19.8b Events of the Cardiac Cycle

How many phases are there in the cardiac cycle?

5:

• Atrial relaxation and ventricular filling

• atrial contraction and ventricular filling

• isovolumic contraction

• ventricular ejection

• isovolumic relaxation.

(1) Atrial relaxation and ventricular filling

• The atria and ventricles are relaxed.

• Blood continues to return to the right atrium through the superior vena cava, inferior vena cava, and the coronary sinus, and to the left atrium through the pulmonary veins.

• Passive filling of the ventricles is under way. No contraction of the atria is needed at this time to assist with the continual ventricular filling.

• The AV valves are open because the pressure exerted by the blood filling the atria is greater than the pressure exerted by the blood in the resting ventricles.

• The semilunar valves are closed because the pressure exerted by the blood in the filling ventricles is lower than the pressure exerted by the blood in the arterial trunks.

(2) Atrial contraction and ventricular filling

This phase is distinguished from the phase of atrial relaxation and ventricular filling by two events: atrial contraction and completion of ventricular filling (phase 2). Atrial contraction is initiated by the SA node stimulating cardiac muscle cells of the atria. Contraction of the atria moves the remainder of blood that was within each atrium into the right or left ventricle.

Ventricular filling is complete upon termination of atrial contraction, and the ventricles now hold their maximum blood volume. This is appropriately called the end-diastolic volume (EDV) because it is the volume of blood within the ventricle at the end of diastole (or relaxation) of the ventricles. In a resting adult, the value of EDV is approximately 130 milliliters (mL) of blood.

Note that during atrial contraction, both the inflow of additional blood from the supplying veins into the atria (i.e., blood flow from the superior vena cava, inferior vena cava, and coronary sinus into the right atrium and blood flow from the pulmonary veins into the left atrium) and the backflow of blood from the atria into these veins are prevented because contraction of the atria compresses the openings for the great veins.

You will find it helpful to remember that at the end of this phase, the atria relax and will remain relaxed until the beginning of the next atrial contraction in the next cycle. The remaining phases (phases 3 through 5) involve the changes associated with contraction and relaxation of the ventricles.

(3) Isovolumic Contraction

Isovolumic (i'só-vol'yu'mik; iso = same) contraction' involves no change in ventricular blood volume when the ventricles are contracting (i.e., the same volume of blood remains within the ventricles during this phase). Two distinctive changes occur in this phase: ventricular contraction and closure of the AV valves (phase

3). Ventricular contraction is initiated by the subendocardial branches of the conduction system stimulating the cardiac muscle cells of the ventricles. As the ventricles begin to contract, the pressure within the ventricles increases and exceeds the pressure within the atria. Consequently, the AV valves are forced closed.

Recall that these valves are braced by tendinous cords to the papillary muscles (see L' section 19.3d). The closure of these valves prevents backflow of blood from the ventricles into the atria. The semilunar valves remain closed because the pressure within the ventricle is still less than the pressure within the attached arterial trunk.

Note that all heart valves are closed at this time. Thus, although the ventricular cardiac muscles are contracting during this phase, no blood is entering or leaving the ventricle and the blood volume within the ventricular chambers remains the same.

(4) Ventricular Ejection

Ventricular ejection is the movement of blood from the ventricles into the arterial trunks. During this phase, ventricular contraction continues and the semilunar valves are forced open (phase 4). As the ventricles continue to contract, the pressure within the ventricles increases and exceeds the pressure within the arterial trunk.

Consequently, the semilunar valves open, which allows blood to be ejected from the ventricles into their associated arterial trunk (i.e., blood is ejected from the right ventricle into the pulmonary trunk and blood is ejected from the left ventricle into the aorta). The AV valves remain closed because the pressure within the ventricles remains greater than the pressure within the atria. The amount of blood pumped out of a single ventricle (either the right or the left ventricle) during ventricular contraction is termed the stroke volume (SV). Generally, this volume is approximately 70 mL of blood.

Not all of the blood in either ventricle is ejected. The blood remaining in a ventricle at the end of systole is appropriately called the end-systolic volume (ESV) because it is the volume of blood in the ventricle at the end of systole (or contraction); ESV is labeled in phase 5. ESV is determined by subtracting SV from EDV (EDV - SV = ESV). In a typical individual at rest, these values are: 130 mL - 70

mL = 60 mL. Thus, 60 mL of blood typically remains within a

ventricle following ventricular contraction.

(5) Isovolumic Relaxation

As its name suggests, isovolumic relaxation involves no change in ventricular blood volume when the ventricles are relaxing. Two distinctive changes occur in this phase: ventricular relaxation and closure of the semilunar valves (phase 5). As the ventricles start to relax and the ventricular chamber expands back to its resting size, pressure within the ventricles decreases below the pressure within the attached arterial trunks. Blood initially flows backward slightly within the arterial trunks but is caught in the semilunar valves, causing them to close. This closure of the semilunar valves prevents blood backflow from the arterial trunks into the ventricles. The AV valves remain closed because the pressure within the ventricles is still greater than the pressure within the atria.

Note that all heart valves are again closed simultaneously. Thus, although the ventricular cardiac muscles are relaxing during this phase, no blood is entering or leaving the ventricle and the blood volume within the ventricular chambers remains the same.

As the ventricles continue to relax, the pressure within the ventricles decreases below the pressure within the atria, and the AV valves open. The opening of these valves allows blood to once again move from the atria into the ventricles for ventricular filling as the cardiac cycle (phase 1) begins again. Amazingly, these events of the cardiac cycle, as organized and described in these five phases, are repeated with each heartbeat!

Ventricular balance

Equal amounts of blood that are typically pumped by the 2 ventricles through the 2 circulations.

In ventricular balance, the right ventricle has to

pump the blood only to the adjacent lungs (a relatively short circuit)

In ventricular balance, the left ventricle has to

pump the blood through the systemic circulation throughout the body (a relatively long circuit)

The left ventricle must be ___ and ___ than the right ventricle to pump the blood farther but it is the same amount of blood pumped from both sides of the heart.

larger; stronger

Sustained pumping of unequal amounts of blood may result in

edema; excess fluid in the interstitial space or within cells.