Conclusion - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 799

distances. The vasculature is composed of curved blood vessels with successive
branchings to irrigate the body's tissues.
Adaptation of blood flow to the body's activity and tissue needs also relies
on the central nervous system. In addition, local mechanisms regulate blood
flows, using three main mechanisms from large arteries to small arterioles: (1)
mechanotransduction-driven postload control; (2) autoregulation; and (3) functional
(reactive) hyperemia.
Both air and blood streams are three-dimensional and developing, as they
are conveyed in entrance length (Vol. 7 - Chaps. 2. Hemodynamics and 3. Air
Transport). Moreover, airways and blood vessels are deformable. Changes in
transmural pressure (the pressure difference between the pressure at the wetted
surface of the lumen applied by the moving fluid on the pipe wall and the pressure at
the external wall side that depends on the activity of neighboring tissues and organs)
can also affect the shape of the vessel cross-section, especially when it becomes
negative. More generally, the change in cross-section shape can result from taper,
possible prints of adjacent organs with more or less progressive constriction and
enlargment, and adaptation to branching (transition zone). These changes can induce
local three-dimensional blood motions displayed by virtual transverse currents, even
if the vessel is considered locally straight.
The magnitude and direction of mechanical stresses applied by air and blood
flows on the wetted surface of respiratory epithelia and vascular endothelia as well
as within the conduit wall vary during the ventilatory and cardiac cycles. Inside
conduit lumens, local changes in the direction of stress components are caused by
flow separations and reversal. Flow separation is set by an adverse pressure gradient
when inertia forces and blood vorticity are high enough, especially in branching
vascular segments. Due to the time-dependent feature of flows, the flow separation
region spreads over a variable length during the physiological cycle and can move.
The location and variable size of the flow separation region depends on the flow
distribution between the branches, which can vary during the physiological cycle.
Flow reversal occurs during the diastole of the left ventricle in elastic arteries, such
as the aorta, and most of the muscular arteries, such as brachial and femoral arteries
(but not in the carotid arteries). Flow reversal can be observed either in a region near
the wall, more or less wide with respect to the position of the local center of vessel
curvature, or in the entire lumen.
During normal tidal breathing, neurohumoral variations are slight. Breathing
influences blood circulation, especially the venous return, by changes in intratho-
racic pressure. Because lungs constantly tend to shrink away from the costal pleura,
the pressure in the pleural cavity is smaller than that of the atmosphere (negative
pressure). Lungs also exert a traction on the pericardium, on arteries in which
the blood pressure is high, especially in those of the systemic circulation, and on
venae cavae in which the blood pressure is low, thereby deforming intrathoracic
veins much more than the intrathoracic arteries. During inspiration, the intratho-
racic pressure lowers, thereby reducing right and left auricular preload and right