FUNCTIONAL DESIGN AND STRUCTURE OF THE TRUNK AS AN INTEGRATED VISCERO-SOMATIC MECHANICAL UNIT
I. Lung and extra-pulmonary airways. Heart and Blood vessels all constitute a visceral integrated mechanical unit for fluid circulation, which behaves as an Autonomous Mechanical System of Fluids, perfectly controlled by the Central Nervous System (CNS)
II. Diaphragm, Costal and Abdominal walls, each one with its own specific and integrated roles constitute a thoraco-abdominal dynamic belt to complement visceral dynamics for a Viscero-Somatic Integrated System of Fluid Mechanics.
The Diaphragm is a striated flat muscle, adhered to the base of the Lung, to which responds via viscero-somatic reflex. Therefore, it becomes the Lung's most important auxiliary muscle, maintaining its structure at different degrees of tension, relative to its elastic properties and expansive forces of the contained mass of air.
The Diaphragm comprises two portions with different partial functions:
The central Diaphragm, innervated by the Phrenic, and the peripheral portion, innervated by the five or six last pairs of intercostals. This topographical innervation announces the role of the Diaphragm in recruiting somatic structures with common innervation, via somato-somatic reflex, thus constituting a contracting-relaxing belt that responds to pulmonary dynamics, as I have proved else were.
Costal walls also have two functional zones: the upper one, down to the six pair of ribs, which has none or scarcely any cyclic mobility, while the inferior zone, formed by the seventh pair of ribs and those placed below, exhibit wide cyclic displacements.
The same pairs of intercostals that innervate the peripheral part of the Diaphragm, and antero-lateral muscles of the abdomen, also innervate intercostal muscles of this inferior part of the costal walls, thus announcing their dynamic integration for successive and joint reflex contraction.
Contraction of these intercostal muscles sum up their forces for costal displacement, also elevating the inferior part of the Sternum, thus widening the diameters of the inferior part of the Thorax.
Contraction of the abdominal muscles diminishes the abdomen's capacity, pressing the abdominal blood towards the thorax.
It is very important to clarify here the moment and reasons for this cyclic widening of the Thorax; in contrast with traditional beliefs according to which, this is performed to expand the Lung, thus enabling its filling with air.
The real reason is quite different: the costal expansion displaces with it, the adhered pulmonary lobes of each side. Therefore widening the inferior mediastinum; better said, the space around the Heart and great vessels.
This fact takes place at the end of the Phrenic Diaphragms contraction, leaving free the Lung for its cyclic elastic retraction. In this very moment the Lung pulls the Diaphragm, removing it from the Abdomen's potential capacity, thus leaving free the space formerly invaded by its contraction, also inverting the effect of abdominal press. Therefore, a greater volume of blood is now arriving to the Heart, coming from the abdominal vessels, as related above.
The formerly said widening of the space around the Heart is carried out precisely to leave it enough free space for an increased expansion in order to receive that increased volume of blood. Without this expansion, the arriving increased volume of blood would be arrested, filling up the abdominal and pelvic vessels ad initio.
III. THE PLEURAS: The parietal leather of the pleura lines the thorax walls, while the visceral leather lines the surface of the pulmonary lobes. As visceral and parietal leathers of the pleura are smooth, wet and polished surfaces in intimate contact, they adhere in accordance with the Laws of Surface Tension. This pleural adhesion is of the utmost importance, since it maintains the elastic structure of the pulmonary lobes and extra-pulmonary airways distended as a unit; sine qua non condition for their uniform cyclic function. In absence of this adhesion the Lung would detach and retract itself, leading to its collapse.
The three analysed parts perform similarly important contributions to enable physiologically balanced Lung and Heart functions as well as a balanced blood circulation. In a concept: Life in Health
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