced
• Hiccup - Sudden inspiration resulting in spasms of the diaphragm
• Yawn - Deep inspiration with open jaws ventilating all alveoli

Pulmonary ventilation includes inspiration (air flowing in) and expiration (air flowing out). Respiratory pressure is relative to atmospheric pressure. Key pressures include intrapulmonary (in alveoli) and intrapleural (in the pleural cavity). Boyle's Law (P1V1=P2V2) explains the relationship between pressure and volume.