inﬂ ow and outﬂ ow is reached. In other words, at any given pump
input, a change in total outﬂ ow resistance must produce changes
in the volume and, thus, the height (pressure) in the reservoir.
This analysis can be applied to the cardiovascular sys-
tem by again equating the pump with the heart, the reservoir
with the arteries, and the outﬂ ow tubes with various arteriolar
beds. As described earlier, small arteries and capillaries offer
some resistance to ﬂ ow, but the major site of resistance in the
systemic blood vessels is the arterioles. Moreover, changes in
total resistance are normally due to changes in the resistance
of arterioles. Therefore, in our discussions we equate total
peripheral resistance with total arteriolar resistance.
A physiological analogy to opening outﬂ ow tube 1 is
exercise: during exercise, the skeletal muscle arterioles dilate,
thereby decreasing resistance. If the cardiac output and the
arteriolar diameters of all other vascular beds were to remain
unchanged, the increased runoff through the skeletal muscle
arterioles would cause a decrease in systemic arterial pressure.
We must reemphasize that it is the total arteriolar resis-
tance that inﬂ
uences systemic arterial blood pressure. The
distribution of resistances among organs is irrelevant in this
illustrates this point. On the right,
outﬂ ow tube 1 has been opened, as in the previous exam-
ple, while tubes 2 to 4 have been simultaneously constricted.
The increased resistance in tubes 2 to 4 compensates for the
decreased resistance in tube 1; therefore total resistance remains
unchanged, and the reservoir pressure is unchanged. Total
outﬂ ow remains 1 L/min, although the distribution of ﬂ ows
is such that ﬂ ow through tube 1 increases, ﬂ ow through tubes
2 to 4 decreases, and ﬂ ow through tube 5 is unchanged.
Applied to the systemic circulation, this process is anal-
ogous to altering the distribution of systemic vascular resis-
tances. When the skeletal muscle arterioles (tube 1) dilate
during exercise, the total resistance of the systemic circulation
can still be maintained if arterioles constrict in other organs,
such as the kidneys and gastrointestinal organs (tubes 2 to 4).
In contrast, the brain arterioles (tube 5) remain unchanged,
ensuring constant brain blood supply.
This type of resistance juggling can maintain total resis-
tance only within limits, however. Obviously, if tube 1 opens
very wide, even complete closure of the other tubes cannot
prevent total outﬂ ow resistance from falling. In that situation,
cardiac output must be increased to maintain pressure in the
arteries. We will see that this is actually the case during exercise.
We have thus far explained in an intuitive way why car-
diac output (CO) and total peripheral resistance (TPR) are the
two variables that determine mean systemic arterial pressure.
This intuitive approach, however, does not explain speciﬁ cally
why MAP is the arithmetic product of CO and TPR. This
relationship can be derived formally from the basic equation
relating ﬂ ow, pressure, and resistance:
Rearranging terms algebraically, we have
Because the systemic vascular system is a continuous
series of tubes, this equation holds for the entire system—that
is, from the arteries to the right atrium. Therefore, the
term is mean systemic arterial pressure (MAP) minus the pres-
sure in the right atrium,
is the cardiac output (CO), and
the total peripheral resistance (TPR).
MAP – Right atrial pressure = CO
Because the pressure in the right atrium is very close
to 0 mmHg, we can drop this term and we are left with the
equation presented earlier:
MAP = CO
This equation is the fundamental equation of cardiovas-
cular physiology. An analogous equation can also be applied
to the pulmonary circulation:
These equations provide a way to integrate almost all the
information presented in this chapter. For example, we can
now explain why mean pulmonary arterial pressure is much
lower than mean systemic arterial pressure. The cardiac output
through the pulmonary and systemic arteries is of course the
same. Therefore, the pressures can differ only if the resistances
differ. Thus, we can deduce that the pulmonary vessels offer
much less resistance to ﬂ
ow than do the systemic vessels. In
other words, the total pulmonary vascular resistance is lower
than the total peripheral resistance.
presents the grand scheme of factors
that determine mean systemic arterial pressure. None of this
Compensation for dilation in one bed by constriction in others.
When outﬂ ow tube 1 is opened, outﬂ ow tubes 2 to 4 are
simultaneously tightened so that total outﬂ ow resistance, total
runoff rate, and reservoir pressure all remain constant.