Structure and adaptation of arteries to pulsatile flow: The case of the ascending aorta

Abstract

Purpose: The objectives are: (i) assess the development of the impedance of some arteries during the first decades of life; (ii) determine the influence of pulse rate in arterial impedance; (iii) compare the structure of some arterial segments with optimized structures with respect to blood flow; and (iv) explain the elongation of the ascending aorta throughout life in healthy subjects. Methods: A model of the arterial network previously developed by the authors, together with data of lengths, diameters, and distensibilities of arterial segments reported in the literature were used. The impedances of the aorta and carotid artery were calculated based on that model. Similarly, the impedances of various arteries corresponding to heart rates of 65 bpm and 120 bpm were calculated. Values observed in arterial segments were compared with the respective optimal values from the viewpoint of hemodynamic performance. This allowed drawing conclusions on the arterial segments that might be critical with regard to hemodynamics. Results: It was found that in healthy people impedances of the aorta and the carotid artery decrease markedly with age especially during body growth. It was also found that impedances of the main arteries do not significantly change with heart rate, even if sharp changes in arterial distensibility are observed. With respect to optimal flow performance, it was found that scaling between diameters of branching arteries is generally close to optimality, while the corresponding length scaling is far from optimality. It was also found that the ascending aorta and aortic arch are among those arterial segments whose lengths are much smaller than the optimum values. An explanation is offered for the age associated elongation of the aorta in healthy people. Conclusions: In healthy subjects, the human arterial system continues to optimize its performance at least until the age of 60.

Heart rate, arterial distensibility, and optimal performance of the arterial tree

“…The same effect has been observed in rats (Mircoli et al, 1999). Though, according to the model above referred (Silva and Reis, 2014) increased heart rate leads to lower arterial impedance, the observed increased arterial stiffness with heart rate actually increases impedance. Apparently, this behavior does not make sense because easing blood flow is sought to be the objective of the circulatory system.…”

“…3). In an artery of diameter D and length L, (aspect ratio x ¼L/D), with pulsatile flow of frequency ω, under pressure difference ΔP the average current I reads (see Silva and Reis, 2014) …”

“…On the other hand, a recent model of pulsatile flow predicts that if distensibility is kept constant, arterial impedance must decrease with pulse frequency (Silva and Reis, 2014). Actually, if pulse frequency (heart rate) and therefore the blood current increases in response to needs of organs in the body, then it makes sense that arterial impedance is reduced to ease the access of blood.…”

“…Actually, if pulse frequency (heart rate) and therefore the blood current increases in response to needs of organs in the body, then it makes sense that arterial impedance is reduced to ease the access of blood. On the other hand, if arterial impedance decreases with distensibility (Silva and Reis, 2014) one would expect that increased blood current would lead to decreased arterial impedance with heart rate.…”

Scaling relations of branching pulsatile flows

“…In total 22 arteries were included in the calculation of J. Based on the present model, a recently published study of the arterial structure [25] also showed that, in general arterial lengths are not optimized with respect to hemodynamic performance, and then an explanation was offered for the elongation of the ascending aorta in healthy people during lifetime. The same study also showed that impedance of the ascending aorta, descending aorta and carotid artery decreases during body growth, therefore suggesting a trend for improvement of hemodynamic performance during that period of life.…”