The T wave occurs just before ventricular relaxation. Examination of an electrocardiogram can determine abnormalities, such as fibrillation, characterized by uncoordinated contraction. Epinephrine from the adrenal medulla is a cardiostimulator. Thyroxin from the thyroid gland causes a sustained increase in heart rate, leading to damage from hyperthyroidism. Blood pressure is the pressure of blood against the walls of blood vessels, measured with a sphygmomanometer.
Blood pressure drops with increasing distance from the left ventricle.
The velocity of blood flow is driven by the blood pressure in the arteries. The movement of blood in veins and venules is due to contraction of skeletal muscles. Valves prevent blood backflow. The pulmonary veins return oxygenated blood to the heart. The systemic circuit includes blood vessels that leave the left ventricle, distribute blood throughout the body, and return blood to the right atrium.