Book excerpt: Biomechanical forces play a major role in organ development, shape and function. When exceeding the physiological range, however, they may become detrimental for organ structure and function. This is probably best exemplified by the cardiovascular system, with both the heart and blood vessels being continuously exposed to the biomechanical forces exerted by the flow of blood. In the heart, it is the build-up of pressure inside the ventricles that allows the ejection of blood into the pulmonary and systemic circulation. The luminal diameter of the small arteries in both parts of the circulation determines the resistance to flow. Hence it also determines the level of blood pressure in both the pulmonary and systemic circulation and thus the afterload for both ventricles of the heart. A narrowing of the small arteries (e.g. due to an increase in tone) therefore leads to an increase in blood pressure in the affected part of the circulation. This will decrease organ perfusion but increase the afterload for the corresponding ventricle of the heart. Consequently, the affected ventricle must build up more pressure to maintain cardiac output. However, if the rise in blood pressure (pulmonary or arterial hypertension) persists the increase in wall tension can no longer be compensated by active constriction, thereby forcing the ventricle to resort to other means to unload itself. Typically, this is achieved by structural alterations in its wall which becomes thicker (hypertrophy) and stiffer (remodelling of the extracellular matrix). Ultimately, this maladaptive response may lead to dysfunction and eventually failure of the ventricle, which would only be able to eject a significantly smaller amount of blood into circulation. The increase in wall tension has resulted in an increased stretching of the cardiomyocytes as well as non-cardiomyocytes, such as cardiac fibroblasts, which in turn alters both their phenotype and their environment. Research into the mechanobiology of the heart aims to unravel the molecular and cellular mechanisms underlying the physiological response of the heart to load to learn what goes wrong when the heart is faced with sustained pressure overload. This may pave the way to therapeutically interfering with this maladaptive response and thus preventing either the initial hypertrophy or its transition into heart failure. While the heart is mainly subjected to pressure hence stretch as a biomechanical force, the mechanobiology of vascular cells is somewhat more complex. Endothelial cells lining the luminal surface of each blood vessel are continuously subjected to the viscous drag of flowing blood (referred to as fluid shear stress). Fluid shear stress mainly affects the endothelial cells of the small arteries and arterioles, maintaining them in a dormant phenotype. If blood flow is disturbed (e.g. at arterial bifurcations or curvatures) fluid shear stress declines and may give rise to a shift in phenotype of the endothelial cells. A shift from anti-inflammatory to pro-inflammatory in combination with the reduced flow at these sites may enable leukocyte recruitment and diapedesis, which results in a pro-inflammatory response in the vessel wall. Endothelial cells and in particular vascular smooth muscle cells are subjected to another biomechanical force: the blood pressure. Volume-dependent distention of the vessel wall (which can be achieved through an increase in blood flow) results in an increase in wall tension, thereby stretching of the endothelial and smooth muscle cells. Like the cardiomyocytes of the heart, the vascular smooth muscle cells of the small arteries and arterioles try to normalise wall tension by active constriction, which cannot be maintained for long. These cells subsequently undergo hypertrophy or hyperplasia (depending on the size of the blood vessel) and remodel the extracellular matrix so that the vessel wall also becomes thicker and stiffer. This in turn raises their resistance to flow and may contribute to the increase in blood pressure in either the pulmonary or systemic circulation. Research into the mechanobiology of the blood vessels aims to unravel the molecular and cellular mechanisms underlying the physiological response of the vascular cells to pressure (wall tension) and flow (shear stress). It also aims to uncover what goes wrong (e.g. in arteriosclerosis or hypertension) and to eventually specifically interfere with these maladaptive remodelling processes. The aforementioned aspects of cardiovascular mechanobiology along with many more facets of this fascinating, timely and highly clinically relevant field of research are addressed by the original research and review articles within this Research Topic.