Vascular adaptation to blood flow and temperature in humans
Exercise training has been shown to decrease the risk of developing cardiovascular disease in humans. It is also known that exercise training induces both macro- and micro-vascular adaptations in humans. Recent papers have suggested the direct action of changes in blood flow, pressure and other haemodynamics during acute bouts of exercise may, in part, explain the cardiovascular benefit of exercise. However, exercise is a highly complex stimulus, involving reflex activation, neurohumoral secretion and elicitation of circulating factors from exercising muscles. The independent and direct role of haemodynamics is therefore difficult to ascertain. The theme of the chapters presented in this thesis was to examine the impact of chronic manipulation of arterial blood flow and shear stress, independent of an exercise stimulus per se. The thesis is divided into two sections. The first lays out the rationale for investigation of the effects of changes in blood flow and shear stress, independent of exercise, on conduit artery function and structure. Section 2 summarises cutaneous microvascular adaptations induced by repeated lower limb heating, in which the independent roles of blood flow and skin temperature on the adaptive responses are examined.
The studies within this thesis utilised exercise-independent modalities and reported beneficial vascular adaptations in both conduit arteries and cutaneous microvessels. These results therefore have implications for clinical and ageing populations where exercise training is contraindicated due to associated increases in physical stress on joints and the musculoskeletal system. Passive heating may provide these groups an alternate modality in which vascular health can be maintained or improved. These findings also have implications for thermoregulatory capacity. Increased capillarisation, resulting in prolongation of the transit time of blood through the cutaneous vasculature, would facilitate greater heat loss, thereby enhancing thermoregulatory efficiency. This, in turn, has potential implications for exercise performance, as it might delay the competition for blood flow between the skin and the working muscles.