The rhythmic contraction and relation of heart is called heart beat. Heart shows autorhythmicity because the impulse for its rhythmic movement develops inside the heart. During embryonic development some of the cardiac muscle fibres become autorthythmic (self-excitable) and generate impulses. The aurorhythmic fibres perform two important functions - they act as pace maker, setting the rhythm for heart and also form conducting system for conducting impulses throughout heart muscles. The components of conducting system are sinoatrial node (SA node), atrioventricular node (AV node), bundle of His, right and left bundle branches and Purkinje fibres. The cardiac excitation begins at SA node, hence called pace maker. The cardiac impulse spreads from the SA node throughout atria causing their contraction. It also stimulates the AV node through internodal pathways. The impulse travels from AV node of Bundle of His, right and left bundle branches and ultimately to Purkinje fibres located in the walls of ventricles causing their contraction.
Blood Pressure : Lateral pressure of force exerted by blood on the wall of arteries is called arterial blood pressure.
Blood pressure is measured with an instrument called sphygmomanometer (sphygmos = pulse, manometer = device to measure pressure). Blood pressure is measured in millimeters of mercury (mm Hg). Blood pressure is of two types - systolic blood pressure and diastolic blood pressure.
Systolic blood pressure : It is the pressure of blood during ventricular systole. It is maximum and responsible for flow of blood in the arteries. Normal systolic pressure is 120 mm Hg.
Diastolic blood pressure : It is the pressure of blood during ventricular diastole. It is minimum. Normal diastolic pressure in 80 mm Hg.
Thus the normal blood pressure is 120/80 mm Hg.