BIOC34H3 Lecture Notes - Lecture 4: Heart Valve, Pulmonary Valve, Aortic Valve
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Lecture 4: The Cardiac Cycle, Heart Pressures and Cardiac Output Heart Rate Regulation
1a. The Cardiac Cycle
The cardiac cycle is simply the cycle of changes in volume and pressure, as well as the opening
and closing of the AV and semilunar valves, that occur during a single contraction and relaxation
of the heart. It is easiest to conceptualize this cycle if we begin during the late period of relaxation,
or late diastole. At this point, the ventricles are filling with blood passively, with blood flowing
from the vena cava, through the right atria and into the right ventricle. Blood also flows from the
lungs, through the pulmonary veins, into the left atria and then into the left ventricle. In the final
phase of late diastole, the atria contract, pumping a little more blood into the ventricles.
The heart then enters its systolic (or contraction) phases of which there are two parts: the first
phase, the isovolumetric contraction phase, involves the heart contracting but, given that all of
the valves are closed, no blood actually leaves the heart at this point. In the second part of systole,
the pressure differentials between the ventricles and the aorta/pulmonary artery become such that
the valves open (the aortic valve on the left; the pulmonary valve on the right), and blood is then
pumped (ventricular ejection) throughout the systemic circulation (on the left) or the pulmonary
circulation (on the right). We then have an isovolumetric relaxation phase, when the muscle
tension in the heart begins to dissipate, whilst the volume stays the same (due to the closing of all
of the valves). Finally, we return to the initial stage (mid to late diastole) - the pressure differential
between the atria and the ventricles becomes such that the AV valve opens, and blood begins to
flow back into the ventricles, setting the scene for the cycle to begin again.
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1b. Stages of the Cardiac Cycle
LV, left ventricle: RV, right ventricle; LA, left atria; RA, right atria
Late Diastole
We consider late diastole to be the first phase of the cardiac cycle. This stage includes both
ventricular filling and atrial contraction. During this stage the AV valves (tricuspid and bicuspid)
are open (so blood can flow from the atria into the ventricles) and the semilunar valves (aortic;
pulmonary) are closed (so there is no blood flow from the LV into the aorta or from the RV into the
pulmonary artery). During late diastole, aortic pressure falls at a constant rate. This will be
discussed further later in the lecture. Atrial pressure and ventricular pressure remain constant
during the first half of late diastole which corresponds to ventricular filling. In the second half of
late diastole, atrial pressure and ventricular pressure increase slightly; this is due to atrial
contraction. Ventricular volume increases due both to passive filling and then atrial contraction.
During late diastole, atrial pressure is greater than ventricular pressure. This keeps the AV valves
open. At the same time, aortic pressure is greater than left ventricular pressure. This keeps the
aortic (semilunar) valve closed.
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Isovolumetric Contraction
The second phase of the cardiac cycle is isovolumetric contraction. Isovolumetric means
constant volume so during this phase the ventricles are contracting but the volume of blood within
them remains constant. In order for this to occur, all valves must be closed.
This phase begins when the ventricles start contracting and left ventricular pressure rises above left
atrial pressure. This pressure difference causes the AV valves to close. Note that aortic pressure is
still greater than left ventricular pressure so the aortic (semilunar) valve remains closed.
Pressure in the ventricle increases substantially during this phase but ventricular volume, for all
intents and purposes, remains constant. There is no significant change in atrial pressure.
Ventricular Ejection (Systole)
The third phase is ventricular ejection. This is the phase when blood is pumped from the LV into
the aorta (and from the RV into the pulmonary artery on the right side of the heart). This phase
begins when the pressure in the left ventricle rises above the pressure in the aorta. This causes the
semilunar (aortic) valve to open and allows blood to flow from the left ventricle into the aorta (and
then to the systemic circulation). This is accompanied by a large decrease in left ventricular
volume as the blood leaves the ventricle. The pressure in the ventricle is still much higher than
pressure in the atria so the AV valves remain closed. During this phase aortic and ventricular
pressure increase parabolically, before cresting and beginning a quick decline. Throughout this
phase, pressure in the LV is higher than pressure in the aorta causing the aortic valve to remain
open.
