output to ensure that blood flow to the muscle is matched to the metabolic needs". The relative and absolute increases in sympathetic nervous system activity during physical activity increase noradrenaline release to increase both heart rate and the amount of ionized calcium released in the sarcoplasmic reticulum. This increase in ionized calcium concentration principally drives the increase in myocardial contractile strength during physical activity, allowing the heart to pump more blood out to the body compared with resting conditions. Physical activity not only drives an increase in myocardial strength but also increases the amount of blood that fills the heart during diastole, namely an increase in the heart's preload. During activity, the respiratory system adapts, in part, by allowing the body to take deeper breaths. With each deep breath and subsequent expansion of the lungs and ribcage, the pressures within the right atrium and right ventricle of the heart decrease. Since the right atrium and right ventricle are anatomically located distal to the superior and inferior vena cava, the blood pressure within these venous structures increases relative to the right-side of the heart, and Bernoulli's Principle dictates an increase in blood flow from the vena cava to the heart, thus increasing the heart's preload. This increase in preload drives an even further increase in cardiac contractility, independent of ionized calcium-driven mechanisms, a phenomenon governed by the Frank-Starling Law. Both heart rate and stroke volume vary directly with the intensity of the exercise performed and many improvements can be made through continuous training. Another important issue is the regulation of blood flow during exercise. Blood flow must increase in order to provide the working muscle with more oxygenated blood which can be accomplished through neural and chemical regulation. This increase in blood flow depends on two factors: increasing the amount of blood pumped out of the heart and disproportionately distributing blood flow to tissues under physical demand. Blood vessels are under sympathetic tone; therefore, the release of noradrenaline and adrenaline will cause vasoconstriction of non-essential tissues such as the liver, intestines, and kidneys, and decrease neurotransmitter release to the active muscles promoting vasodilatation. Also, chemical factors such as a decrease in oxygen concentration and an increase in carbon dioxide or lactic acid concentration in the blood promote vasodilatation to increase blood flow. As a result of increased vascular resistance, blood pressure rises throughout exercise and stimulates baroreceptors in the carotid arteries