8.2 Ventilation and respiratory mechanics

Breathing is a complex process involving various muscles and pressures. We'll explore how air moves in and out of our lungs, the volumes involved, and the key muscles that make it happen. It's all about expanding and compressing our chest cavity.

Lung mechanics play a crucial role in how we breathe. We'll look at factors like lung elasticity and compliance, which affect how easily our lungs expand. We'll also learn about surfactant, a substance that helps keep our tiny air sacs open and functioning properly.

Ventilation

Breathing Volumes and Rates

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• Tidal volume represents the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied
• Respiratory rate measures the number of breaths taken per minute
• Minute ventilation equals tidal volume multiplied by respiratory rate and represents the total volume of air entering the lungs each minute ($\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Respiratory Rate}$)

Breathing Pressures

• Negative pressure breathing occurs when air flows into the lungs due to the pressure in the lungs being lower than the atmospheric pressure, which is the normal breathing process for humans and most other mammals
• Positive pressure breathing happens when air is forced into the lungs by increasing the pressure of the air in the airways, which is seen in mechanical ventilation (ventilators) or during the use of continuous positive airway pressure (CPAP) devices

Respiratory Muscles

Inspiratory and Expiratory Muscles

• Inspiratory muscles contract during inhalation to expand the thoracic cavity and allow air to flow into the lungs (diaphragm, external intercostals)
• Expiratory muscles contract during exhalation to compress the thoracic cavity and force air out of the lungs (internal intercostals, abdominal muscles)

Key Respiratory Muscles

• Diaphragm is the primary muscle of inspiration that contracts and flattens, increasing the vertical dimension of the thoracic cavity
• Intercostal muscles are located between the ribs and assist in breathing
  • External intercostals contract during inspiration, pulling the ribs upward and outward
  • Internal intercostals contract during expiration, pulling the ribs downward and inward

Lung Mechanics

Factors Affecting Lung Expansion

• Compliance refers to the ease with which the lungs can expand and is determined by the elasticity of the lung tissue and the surface tension of the alveolar fluid
  • Increased compliance allows the lungs to expand more easily (emphysema)
  • Decreased compliance makes it harder for the lungs to expand (fibrosis)
• Airway resistance is the measure of how difficult it is for air to flow through the airways and is affected by factors such as airway diameter, inflammation, and mucus production
  • Increased airway resistance makes it harder for air to flow (asthma, bronchitis)

Lung Elasticity and Surfactant

• Elasticity is the ability of the lungs to recoil and return to their original shape after being stretched during inhalation, which is essential for exhalation
• Surfactant is a mixture of lipids and proteins secreted by type II alveolar cells that reduces the surface tension of the alveolar fluid
  • Surfactant prevents alveolar collapse by lowering surface tension, making it easier for the lungs to expand during inhalation and preventing the alveoli from collapsing during exhalation
  • Insufficient surfactant production can lead to respiratory distress syndrome (RDS) in premature infants