Lecture 4: The Cardiac Cycle, Heart Pressures and Cardiac Output Heart Rate) Regulation
1. The Cardiac Cycle
See lecture 3 notes for the detailed description of 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 beginduring 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 the diastolic part of the cardiac cycle, the atria contract, pumping a little more blood into
The heart then enters its systolic (or contraction) phases of which there are two parts: the first
phase, the isovolumetric 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
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 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.
We looked before at the cardiac cycle using a diagram which displayed four lines of data - atrial
pressure, ventricular pressure, aortic pressure (pulmonary pressure on the right side), and 2
ventricular volume. For a more detailed discussion of this diagram, consult the previous lecture.
However, the key points on this chart are when ventricular pressure goes above or below atrial
pressure, and when ventricular pressure goes above or below aortic (or pulmonary) pressure.
These points are important because they mark the opening and closing of thevarious valves of the
2. A Closer Look at Aortic Pressure
Several interesting features of aortic pressure occur during the cardiac cycle - particularly the
abrupt increase in aortic pressure that occurs during the isovolumetric relaxation phase - this we
call the dicrotic notch.
The first important point with regard to the aorticpressure trace is the slow, steady decrease during
diastole (when the semilunar valves are closed). This reduction in aortic pressure occurs because
although no blood is being pumped into the aorta, it is still leaving the aorta and entering the
systemic circulation. Blood can continue flowing, even during the heart's relaxation phase,
because arteries are what we call pressure reservoirs: and the larger the artery, the greater the
pressure differential between the large artery and the smaller arteries that are found downstream.
So what happens when the heart is contracting, and blood is flowing from the left ventricle to the
aorta, is a very large movement of blood into theaorta. Under healthy conditions, the aorta is fairly
elastic and so it expands as blood enters it. Therefore, when the heart is contracting, blood flows
into the systemic circulation but some also remains in the aorta , causing it to bulge outward. As
the heart relaxes, the aorta reverts back to its 'regular size', forcing blood forward again. This
allows for blood to constantly flow from the heartinto the systemic circulation even when the heart
is in diastole. A large artery that acts as a pressure reservoir and helps to maintain a constant flow
of blood is called a Windkessel vessel.
In the ventricular ejection phase, and also in the isovolumetric contraction before it, we see
increases in ventricular pressure. However, aortic pressure only begins to increase in the
ventricular ejection phase. This rapid rise in aortic pressure is due to blood flowing from the left
ventricle into the aorta at a far greater rate thanblood is flowing out of it. In the ventricular ejection
phase, as blood is pumped throughout the aorta, pressure will rise until it reaches its maximum
value, before it begins to fall quite gradually as the phase ends. When ventricular pressure falls 3
below aortic pressure at the start of the isovolumetric relaxation phase, the aortic semilunar valve
will close, prompting an interruption in the smooth decrease of aortic pressure. This blip, a little
increase in aortic pressure, is called the dicrotic notch; and it is essentially a pressure
reverberation that is associated with the closing of the aortic valve. When the valve snaps shut, a
little shock wave is created in the blood in the aorta.After the dicrotic notch,aortic pressure begins
to decrease in an essentially linear fashion, until the cycle begins again.
3. Systolic and Diastolic Pressure; Pulse Pressure; Mean Arterial Pressure
We can define pressure in the aorta, and blood pressure in general, in a number of different ways.
When we look at blood pressure regulation, we talk about mean arterial pressure. But as we have
seen, aortic pressure is not at all constant, but instead changes throughout the cardiac cycle. So,
what then, is the mean arterial pressure that the body is trying to maintain and regulate? Well, it
turns out to be an amalgamation of maximum and minimum pressures.
When we look at the aortic pressure trace we see that aortic pressure is at its minimum level just
before the start of the ventricular ejection phase. This value is called the diastolic pressure (even
though it is not in fact achieved until slightly into systole). The maximum pressure, which occurs
halfway through the ventricular ejection, is called thesystolic pressure. These values, in a typical
healthy human, are approximately 120 mm of mercur y (Hg) for systolic pressure and 80 mmHg for
diastolic pressure - thisis where you get the 120/80 number for a “typical value” of blood pressure.
When we calculate mean arterial pressure, we will use these two numbers.
The difference between systolic pressure and diastolic pressure is referred to as pulse pressure.
This value has several uses: it can be used in the calculation of mean arterial pressure, as we shall
see, but it can also be used to diagnose cardiovascular disease such as atherosclerosis (the
hardening of the arteries). So, if we have a norm al blood pressure of 120/80,then we have a normal
pulse pressure of 40. This will be determined inpart by how well arteriescan expand and contract
when filled with blood. As we saw with the aorta earlier, arteries need to be elastic in order to 4
handle differing volumes of blood. However, as they get older, arteries begin to lose their
elasticity, and this is referred to as 'hardening' - generally thisis caused by buildup of fatty deposits
and plaque on the inside of the arteries. This, in turn, leads to an increase in blood pressure: both
systolic and diastolic values increase, but particularlysystolic, as it means more blood is trapped in
the aorta waiting to flow into the systemic circulation. An increase in pulse pressure then is an
indicator of atherosclerosis, or simply the normal hardening of the arteries due to age.
Mean arterial pressure (MAP) is the blood pressure value we are most concerned with when
looking at cardiovascular regulation - as said before, the task of almost all cardiovascular
regulators is to prevent blood pressure from falling to low. Mean arterial pressure is not simply the
arithmetic mean of systolic and diastolic pressure- it is more complicated,because aortic pressure
is closer to its minimum value for much longer thanit is closer to the maximum value. Thus, mean
arterial pressure is generally calculated with the following formula: MAP = 1/3 of systolic
pressure + 2/3 of diastolic pressure. So at normal, 120/80 blood pressure, we will add 40+53,
and so mean arterial pressure will be 93 mmHg - much closer to the diastolic pressure value than
the systolic pressure value, in order to more accurately reflect the average pressure of the aorta. A
different way of calculating MAP is to take one's pulse pressure, and follow this formula: MAP =
1/3 of pulse pressure+ diastolic pressure. So, for a normal resting person with SP/DP of 120/80,
the formula goes 40/3 = 13; 13+80 = 93 mmHg. These calculations are important, because mean
arterial pressure is the driving force for blood flow in the systemic circulation.
One should keep in mind that blood pressure is rarely (or never) measured by a catheter placed
directly into the aorta - instead, a cuff is usually placed upon the upper arm, and pressure is
measured in the brachial artery. This is a fairly g