The Circulatory System
 

Transportation Of BloodThe Cardiac Muscle  Impluses  Transportation In Blood  Plasma  Red Blood Cell
White Blood Cell  Platelets Blood Vessles  Arteries Capillaries  Vein Distribution Of Blood
 

The Cardiac Muscle

The alternating contraction and relaxation of the heart chamber is called the cardiac cycle. The two atria contract in synchrony,emptying their contents into the ventricle. A fraction of a second later, the two ventricles contract simultaneously, forcing blood into the arteries leaving the heart. Both chambers then relax briefly before the cycle is repeated. The period of ventricular contraction is called systole. The rest of the cycle, including relaxation of all the chambers followed by contraction of the atria, is called diastole. At normal resting heart rate, systole lasts about 0.3 second and diastole about 0.5 second. When blood pressure is measured, the higher of the two readings is the systolic pressure, which is measure during ventricular contraction, and the lower reading is the diastolic pressure, measured between contraction.