At S10, the heart begins to beat (detectable at E23 by means of transvaginal ultrasound). The impulse starts at the sinus venosus and continues towards the outflow tract. Blood movement is initially an ebb-and-flow. Circulation starts from S12 onward. The heart rate starts at around 70 beats per minute and reaches 160 to 180 beats per minute at the end of the embryonic period. Blood flow is at first serial, through all portions of the heart, from the venous to the arterial end (animation: development of the endocardial tube).