**2. Biochemical biomarkers of vascular (endothelial) function**

Over the last three decades, a number of methodological approaches were developed in order to evaluate and measure (patho)physiological function of the endothelium in humans [1, 2]. Evidently, these new methods intensified research and brought novelties in the field of vascular physiology and pathophysiology, but still are not implemented as clinical tools in daily practice. The approaches for endothelial function assessment were designed to provide insight into vascular/endothelial function in different sites (vascular beds) and different blood vessel types (conductive, resistant, and microcirculation). Earlier methods were more invasive (e.g., intracoronary infusion of acetylcholine (ACh), and later developed techniques that were less invasive have focused on peripheral circulation (forearm circulation) as a surrogate for coronary arteries [3–5]. As expected, all of these methods have their advantages and accepted limitations, and neither of the developed methods does present the absolute standard for the evaluation of endothelial function, in both macro- and microcirculation.

There is an extensive body of evidence reporting that generalized endothelial dysfunction exhibited virtually in every arterial bed presents an early manifestation of a variety of cardiovascular diseases (CVDs) [6, 7]. Still, when investigating endothelial function in different CVDs, diverse (patho)physiological role of large conductance vessels and small microvasculature should be considered.

There are many various molecules which have been denoted as vascular or endothelial markers, e.g., lipids, cytokines, ADMA, hsCRP, MPO, CAMs, markers of coagulability, markers of oxidative stress, chemokines, microparticles, and endothelial progenitor cells. It has been demonstrated that reduced bioavailability of nitric oxide (NO) plays a central role in impaired vascular/endothelial response (endothelial dysfunction) in conduit arteries, while NO in the microcirculation primarily modulates tissue metabolism [8]. On the other hand, a number of studies indicate that endothelium-derived hyperpolarizing factor (EDHF) plays a major role in vasodilation in skin microcirculation [9], whereas the results are still conflicting concerning the implication of prostaglandins [10–12]. A study on coronary endothelial function in young smokers reported that they had epicardial coronary endothelial dysfunction but preserved microvascular endothelial function [13].
