exceeds the external, residual pressures in the adjacent trunks of both the pulmonary artery and the aorta; this stage, in turn, causes the pulmonary and aortic valves to open. blood is then ejected from the two ventricles, pulsing into both the pulmonic and aortic circulation systems. mechanical systole causes the pulse, which itself is readily palpated (felt) or seen at several points on the body, enabling universally adopted methods—by touch or by eye—for observing systolic blood pressure. the mechanical forces of systole cause rotation of the muscle mass around the long and short axes, a process that can be observed as a "wringing" of the ventricles. physiological mechanism systole of the heart is initiated by electrically excitable cells situated in the sinoatrial node. these cells are activated spontaneously by depolarization of the electrical potential across their cell membranes, which causes voltage-gated calcium channels on the cell membrane to open and allow calcium ions to pass through into the sarcoplasm (cytoplasm) of cardiac muscle cells. calcium ions bind to molecular receptors on the sarcoplasmic reticulum (see graphic), which causes a flux (flow) of calcium ions into the sarcoplasm. calcium ions bind to troponin c, causing a conformational (i.e., structural) change in the troponin-tropomyosin protein complex, causing the myosin head (binding) sites on f-actin filamentous proteins to be exposed, which causes muscle contraction to occur. the cardiac action potential spreads distally (or outwardly) to the small branches of the purkinje tree via the flux of cations through gap junctions that connect the sarcoplasms of adjacent myocytes. the electrical activity of ventricular systole is coordinated by the atrioventricular node, which is a discrete collection of cells that receives electrical stimulation from the left and right atria and can provide an intrinsic (albeit slower) heart pacemaker activity. the cardiac action potential is propagated down electrical pathways through the bundle of his to the purkinje fibres; this electrical flux causes coordinated depolarisation and excitation-contraction coupling from the apex of the heart up to the roots of the great vessels. clinical notation when blood pressure is stated for medical purposes, it is usually written with the systolic and diastolic pressures separated by a slash, for example, 120/80 mmhg. this clinical notation is not a mathematical figure for a fraction or ratio, nor a display of a numerator over a denominator. rather, it is a medical notation showing the two clinically significant pressures involved (systole followed by diastole). it is often shown followed by a third number, the value of the heart rate (in beats per minute), which typically is measured jointly with blood pressure readings.