16B

The Respiratory System: Pulmonary Ventilation

Chapter Overview

  • Focus on the mechanics of breathing and the muscles involved in respiration.

  • Understanding of pulmonary pressures and their roles in ventilation.

Mechanics of Breathing

Inspiration

  • Pressure in lungs (Palv) becomes less than atmospheric pressure (Patm).

  • Air flows into lungs until Palv equalizes with Patm.

Expiration

  • Pressure in lungs (Palv) becomes greater than atmospheric pressure (Patm).

  • Air flows out of lungs until Palv equalizes with Patm.

Forces for Pulmonary Ventilation

  • Ventilation is driven by pressure gradients.

  • Bulk flow of air is established by differences in pressure between lungs and atmosphere.

Respiratory Pressures

Atmospheric Pressure (Patm)

  • Refers to the pressure exerted by the weight of the air in the atmosphere.

  • Standard value: 760 mm Hg at sea level.

    • Value decreases with altitude and increases underwater.

Intra-alveolar Pressure (Palv)

  • Pressure of air within the alveoli of the lungs.

  • Changes during different phases of respiration:

    • Inspiration: Palv is negative (< Patm).

    • Expiration: Palv is positive (> Patm).

Intrapleural Pressure (Pip)

  • Pressure within the pleural sac.

  • Under normal conditions, it is always negative (less than Palv).

    • Typically starts at –4 mm Hg at the beginning of the respiratory cycle.

  • During inspiration, Pip becomes more negative.

  • During expiration, Pip becomes less negative.

Transpulmonary Pressure (Ptp)

  • Defined as Ptp = Palv - Pip.

  • Represents the distending pressure across the lung wall.

  • Inspiration: Increased Ptp causes alveoli to expand, decreasing Palv.

  • Expiration: Decreased Ptp results in lung recoil, increasing Palv.

Effect of Pneumothorax

  • Occurs when air enters the pleural space (can be open or closed).

  • Equalizes Pip with Patm, resulting in loss of distending pressure across lungs leading to lung collapse.

Breathing Cycle

  • Consists of inspiration and expiration.

  • Involves changes in pressure gradients that facilitate air movement in and out of the lungs.

Resistance to Airflow

  • Airflow (flow) during breathing is influenced by:

    • Pressure gradient (∆P).

    • Resistance (R), impacted by airway radius and mucus.

Boyle's Law

  • States that pressure of a gas varies inversely with its volume:

    • Increase in volume (↑V) results in decrease in pressure (↓P).

    • Decrease in volume (↓V) results in increase in pressure (↑P).

Intra-alveolar Pressure Determinants

  1. Quantity of air in alveoli (functional residual capacity).

  2. Volume of alveoli changes with lung expansion and recoil.

Inspiratory and Expiratory Muscles

Inspiratory Muscles

  • Diaphragm and external intercostals increase thoracic cavity volume during inspiration.

Expiratory Muscles

  • Passive expiration occurs when inspiratory muscles stop stimulating.

  • Active expiration involves internal intercostals and abdominal muscles to forcefully decrease thoracic cavity volume, significantly increasing Palv.