**2. Fundamentals of cardiac physiology**

Understanding of counterpulsation, requires a knowledge of relevant cardiovascular physiology. The heart works as a series circuit of pumps, the left system and the right system.

The right atrium (RA) receives blood from the inferior and superior vena cava and coronary sinus, most of which flows passively into the right ventricle (RV) through the tricuspid valve. An additional 20% of ventricular filling occurs through the atrial kick corresponding to the "p" wave of the Electrocardiogram (ECG). *(Quaal 1993)* 

The right system feeds the low pressure pulmonary vasculature which offers little resistance to the blood ejected from the right ventricle (RV). Consequently the right ventricular musculature is one third the girth of the Left Ventricle. The left atrium receives blood from four pulmonary veins and passively empties the blood into the left ventricle through the mitral valve. Since there are no valves in the pulmonary veins, elevation of left atrial pressure results in an increase in pulmonary vascular resistance (PVR).

Generation of high pressure is required by the left ventricle, most of the pressure being generated occurs during isovolumetric contraction (Fig2-phases of contraction-electrical and mechanical) in order to open the aortic valve, and overcome SVR and aortic end diastolic pressure (AEDP), which is a function of the systemic vascular resistance.

The intraventricular septum also contributes to the left ventricular ejection along with the thick circular posterior and lateral walls.

Some of the energy imparted to the blood through ventricular ejection is stored in the proximal aorta and large arteries as potential energy during their peak expansion. This is known as the Windkessel effect. In diastole, this energy is transformed into kinetic energy by the aorta and large arteries causing a recoil, which maintains a pressure head in the aorta. This in turn maintains a runoff during diastole into the peripheral arteries. It's important to note that coronary arteries fill during diastole.

Fig. 2. Phases of electrical and mechanical contraction
