Human Physiology - Week 10: Respiratory System Notes

Pulmonary Pressures

• Intra-alveolar pressure (Palv)

• The pressure of air within the alveoli, relative to atmospheric pressure. This pressure is crucial for the mechanics of breathing and influences the airflow into and out of the lungs.

• Changes during respiration:

  • Inspiration: During inhalation, the diaphragm contracts, and the thoracic cavity expands, leading to a decrease in Palv, creating a negative pressure (Palv < Patm) that allows air to flow into the lungs.

  • Expiration: During exhalation, the diaphragm relaxes, and the thoracic cavity volume decreases, resulting in an increase in Palv, creating positive pressure (Palv > Patm) that drives air out of the lungs.

• The pressure difference between Palv and Patm drives ventilation, ensuring adequate gas exchange in the alveoli by facilitating the movement of oxygen into and carbon dioxide out of the bloodstream.

• Intrapleural pressure (Pip)

• The pressure within the pleural sac surrounding the lungs, which is always negative under normal conditions. This negative pressure is essential for maintaining lung expansion against the chest wall.

• Varies with respiration phases:

  • At rest, the Pip is typically around -4 mm Hg, although it can fluctuate during different phases of the respiratory cycle.

  • The negative pressure is maintained due to the elastic recoil of both the lungs and the chest wall, which prevents the alveoli and pleura from separating and collapsing the lung.

• Transpulmonary pressure (Ptp)

• Defined as Ptp = Palv - Pip, this pressure is a critical factor in respiratory mechanics.

• It represents the distending pressure across the lung wall; an increase in Ptp correlates with an increase in lung volume, which facilitates greater air entry during inspiration.

Mechanics of Breathing

• Movement of Air:

• Air flows in and out of the lungs due to established pressure gradients between the intra-alveolar pressure and atmospheric pressure.

• Boyle's Law: This law states that pressure and volume are inversely related, meaning that as volume increases, pressure decreases. Changes in alveolar volume alter pressure gradients between Palv and Patm, allowing for airflow.

• Components Influencing Intra-Alveolar Pressure:

• Volume of Air in Alveoli:

  • An increased volume of air reduces Palv, facilitating air entry into the lungs. Conversely, a decreased volume raises Palv, pushing air out of the lungs.

• Mechanism of Breathing:

  • Inhalation involves the contraction of inspiratory muscles, including the diaphragm and external intercostal muscles, which leads to an increase in thoracic cavity volume.

  • As the thoracic volume increases, the alveoli expand, decreasing Palv, and allowing ambient air to flow in through bulk flow.

  • During expiration, the lungs recoil passively due to elastic forces, while active expiration incorporates the use of expiratory muscles (e.g., abdominal muscles) to compress the thoracic cavity further.

Factors Affecting Pulmonary Ventilation

1. Lung Compliance:

   • Lung compliance is a measure of how easily the lungs can be stretched. Higher compliance indicates that the lungs can inflate more easily with less effort.

   • Compliance is influenced by several factors:

     • Elasticity: The inherent elasticity of lung tissue impacts how well the lungs can expand during inhalation.

     • Surface Tension: The surfactant in the alveoli reduces surface tension, increasing compliance by preventing alveolar collapse during expiration.

2. Airway Resistance:

   • Increased airway resistance can significantly impede airflow, making breathing more difficult.

   • Resistance levels are determined primarily by the geometry and size of the airways:

     • Smaller diameters and narrowed airways result in increased resistance.

   • Resistance can also be modulated through the contraction and relaxation of smooth muscle surrounding the airways and the secretion of mucus, which can obstruct airflow.

Smooth Muscle Control of Airway Resistance

• Bronchoconstriction vs. Bronchodilation:

• The smooth muscle surrounding the airways plays a vital role in controlling airway resistance:

  • Bronchoconstriction: When the smooth muscle contracts, the airway radius decreases, resulting in increased resistance and reduced airflow.

  • Bronchodilation: In contrast, smooth muscle relaxation increases the airway radius, leading to decreased resistance and enhanced airflow.

• Regulatory Mechanisms:

• Extrinsic Control:

  • Autonomic Nervous System:

  • Sympathetic stimulation results in bronchodilation through Beta-2 adrenergic receptors.

  • Parasympathetic stimulation leads to bronchoconstriction through Muscarinic receptors.

  • Hormonal:

  • Hormones such as epinephrine can promote bronchodilation, facilitating airflow during stress or physical activity.

• Intrinsic Factors:

  • Histamine Release: In conditions like asthma or allergies, histamine can cause bronchoconstriction, leading to increased airway resistance.

  • CO2 Levels: Elevated carbon dioxide levels in the blood can lead to bronchodilation as the body attempts to enhance airflow to expel CO2 more effectively.