Either of two physiological principles first described by the British physiologist Ernest H. Starling (1866–1927). Starling’s law of the heart states that the more the muscle fibres of the heart are stretched at the end of diastole, the stronger their contraction during the following systole part of the cardiac cycle. This principle generally operates under normal conditions and ensures that the amount of blood pumped out of the heart (cardiac output) automatically adjusts to the amount of venous blood returning to the heart. This relationship produces an S-shaped curve (Starling’s curve) when stroke volume is plotted against ventricular pressure at the end of diastole. Starling’s law of the capillaries describes the movement of water and small solutes between the capillaries and the surrounding intercellular space, as well as how blood volume is maintained in capillaries. At the arterial end of the capillaries, blood pressure is relatively high, and fluid is forced out through the capillary walls by hydrostatic pressure. Towards the venous end, blood pressure falls, and much of the fluid returns to the capillaries due to the inwardly directed osmotic pressure created by the large proteins retained in the capillaries. There is roughly a net 10% loss of fluid from the capillary bed, but this eventually returns to the venous blood via the lymphatic vessels.