Overview of the normal structure and function of the macrocirculation and microcirculation

Abstract

The function of the left ventricle (LV) is to pump oxygen- and nutrient-rich blood through the large arteries (macrocirculation) of the systemic arterial tree to the arterioles and capillaries (microcirculation) to meet the metabolic requirements of surrounding tissues. The walls of arteries are composed of living cells, including smooth muscles cells (SMC), stiff collagen fibers, and compliant elastic fibers. The proximal aorta has large amounts of elastic fibers with fewer collagen fibers, while the lower descending aorta has fewer elastic fibers and more collagen fibers; muscular arteries have an abundance of SMC. The large elastic arteries serve as a buffering reservoir or Windkessel that damps pulsations and stores blood during systole and expels it to the peripheral circulation during diastole so that the capillaries receive a steady or continuous flow of blood during the entire cardiac cycle. The arterioles, by changing their caliber, alter peripheral resistance and aid in the maintenance of mean arterial blood pressure. If the LV is ideally matched to its afterload, ventricular/vascular coupling and cardiac efficiency are optimal and myocardial oxygen consumption is minimal. The amount of energy and thus the oxygen used by the LV to produce the cardiac output are dependent upon the physical properties of the arterial system. With increasing age some elastic fibers fragment and become dysfunctional, while the number of collagen fibers increases. This process increases elastic artery stiffness and causes increased wave reflections which increases pulsatility and elevates LV afterload and myocardial oxygen demand.