Blood Pressure and the Mechanical Behavior of the Arterial Wall

The elastic thoracic aorta takes origin from the left ventricle and almost immediately curves, in a three- dimensional way, dividing into musculo-elastic and muscular branches to the heart, head, and upper and lower limbs (macrocirculation). Beyond the early branches, the total cross-sectional area of the arterial tree begins to expand markedly. Whereas total cross-section increases, the average diameter is reduced, reflecting the increased number of bifurcations toward arterioles and capillary network [1, 2] (fig. 1). The microcirculation begins when the arteriolar diameter is <150 ^m (fig. 1). Along the arterial and arteriolar tree, the forces governing flow are exclusively interested in the pressure generated by the heart. This quantity, which is the difference between the actual pressure and its hydrostatic component, is commonly referred to as 'blood pressure' (BP) [1]. It is the gradient of excess pressure which drives the flow. The distribution of this excess pressure through the circulation, which is largely dissipated in forcing the blood through the microcirculation, is at the origin of the so-called 'vascular resistance'. The behavior of BP along the arterial tree and its consequences on the arterial wall is the first objective of this chapter, taking into account that the heart is an intermittent and not a steady pump.

Cardiac Contraction and Aortic Consequences of Intermittent Ventricular

Ejection [1-3]

There are three main consequences on large vessels of the intermittent ventricular ejection of the heart: (1) as a result of the systolic contraction of the left ventricle, coronary blood flow is interrupted and the coronary arteries are perfused only during diastole [1-3]; (2) the thoracic aorta during systole acts

Proximal arterial compartment

Distal arterial compartment

Arterioles Capillaries

Diameter: Endothelium: Medial layer: Function:

Elastic arteries

Conduit; compliance

Muscular arteries

Arterioles

Capillaries

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