The period that begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle. The period of contraction that the heart undergoes while it pumps blood into circulation is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.

Pressures and Flow

Fluids are materials that flow according to pressure gradients, moving from high pressure areas to low pressure areas. Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the veins, which are higher in pressure. As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles. When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles. During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle.

Phases of the Cardiac Cycle

The cardiac cycle which begins when both the atria and ventricles are relaxed (diastole) can be broken up into four phases – filling, isovolumic contraction, ejection, and filling. Several variables change during each of these phases. We will discuss the changes to ventricular volume, ventricular pressure, the opening and closing of the atrioventricular and semi-lunar valves, the presence/absence of heart sounds, and the electrical activity of the heart as measured by an electrocardiogram (ECG) (Figure 36.1).

Filling

Blood constantly flows from the right atrium from the superior and inferior venae cavae and the coronary sinus. Blood flows into the left atrium from the four pulmonary veins. During this phase, atrial pressure exceeds ventricular pressure which allows the  two atrioventricular valves, the tricuspid and mitral valves, to open. Consequently, blood passively flows unimpeded from the atria and into the ventricles. Approximately 70–80 percent of ventricular filling occurs this way. The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left. During filling ventricular pressure increases, while atrial pressure decreases.

Towards the end of filling, atrial contraction (atrial systole) occurs starting with the depolarization of the atria, represented by the P wave of the ECG, followed by contraction. This contraction increases atrial pressure, which pumps more blood into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during atrial diastole. Atrial contraction contributes the remaining 20–30 percent of filling (Figure X.X.X). The volume of blood at the end of filling, which is the end of ventricular diastole, is referred to as the end diastolic volume (EDV) or preload. EDV is typically ~130 ml blood in a resting adult who is standing. Filling ultimately results in an increased ventricular volume and pressure, while atrial volume and pressure decrease. Consequently, the atrioventricular valves slam shut, resulting in the audible first heart sound (S₁), which is referred to as “lub.”.

Isovolumic contraction

Ventricular systole starts with the closing of the atrioventricular valves, followed by the ventricular depolarization, represented by the QRS complex in the ECG. This depolarization results in ventricular contraction, which can be divided into two phases, the first of which is isovolumic contraction. Initially, as the ventricles contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart. Consequently, ventricular volume remains constant, which is why this phase is called isovolumic contraction – the ventricles are contracting, but volume is unchanged.  Additionally, this increased ventricular pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves. Ultimately ventricular pressure increases until is greater than aortic pressure, which results in the opening of the semi-lunar valves, which signifies the end of this phase.

Ejection

In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has increased ventricular pressure so that it exceeds pulmonary artery and aortic pressures. This allows blood to be pumped from the heart, pushing open the pulmonary and aortic semilunar valves. Ventricular pressure increases during the first half of ejection and decreases during the second half of ejection. Although blood is constantly leaving the ventricles during ejection, pressure continues to increase during the first half of ejection due to the force of the contraction. However, during the second half of ejection, despite ongoing ventricular contraction, pressure decreases due to blood exiting the ventricles. Ultimately this causes ventricular pressure to fall below aortic pressure, which allows the semi-lunar valves to audibly close creating the 2nd heart sound (S₂) “dub”. The closing of the semi-lunar valves traps ~50-60 ml of blood in the ventricles, which is referred to as end systolic volume (ESV).

Isovolumic relaxation

Similar to ventricular systole, ventricular diastole can be divided into two phases (isovolumic relaxation and filling) which last ~430 ms. Isovolumic relaxation follows repolarization of the ventricles and is represented by the T wave of the ECG.  During isovolumic relaxation, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall. When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in blood pressure tracings. The semilunar valves close to prevent backflow into the ventricles. This phase is called isovolumic relaxation since the atrioventricular valves remain closed at this point, resulting in no change in ventricular volume. This phase continues until the ventricular muscle has relaxed so much that ventricular pressure drops below atrial pressure. When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves When these valves open, the cardiac cycle is complete and the next cycle begins with the filling phase.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 19