Physiological responses to intense aerobic exercise generally increase blood flow, and therefore oxygen delivery, to active muscles. In muscles, sympathetic nerve impulses stimulate vasodilation, opening more capillaries. At the same time, vasoconstriction diminishes blood flow where it is not immediately needed, such as to the digestive tract. Blood flow, however, is maintained in the brain and kidneys, which need a steady stream of oxygen and nutrients to function. Respiratory rate rises, increasing the rate of venous return to the heart. Heart rate increased in response to decreased parasympathetic stimulation of the S-A and A-V nodes and to sympathetic reflexes triggered by stimulation of proprioceptors in the skeletal muscles and of stretch receptors in the lungs. As the volume of blood returning to the heart increases, ventricular walls stretch, stimulating them to contract with great force.