Figure 2.1
Normal, transparent Pleura: Left subclavian artery and phrenic nerve are visible
Figure 2.2
Layer with arterial and venous capillaries
2.2.2 Vessels
The pleural vessels run in the soft tissue main layer. The blood supply of the visceral pleura is located in the area of the ribs and the diaphragm via the pulmonary arteries and bronchial arterial branches which create anastomoses with the pulmonary arteries in all other areas. The parietal pleura is fed by the intercostal arteries in the regions of the ribs and by the rami intercostales of the internal mammary arteries.
2.2.3 Lymph Vessels
The lymphatics of the visceral pleura are located close to the lung in the soft tissue main layer of the pleura. Of special importance are lymphatic stomata which are 2–8 μm in diameter in the parietal pleura. In these areas the pleural space is directly connected to the draining lymphatic system [1]. The pleural fluid circulates at a speed of 0.2 ml/kg/h, which allows for a total exchange of the fluid within one hour.
2.2.4 Mesothelium
The mesothelium is a tissue with a high metabolism that is able to produce different mediators and soluble factors for chemotactic activity and phagocytic activity. Mesothelial cells not only play an active roll in inflammatory processes but also participate in tissue repair as they synthesize collagen types I, II, and IV. Under physiologic circumstances, the pleural cell surfaces have mononuclear phagocytes that produce cytokines IL-1P, TNF-a, IL-8, and LTB-4 [3].
2.3 Physiology of the Pleural Space
2.3.1 Intrapleural Pressure
The position of the body and the phase of the breathing cycle create different pressure ratios in the pleural space. In principle a “negative pressure” is created by the elastic recoil of the lung. “Elastic recoil” is the tendency of the lung to shrink to a smaller volume than is available in the chest cavity. The lung adjusts to the significantly less elastic chest wall in regards to shape and volume in a way that the “negative” pressure is significantly higher than the elastic recoil of the lung itself. The forces that keep the lung expanded are created by the continuous evacuation of pleural fluid through the lymphatic vessels and the subpleural capillaries which prevent the intrapleural fluid from increasing. Adhesion forces between the surface of the lung and the chest wall work like a homogenous vacuum spread over the pleural surface forcing the lung to follow the chest wall during inspiration and expiration.
2.3.2 Basics in Physics [4]
The pressure (P) describes a physical condition defined for each place in space as a force being affected in all directions. Natural law (2nd law of thermodynamics) defines that the pressure between two adjacent spaces is equalized if the bounding surface allows. Negative pressure does not exist; we always look at the pressure difference (ΔP) between two different spaces (i.e. lung and pleural space). Reference pressure is the atmospheric pressure. The weight of air over the earth is 10 tons per square meter. All pressures measured in or at the human body are differences compared to the atmospheric pressure, i.e. higher or lower than the atmospheric pressure. The units of measure frequently used in medicine are listed in Table 2.1.
Table 2.1
Physical unit of measurement regarding pressure in medicine
Bar | m WS | Torr | Physical atmosphere | |
|---|---|---|---|---|
1 mbar | 1 cmH2O | 1 mmHg | 1 atm | |
1 atm | 103 | 103 | 750 | 1 |
1 mmHg | 1.3 | 1.3 | 1 | 1.3*10-3 |
1 cmH2O | 1 | 1 | 0.75 | 10 |
1 mbar | 1 | 1 | 0.75 | 10 |
2.3.3 Changes in Pleural Pressure During Breathing
Lung tissue is very elastic and is able to both increase and decrease its volume. As there is equilibrium or a pressure difference between intrapulmonary and intrapleural pressure (i.e. in atelectasis, in a pneumothorax, during single lung ventilation) the lung volume decreases from Vtot (2500 cm3) to V0 (700 cm3). Elasticity of the lung or elastic recoil as well as the distension of the lung in the pleural cavity are responsible for the so called “negative pressure” in the pleural space. In a healthy lung with normal elastic recoil intrapleural pressure oscillates around 10 mbar.
Direct measurement of the intrapleural pressure can only be done by invasive procedures. In daily practice intrapleural pressure is determined via the esophagus, presuming that the intrapleural pressure is the same as the pressure in the esophageal area near the mediastinal pleura. The clinical relevance of the absolute measured value can be neglected.
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