Fluid Dynamics in Normal Pleura - The Determinants of Dynamics

Introduction

The pleural fluid serves various functions and helps diagnose and evaluate diseases, trauma, and other abnormalities. The volume and composition of the pleural fluid play a significant role in maintaining an efficient coupling between the chest wall and the lungs. The pleural fluid in the normal pleura represents a balance between various forces acting on them. A disruption in fluid dynamics and homeostasis results in pleural effusions.

What Does Pleura Mean?

Pleura is a thin layer of tissue that lines the lungs and the inner wall of the thoracic (chest) cavity. It forms the essential component of the respiratory tract. The pleura is a two-layered membrane consisting of the parietal pleura and the visceral pleura covering each lung. The pleura cushions the lungs and reduces the friction that may develop between the thoracic cavity, rib cage, and lungs. The parietal pleura and the visceral pleura are separated by a small amount of thick fluid called pleural fluid.

What Is Pleural Fluid?

Pleural fluid is a lubricant that is present between the two layers of the pleura. It keeps the pleural layers moist and helps reduce friction between the layers during respiration. The parietal circulation continuously produces the pleural fluid in a bulk flow. Also, the lymphatic system continuously reabsorbs the pleural fluid through the stomata present in the parietal pleura. In a healthy state, 0.03 to 0.67 fl oz (fluid ounce) of pleural fluid is present within the pleural space. This fluid is filtered from systemic microvessels at the parietal pleural level to the pleural and extrapleural space down a pressure gradient. The stomata in the parietal pleura provide most of the drainage. Also, the visceral pleura does not account for any fluid drainage under normal conditions.

The pleural fluid acts as a lubricant and allows the pleural layers to glide smoothly during respiration. The pressure in the pleural fluid is subatmospheric, and it maintains the negative pressure between the thoracic cavity and the lungs. This maintenance of pressure is essential for inhalation and for preventing lung collapse.

What Does Fluid Dynamics in Normal Pleura Mean?

Fluid dynamics refers to the description and study of the flow of liquids and gases. Pleural fluid dynamics describes the flow of pleural fluid and its pressure gradient. The pleural surface forces exerted due to the opposing elastic recoils of the chest wall and the lungs are the primary determinants of pressure in the pleural liquid. The pressure on the pleural surface is the force per unit area of the pleural surface, equal to the pressure in the pleural liquid. The viscous flow of the pleural fluid within the pleural space is due to the difference between the vertical gradient in pleural pressure and the hydrostatic value, which is 0.394 inch H20 / inch height. The ventilatory and cardiogenic motions and gravity drive the recirculation of the pleural fluid to maintain the uniform thickness of the lubricating fluid. The fluid dynamics in a normal pleura include the pleural pressure, regional volume, and the different types of pleural pressure measurements.

Hydrostatic Equilibrium Vs. Viscous Flow:

Two scientists explained the transmission of forces between the chest wall and the lungs across the pleural space. It was proposed that the pleural liquid was in hydrostatic equilibrium with a vertical pressure gradient. Also, the hydrostatic equilibrium was achieved by the absorption of the pleural liquid into the blood until a balance of forces occurs between the blood and the pleural fluid. This pressure difference is attributed to the point of contact between the pleural surfaces.

The second concept states that the surface pressure acting to expand the lung acts equal and opposite to the pressure that is acting to expand the chest wall and is transmitted through the pleural space with the surface pressure equal to the pleural fluid pressure. Hence, the pleural surface is the major determinant of the pleural fluid pressure. Also, the viscous fluid of the pleural fluid is due to the difference between the pleural surface pressure and the hydrostatic value. Therefore, the dynamics of the pleural fluid in the pleural space require a continuous space for the pleural fluid with no pleural contact.

Pleural Pressure:

• Regional Lung Volume and Pleural Pressure: In an intact thorax, the regional lung volume varies with body position relative to the gravitational forces. The interaction between the forces in the lungs, thoracic, and abdominal cavities attenuated by the effects of gravitational forces that change with the body position determines the regional lung volume distribution. As the vertical gradient in the pleural pressure is less than the hydrostatic value, part of the gravitational force drives the viscous flow of the pleural fluid downward in the pleural space and does not affect the pleural surface pressure, which is determined by the forces between the lung and the adjacent structures.

• The Pleural Contact: Studies have shown that some point of contact exists between the pleural surfaces to compensate for the difference between pleural fluid pressure and the pleural surface pressure. Though some contact exists in the pleural space, it is not known if its magnitude is sufficient to affect the equality between surface pressure and fluid pressure.

• Pleural Pressure Measurements: Several techniques, such as esophageal balloons, intrapleural catheters, and flat air-filled pleural balloons, are available to measure pleural pressure. It can be measured by comparing regional alveolar volume to the pressure-volume behavior in the isolated lung.

• Pleural Fluid Thickness And Lubrication: The pleural fluid thickness is essential for the evaluation of pleural contact and the magnitude of shear stress-induced during ventilation. Two types of lubrication have been postulated. One involves lubrication based on viscous shear stresses in the fluid layer interposed between the sliding surfaces. The second is lubrication, where the fluid layer is thin so that the pleural surfaces come into contact.

Conclusion

The pleural fluid dynamics is the mechanics of the fluid movement in the pleural space. Under normal circumstances, homeostasis is achieved by an adjustment in the thickness of the pleural fluid to the filtration rate that is matched by an outflow through the lymphatic stomata. The fluid dynamics and the equilibrium is disrupted in cases of pleural disorders. The normal flow of pleural fluid and the homeostasis of the pressure gradient is essential to prevent lung collapse during respiration.