INTRODUCTION TO THE
THE PLEURAL SPACE
The pleural space constitutes a topographical zone for the potential convergence of the effects of pulmo-thorax dynamics, leading to air renovation from the atmosphere, as well as to alveolo-capillary gas exchange. This space must offer the mechanical conditions for the elastic play of the pulmo-thorax structures, in order to make possible the mechanics of low intensity that characterise the pulmonary peripheral area, represented by the lobular and alveolo-capillary structures or respiratory zone, as referred to above.
The interpretation of the dynamic phenomena, as they are detected at the pleural level, leads to the interpretation of the actions and reactions whose effects are propagated at that level, and the sequence of these effects enables us to interpret the kinematics and kinetics of the pulmo-thoracic apparatus, basing ourselves in anatomical and histological knowledge. This also applies to the mechanical function of the structures, and finally, to their action potential, whose co-ordination in time and space, leading to the function, must also be borne in mind. This latter role corresponds to the central nervous system and its complex network of information and responses.
The pleural space is the potential cavity circumscribed by the two-pleural surfaces, each of which is adjoined to the surface of the pulmo-thoracic structure which ít lines. The pleural cavity is the space for the potential integration of the dynamics of the parts, made possible by means of the pleural content and the force of adhesion generated in their integration. In this way, two pleural surfaces are defined as:
1. Parietal pleura, which lines the thorax walls (costo-diaphragmatic walls).
2. Visceral pleura, which lines the pulmonary surface.
The two-pleural surfaces are in intimate contact, moistened permanently by the pleural vapour condensed on them, enabling the physical adhesion of the two contacting walls. The non-saturator pleural vapour is as perfectly elastic as any gas, enabling the increase and decrease of the distance between them (pleural diameter).
Both pleural surfaces have a "moulding-moulded" relationship in their macroscopic structure. The visceral surface (pulmonary surface) shows delimited areas, as observed in the lungs of adults, where the figure of a polygonal mosaic is evident. This corresponds to the exterior surface of the lobuli at that level; their boundaries, which are depressed, are the inserting points of the elastic and colagen fibres which constitute the peri-lobular septum, spread out from the bronchial wall at the point where the centre-lobular bronchiole is generated. From. these points, they fan out; therefore, we can imagine the lobular surface and the septum that limit them, as an opened parachute, with a flattened dome and, septum or "tensors" at the stress points that corresponds to the degree of expansion, according to the dynamic point of the lobular cycle (as described elsewhere).
When the lobulus fills up with renewed air, during the phase of lobular ventilation or first half of the cycle, its exterior surface is expanded, acquiring the shape of a dome as a result of coincident actions: 1. The air ejection through the bronchioli, directed towards the pulmonary surface. 2. The simultaneous traction of the visceral pleura determined by bronchiolar -constriction and tensile elastic action of the inter-lobular septum.
These actions generate simultaneous sectorial forces, with the same direction and opposite sense, applied to the bronchiolar surfaces and their boundaries, with two main results: 1. The increase of the pleural diameter. 2. The increase in the force of air injection to fill up the alveoli. Consequently, the septums represent Sectorial forces whose direction and sense are towards the centre-lobular bronchiole, and further on to the pulmonary hilium, shortening the airways while the displaced mass of air acquires an acceleration in the same direction but in the opposite sense, towards the pulmonary periphery. The consequence of this is the ejection of air into the alveoli where it expands, acquiring the physiological pressure for the dynamic exchange.
The Resultant of the forces at the pleural level are detected by the water-filled balloon placed in the pleural space, as observed in the graph and analysed from a mechanical point of view.
The above explained dynamics determine:
1. The increase of the pleural diameter, and in consequence, the increase of the pleural capacity, through contraction of the bronchial muscles, which at the same time that they diminish the diameter of the airways and their longitudinal axis, retract the inter-lobular septums during the alveolar filling.
2. The simultaneous filling of the space created at the pleural level, by expansion of the lobular periphery during the first stage, and by the alveolo-capillary units in the second stage.
3. The increase of the pleural space simultaneously determines the decrease in the intrapleural pressure. This is also a factor favouring the alveolo-capillary currents, which are low intensity impulses. (The decrease in the, sub-atmospheric intra-pleural pressure is related to the behaviour of pleural vapour as a gas).
When capillary circulation ends and gas exchange is carried out, lobular relaxation takes place. Therefore, the structure previously shortened, expands now by elasticity (reaction); this expansion determines a pressure on the alveolo-capillary zone which helps to "squeeze" the contained fluids, and is a factor which favours the exit of the air to the upper airways. As the pleural diameter diminishes, the alveolo-capillary fluids follow the path of least resistance, which is now in the direction of the pulmonary hilium. These conditions now enable a new cycle of gas exchange to commence.
Throughout this brief description, it has been established that "expiration" as the second phase of the ventilatory cycle according to the classical scheme, does not correspond to the process which actually takes place, since it has only the appearance of a steady flow at the nasal level, but is in fact a cyclic ejection, with cardiac rhythm, at the lobular level, of the air used during each alveolo-capillary cycle.
In summary: The interlobular septums work like tensors which exert Sectorial forces from the centro-lobular bronchioli, while the lobular surfaces are distended by the air ejected through the respiratory airways, generating sectorial forces towards the pulmonary periphery; i.e., the Resultant forces are in a permanent state of dynamic imbalance, produced at each moment in the performance of the function. Hence, the creation of space at the pleural level is followed by the occupation of a part of it by the alveolo-capillary units which expand.
The consequences of these actions can be appreciated in fig. 17. When the lobuli are deflated
they appear flat and shallow and, the visceral surface appears even. Under these circumstances, the pleural capacity is equal to the surface of the pulmonary lobes, multiplied by the pleural diameter. When the lobular zone retracts the diameter increases; hence, pleural capacity increases. The simultaneous expansion of the alveolo-capillary units occupies a part of this space, leaving free space around the expanded hemispheric alveolo-capillary units; Consequently, the pleural capacity is still increased.
It is evident that the increase of the diameter of the pleural space is maximal around the circular base of these hemispherical units, and minimal in the upper point of the dome and, is much larger than could be deduced from the real increase of the pleural capacity, since a part of it is filled by the hemispheres of the expanded alveolo-capillary units.
The volume of the expanded alveolo-capillary units can be measured by the increment of the sub-atmospheric intra-pleural pressure, since this is inversely proportional to the space now occupied by the respiratory units.
The creation of pleural space is the effect of the traction determines by the Resultants of the forces exerted by the taut interlobular septums from their points of insertion in the sub-pleural tissue. The source of these forces is the muscular contraction of the bronchioli, which also determine a simultaneous increase in air pressure in the narrowed and shortened intra-lobular airways.
Two major simultaneous conditions have been clarified in the interpretation of the expansion of gas towards the alveoli, and the simultaneous circulation of blood in the alveolar capillaries. These are:
1. The forces generated and applied during these events.
2. The results of the kinematics of the function for gas interchange, or alveolo-capillary respiratory cycle.
The lobular units as a whole are subjected to the effects generated during these dynamics. All this has been interpreted through the study of the graphs which translate the effects of forces and movements en their surfaces, i.e., at the pleural level.
These facts show the great importance acquired by the division of work in the performance of this very complex and important vital function.
This division of work is effected by successive divisions and sub-divisions of the pulmonary structure up to the alveolo-capillary units, all of which ensures that a balanced force is exerted at each point of the pulmonary surface and what is still more important, in each alveolus.
All this is very important for a uniform maintenance of tension in the pulmonary structure, as well as in intra-pulmonary air-pressure, both of which are indispensable conditions for a uniform capillary circulation and, lastly, for a steady filling of the heart cavities.
The fact that the lungs are two units also divided in lobes, serves as a base to understand the need for -a uniform division of the work, since the pulmonary surface increases more than one large lobe would. Furthermore, an easier displacement of the whole is assured with a better dynamic adaptation to the interior surface of the thorax walls. The length of each bronchus is reduced, the number of bronchi is multiplied and their radial distribution in several spheres leads to an optimal distribution of the displaced volume of air. Smaller fractions of masses, and shorter ducts through which they are displaced, demand weaker forces in each segment to produce uniform impulses in the totality of the alveoli.
Consequently, the tiny airways at the end of the respiratory tree exert an enormous Resultant force if taken as a whole.
The division and subdivision in lobes, lobuli, alveolar completes and alveoli, multiply the number of vectorial forces distributed in the pulmonary periphery (60.000.000 alveoli), each of these forces corresponds to a fraction of pressurised air-mass which first expands the alveoli, and is then subjected to gas interchange with the proportional fraction of blood circulating through the corresponding capillaries. This is also guarantee for a function which is extensive in space (60 M2) and intensive in time (0.2 sec.), owing to the co-ordination of the unitary actions by the central nervous System.
INTRODUCTION TO PULMO T'HORAX KINEMATICS
The interpretation of the dynamics for gas exchange is based on a critical analysis of the factors that enter into play, summarised here.
1. The Anatomo-histo-physiology of the lobular units designed to fulfil dynamic cycles for gas exchange with the blood. These cycles have been discovered by the Author, who has named them, "Lobular-alveolo-capillary cycle".
2. The Anatomo-histology and kinetics of the lungs, in their integration with the organism as a whole, to fulfil the circulation of the intrapulmonary air-mass as well as its renovation from the atmosphere. This ventilatory cycle supplies the dynamic conditions and the volume-mass of air to enable the performance of the small alveolo-capillary cycles comprised in each ventilatory cycle.
3.The airways as active ducts to display a mass of air by the action of Resultant forces having specific direction and sense, as well as a chronologically and precisely determined sequence. These forces are primarily generated by the contraction of the smooth muscles of the respiratory tree, working from bases located on the pulmonary periphery. Among the effects determined by this primary force is the evocation of reflexes which also determine actions and reactions, from which new reflexes with their consequences are generated. All this is linked to a common purpose.
4. The co-ordination of actions to accomplish the function within physiological limits, is closely related to the information supplied by the specific receptors, located at different levels, and with different thresholds of sensitivity, as well as to the different conducting velocity of many kinds of nerve fibres which carry information and orders.
5. The elastic pulmonary structure which works like tensors permanently tightened and, stretched or shrunk when the lungs expand or retract, according to a programme for the circulation, of the air in the lungs. Similar dynamics are in operation at the pulmonary periphery, in the lobular or "respiratory" zone during each lobular cycle, ejecting the volume-mass of air towards the alveoli to be used by each alveolo-capillary cycle.
This structure is disposed as septums around the lobular borders, as a very important part of the design, in order to guarantee freedom of action for the lobular units on the general dynamics for air circulation, which is accomplished by the ventilatory infra-structure, both in accordance with the principle of "the independence of forces and movements".
6. The thorax walls, as active support of the lungs, limiting the primary active pulmonary retraction, and then resisting the strength of the elastic pulmonary structure during the expansive phase of inspiration, and also during the intervals of elastic retraction within each two inspirations.
The thorax walls play their own specific reflex role during pulmonary expansion (diaphragm) and the elastic retraction (costall wall) which follows.
The role of the diaphragm is to make way toward the abdomen, enabling pulmonary expansion. The costal wall stretches the pulmonary structure during its elastic retraction. Both activities have a common purpose: that of impeding the pulmonary periphery from ever being pressed against the thorax walls, or retracted this simultaneously guarantees an appropriate pleural diameter for the incessant succession of cyclic alveolo-capillary dynamics.
The pulmonary elastic structure and the thorax walls constitute a functional unit whose permanent unstable equilibrium, makes the dynamics of the alveolo-capillary micro-structure possible.
7. The pleural membrane, which forms the functional unit, integrating the two very different components: The thorax walls and the lungs, the first ascribed to the somatic system, and the second to the vegetative system.
Each pleural surface is also ascribed to the sector it lines maintaining absolute anatomical independence. The functional synthesis of the two pleural surfaces is only possible by means of a neutral elastic medium whose physical properties could be a guarantee of the permanent adhesion of the sectors of the anatomical complex. This medium is also responsible for both the smooth steady displacement of the two surfaces, and the alternating increase and decrease in the pleural diameter. This medium has to be perfectly elastic.
The author has stated that this medium behaves as gases, reflecting the consequences of the cyclic movements of the containing walls, which modify pleural capacity. This fact, among others, has enabled the author to differentiate two classes of cycles with detestable effects at the pulmonary surface, and consequently, to identify two anatomo-dynamic sectors responsible for these cyclic dynamics:
1. VENTILATORY ZONE. Comprises the airways up to their limits with the lobuli where each centre-lobular bronchiolus has its origin. This zone is directly responsible for ventilatory dynamics.
2. RESPIRATORY ZONE. Constituted by the lobular area, which is directly responsible for the small cycles during which gas exchange with the blood is achieved.
The respiratory zone develops a complex cycle whose components can be individualised:
1. LOBULAR CYCLES, which have a period similar to that of the cardiac cycle, and two phases of equal duration. The objective of the first phase is the delivery of the air-mass fraction to be interchanged, taking it from that delivered by the ventilatory zone.
2. ALVEOLO CAPILLARY CYCLE or second half of the lobular cycle whose sole objective is gases exchange with the blood.
This complexity of actions for a common purpose sometimes makes it difficult to refer to a "lobular cycle" or to an "alveolo-capillary cycle" with absolute accuracy. In view of this the term "Lobular cycle" is chosen to refer to the whole cycle.
The "respiratory cycle" has now been clarified as a complex comprehensive cyclic activity, which encompasses several lobular cycles in one ventilatory cycle. Each one of the lobular cycles also embraces one alveolo-capillary cycle.
This functional scheme allows us to elaborate a parallel dynamic conception, which refers to a ventilatory infra-structure whose specific role is to put the intra-pulmonary volume-mass of air into circulation, in order to renew a volume-mass of air similar to that exhausted during the previous ventilatory cycle. This determines a strong primary pulmonary retraction, and evokes the reflex diaphragm contraction while accumulating potential energy.
The aim of the ventilatory cycle is to guarantee steady dynamic conditions and supplies to the respiratory zone, which is now identified as a "supra-structure". The first role of this supra-structure, chronologically speaking, is to make way for capillary circulation, and to eject a mass of air to the alveoli, providing the dynamic conditions for gas exchange when both kinds of fluids converge simultaneously in the alveolo-capillary units. Other factors not studied here are concerned with this exchange.
It is worth noting that the first part of the lobular cycle bears a strong resemblance to the pulmonary ventilation, since the before mentioned air-mass fraction is also displaced for gas exchange; the differences being mainly topographical, and relative to the magnitude of the displaced mass of air and the force generated to accomplish it.
In relation to their topography, the lobular cycles take place at the pulmonary periphery. Concerning the dynamics, the intensity of their impulses and the time used to perform the cycle, constitute a fraction of the inspiratory impulse and period of a ventilatory cycle, their relation being similar to the quotient of their respective rhythms.
Only the alveolo-capillary phase of the lobular cycle, which constitutes a cycle within the other, has a specific-biological function: gas exchange with the blood.
The alveolo-capillary capacity needed for the reception of fluids being conveyed is represented by the expansion of its structures towards the potential space previously created in the pleural space for this purpose.
The duration of the alveolo-capillary cycle and the magnitude of its impulses are correlative to the period of blood injection by the heart, and to the strength of the impulse necessary to create the pressure-gradient needed for the diffusion of gases. The responsibility for the co-ordination of these events rests on the alveolo-capillary reflex.
The successful convergence of air and blood depends not only on the potential conditions of the atmosphere where life exists, i.e., the dynamics of the living organism is a dependent variable of the conditions of the atmosphere which supply the volume-mass of air to be inspired. In other words, the organism is a potential structure for the permanence of life. The organic structure is not self-sufficient, and is thus open to the atmospheric environment.
From the point of view of respiration, the organism makes use of both the inertial property of the air in the atmosphere to which the pulmothorax dynamic-structure is adapted; and the relative composition of the air mass.
This relates to the needs of biological phenomena.
The physic-natural, organo-physical and biological-dynamic phenomena are so closely correlated that the evolution of the species is implicit.
Also the proportional growth of functional structures is correlative to organic growth during the natural evolution of each individual.
Physical-Nature and Natural-Biology are aspects of a universal dynamic-balance, which is in permanent evolution.
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