Normal Heart Physiology
On returning to the heart blood collects in the atria. At the end of systole (ventricular contraction) the pressure in the ventricles falls below that in the atria allowing the tricuspid and mitral valves to open and blood to flow from the atria to ventricles. Towards the end of this filling phase (called diastole), the atria contract expelling the last remnants of the blood in them to the ventricles. This sequential contraction (atria first then ventricles) is controlled by the cardiac pacemaker, called the sinus node, which is situated high up in the right atrium. It initiates an electrical impulse which triggers atrial contraction; the impulse then traverses to the ventricles through the atrio-ventricular node where it is delayed for a short time, before flowing down the bundles of His to the ventricular myocardium initiating contraction of the ventricles from the bottom upwards. Normal cardiac rhythm is called sinus rhythm.
The left ventricle is the main pumping chamber providing sufficient contractile force to expel blood round the body through the arteries. This is called the systemic circulation and is characterised by a mean blood pressure of more than 100mmHg. The blood pressure (BP) is usually measured by auscultation with a stethoscope and recorded as systolic/ diastolic rather than mean. A blood pressure range of 85-150mmHg systolic and 60-90 mmHg diastolic would be expected in a normal person. Low BP or hypotension is commonly defined as a systolic pressure of less than 100mmHg; high blood pressure used to be defined as greater than 150/90 but now is considered to be anything above 130/80 in certain disease states like diabetes.
The right ventricle is the pumping chamber which delivers blood to the lungs through the pulmonary artery and its tributaries, and is called the pulmonary circulation. This is a low pressure system characterised by a pressure of 15-28/5-14mmHg on average.