Introduction:
The lungs have elastic recoil property and a tendency to collapse, whereas the chest wall or the thoracic cavity also has an elastic recoil property but a tendency to expand. The two forces working simultaneously but in opposite directions nullify the effect of each other. Thereby creating a negative intrapleural pressure.
What Is Intrapulmonary or Intra-alveolar Pressure?
The pressure developed within the alveoli is known as intrapulmonary or intra-alveolar pressure (IAP). As alveoli are in connection with the atmosphere, at the end of normal expiration, intrapulmonary pressure is equal to atmospheric pressure (Patm). Atmospheric pressure is similar to 760 mmHg.
Inspiration:
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At the beginning of inspiration, intrapulmonary pressure equals atmospheric pressure.
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During inspiration, expansion of thoracic volume occurs due to the contraction of the diaphragm and the abdominal contents are compressed by the descent of the diaphragm. It decompresses the contents of the thoracic cavity, and the lungs will expand, resulting in an increase in lung volume and a decrease in intrapulmonary pressure.
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In mid-inspiration, intrapulmonary pressure decreases and becomes -1, less than the atmospheric pressure. As the intrapulmonary pressure is sub-atmospheric, air enters the lungs.
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By the end of inspiration, the intrapulmonary and atmospheric pressure becomes equal to 0 mm Hg.
Expiration:
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At the beginning of expiration, the cycle is reversed as deflation occurs as a result of relaxation of the inspiratory muscles, and the inward elastic recoil of the lungs. The lungs and chest wall function as one unit during deflation. Airflow out of the lungs stops when alveolar pressure equals atmospheric pressure (0 cm H2O).
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According to Boyle’s law, in a closed chamber, the number of gas molecules is always equal to the constant, at a constant temperature, the pressure exerted by a gas varies inversely proportional to the volume of the gas. Hence, the pressure exerted by the gas reduces as the volume of gas increases and vice versa. So, as the lung volume decreases, there is an increase in the intrapulmonary pressure, and it becomes +1 at mid-inspiration. The intrapulmonary pressure is higher than the atmospheric pressure, and air moves out of the lungs till the pressures on both sides become equal.
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By the expiration's expiration, intrapulmonary pressure and atmospheric pressure equal 0 mm Hg.
What Is Intrapleural Pressure?
Intrapleural pressure (IPP) is the pressure developed between the two layers of the pleural membrane - visceral and parietal. This pressure is always sub-atmospheric or consistently negative. Normal intrapleural pressure is -4 to -7 mm Hg.
The reason behind negative intrapleural pressure:
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The elasticity of the lungs.
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Surface tension.
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The elasticity of the chest wall.
The Elasticity of Lungs: It offers resistance to stretch. It pulls the visceral pleura away from the parietal pleura. It always tries to recoil or deflate the lungs.
Surface Tension: The alveolar epithelium is lined by a thin, aqueous liquid layer with associated surface tension. The surface tension acts parallel with the lung tissue elasticity to tend to collapse the alveoli. It pulls the visceral pleura away from the parietal pleura.
The Elasticity of the Chest Wall: it always tries to push the chest wall, thereby expanding it. It pulls the parietal pleura away from the visceral pleura. Due to the dynamic interplay between these forces, the volume of the pleural cavity is. As per Boyle's law, the pressure in the pleural cavity decreases and becomes negative.
What Is the Importance of Negative Intrapleural Pressure?
The importance of negative intrapleural pressure is as follows:
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Maintains alveolar stability.
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Maintains airway patency.
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Prevents the collapse of the lungs.
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Increases venous return. During inspiration, the increased negative pressure within the mediastinum pulls the blood toward the great veins and heart.
What Is Transpulmonary Pressure or Transmural Pressure?
Transmural pressure is the pressure inside relative to the pressure outside a compartment. The transmural pressure equals the elastic recoil pressure of the lung compartment, under static conditions. The transmural pressure of the lungs is also referred to as transpulmonary pressure. As, the lungs tend to recoil inwards, inflating them requires increased transpulmonary pressure.
Transpulmonary pressure can be increased by either:
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Increasing the pressure inside according to the pressure outside the lungs, or
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by decreasing the pressure outside according to the pressure inside the lungs.
If the lungs were removed from the thoracic cavity, the chest wall would recoil or spring outward and expand to a larger size. Increasing or decreasing the volume of the chest wall from this resting position requires changes in the external or internal pressures acting on the chest wall. The chest wall expands from its resting position as result of contraction of inspiratory muscles.
It is the intra-alveolar pressure minus the intrapleural pressure [IAP - IPP = 0 - (-4) = +4 mm Hg]. It is always positive, which helps in keeping the lungs inflated.
What Is the Clinical Significance of Transpulmonary Pressure?
The clinical significance of transpulmonary pressure is as follows:
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In pneumothorax, because of the penetrating injury to the thoracic wall, the intrapleural pressure is equal to the atmospheric pressure, that is, it goes from -4 mm Hg to 0 mm Hg. The TTP is nullified as the intra-alveolar pressure (0 mm Hg) minus the intrapleural pressure (0 mm Hg). When TTP is eliminated, the lung collapses.
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In pleural effusion, fluid accumulates within the pleural cavity, which leads to increased intrapleural pressure as excess fluid limits inspiration. So, inspiration becomes more difficult as the lungs overcome the increased intrapleural pressure and resistance to expansion.
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In obstructive lung diseases such as emphysema, chronic bronchitis, and asthma, air trapping leads to difficulty in expiration. During forced expiration, the intrapleural pressure becomes positive, resulting in airway compression and further worsening the air trapping. As the air accumulates within the lung due to airway obstruction, lung compliance increases to accommodate the increased air volume. However, due to airway obstruction, the intra-alveolar pressure remains the same, decreasing the transpulmonary pressure gradient and causing lung expansion and air retention.
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In restrictive lung diseases like pulmonary fibrosis, increased effort is necessary for lung expansion due to pathologic lung remodeling. Decreased lung compliance leads to a decreased lung volume, and transpulmonary pressure increases.
Conclusion:
Transpulmonary pressure is a difference in pressure between the intra-alveolar and intrapleural space. It is a measure of the elastic forces within the lungs that has the tendency to collapse the lungs. Therefore, it prevents lung collapse. The higher the volume of the lungs, the higher the tendency to recoil. It is also helpful in understanding the stress on the lung parenchyma.
