An important determinant of blood flow in the cardiovascular system is the principle that fluid flows from a region of higher pressure to one of lower pressure. The pressures responsible for blood flow in the normal circulation are generated during systole and diastole. This is Cardiac Hemodynamics.
Starting with systole, the pressure inside the ventricles quickly rises, forcing the atrioventricular valves to close. As a result, blood stops to flow from the atria into the ventricles and regurgitation of blood into the atria is prevented. The swift rise of pressure inside the right and left ventricles forces the pulmonic and aortic valves to open, forcing the entry of blood into the pulmonary artery and aorta. The exit of blood is at first rapid; but as the pressure in each ventricle and its corresponding artery equalizes, the blood flow progressively decreases. At the end of systole, pressure within the right and left ventricles decreases; this leads to the lowering of the pulmonary artery and aortic pressures, hence, causing the closure of the semilunar valves. These represent the start of diastole.
During diastole, the ventricles are relaxed and the atrioventricular valves are open, blood coming from the veins gushes into the atria and then into the ventricles. Toward the end of diastole, the atrial muscles contract in response to the electrical impulse initiated by the SA node. The muscle contraction increases the pressure inside the atria, ejecting blood into the ventricles. Hence, ventricular systole begins.
The pressure generated during ventricular systole (15 to 25 mmHg) exceeds the pulmonary artery diastolic pressure (8 to 15 mmHg) in the right side of the heart; ejecting blood into the pulmonary circulation. During diastole, venous blood flows into the atrium due to the pressure in the superior and inferior vena cava (8 to 10 mmHg); which is higher than that in the atrium. Blood flows through the open tricuspid valve, into the right ventricle thereafter until the two right chamber pressures equalize (8 to 10 mmHg).
In the left side of the heart, although higher pressures are generated, what happens is the same. Pressure generated by left ventricular systole (110 to 130 mmHg) surpasses the resting aortic pressure (80 mmHg), ejecting blood to the aorta. During left ventricular ejection, the ensuing aortic pressure (110 to 130 mmHg) forces blood progressively through the arteries. Forward blood flow into the aorta stops as the ventricle relaxes and pressure drops. Oxygenated blood returning from the pulmonary circulation via the four pulmonary veins flows into the atrium throughout diastole, where the pressure remains low. Blood flows into the left ventricle since ventricular pressure is low. At the end of diastole, pressure in the atrium and ventricles equilibrates (4 to 12 mmHg).
Chamber pressures are measured using special monitoring catheters and equipments. Hemodynamic monitoring aids with the diagnosis and management of pathophysiologic conditions that affect patients.