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.
Inspiration is an active process that occurs due to the contraction of inspiratory muscles, whereas expiration is a passive phenomenon that occurs due to the elastic recoil of the lungs. Contraction of the inspiratory muscles causes expansion of the thoracic cavity, which leads to reduced intrapleural pressure. A drop in intrapleural pressure helps the lungs to expand. The expansion of the lungs decreases intrapulmonary pressure to the subatmospheric level, due to which air from the atmosphere is sucked into the lungs.
What Are the Muscles of Respiration?
Main muscles for inspiration:
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Diaphragm.
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External intercostal muscle.
Main muscles for expiration:
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Internal intercostal muscle.
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Pectoral muscle.
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Scalene muscle.
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Sternocleidomastoid muscle.
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Neck and back muscles.
The diaphragm is the major muscle in inspiration. The contraction of the diaphragm can lead to an inflation of the lungs. Contraction of the diaphragm expands the thoracic cavity in two ways:
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The diaphragm is a dome-shaped muscle and attaches itself to the lower six ribs and the xiphoid process of the sternum. Thus, when it contracts, the dome becomes flattened, and abdominal contents are compressed downward so that the thoracic cavity increases to its rosto-caudant extent. Thus, the vertical diameter of the thoracic cage increases.
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Contraction of the diaphragm also pushes the rib cage outward, which enlarges the thoracic cavity in its anteroposterior and lateral planes.
External intercostal muscles are obliquely present between ribs in forwarding and downward direction. Their attachment to lower ribs is anterior to the axis of rotation. Thus, the contraction of these muscles raises the lower rib adequately.
Contraction of these muscles has two effects:
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Bucket-Handle Effect: Higherthe transverse diameter of the thoracic cavity.
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Pump-Handle Effect: Enhances the thoracic cavity's vertical diameter, though anteroposterior diameter also increases to some extent.
What Happens During Inspiration?
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At the beginning of inspiration, intrapulmonary pressure equals atmospheric pressure.
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During inspiration, the diaphragm contracts and descends downwards, expanding the thoracic cavity volume. The diaphragmatic descent compresses the abdominal contents. 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 subatmospheric, air enters the lungs.
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By the end of inspiration, the intrapulmonary and atmospheric pressure becomes equal to 0 millimeters of mercury.
What Happens During Expiration?
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At the beginning of expiration, the cycle is precisely reversed. The inspiratory muscles relax, and the inward recoil of the lungs causes deflation. During deflation, the chest wall and the lungs move as a single unit. Airflow out of the lungs stops when the alveolar pressure and the atmospheric pressure (0 centimeter H2O) reach equilibrium.
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At any constant temperature, in a closed system, where the number of gas molecules is constant, the pressure exerted by a gas is inversely proportional to the volume of the gas, as per Boyle's law. Therefore, as the gas's importance increases, the gas's pressure decreases. Contrary, the pressure increases as the volume decreases. 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 greater 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 millimeters of mercury.
What Are the Pressures in the Thoracic Cavity?
Changes in different pressure in the thoracic cavity that result in breathing are:
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Intrapulmonary or intra-alveolar pressure.
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Intrapleural pressure.
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Transpulmonary pressure or transmural pressure.
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 millimeters of mercury. During inspiration, alveolar pressure decreases, which draws air into the lungs, and during expiration, the alveolar pressure increases, which removes the air from the lungs. The normal intra-alveolar pressure value during inspiration is approximately -1 millimeters of mercury and, during expiration, +1 millimeters of mercury.
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 millimeters of mercury. The normal value in quiet breathing the intrapleural pressure during expiration is about -2.5 to -4 millimeters of mercury, and during inspiration is about -6 millimeters of mercury. However, intrapleural pressure becomes positive during forced expiration, and during forced inspiration, it becomes further negative, maybe up to -30 millimeters of mercury. 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 the chest wall. 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. The significance of intrapleural pressure is that loss of normal intrapleural pressure results in lung collapse and a barrel-shaped chest.
Transpulmonary Pressure is the pressure difference across an airway or the lung wall. It is the intra-alveolar pressure minus the intrapleural pressure [IAP - IPP = 0 - (-4) = +4 millimeters of mercury]. It is always positive, which helps in keeping the lungs inflated and prevents the lungs from collapsing.
Conclusion:
The breathing mechanism involves air flowing in during inspiration and airflow out of the lungs during expiration. The pressure gradient between the atmosphere and gases within the lungs causes the air to flow in or out of the lungs. The breathing process is complex and requires a dynamic interplay between pressure differences.
