the left atrium by way of the pulmonary
veins. Blood then flows from the left atrium
into the left ventricle. The left ventricle ejects
the blood into the aorta, which then distrib-
utes the blood to all the organs via the arterial
system. Within the organs, the vasculature
branches into smaller and smaller vessels,
eventually forming capillaries, which are the
primary site of exchange. Blood flow from the
capillaries enters veins, which return blood
flow to the right atrium via large systemic
veins (the superior and inferior vena cava).
As blood flows through organs, some of
the fluid, along with electrolytes and small
amounts of protein, leaves the circulation and
enters the tissue interstitium (a process termed
fluid filtration). The lymphatic vessels, which
are closely associated with small blood vessels
within the tissue, collect the excess fluid from
within the tissue interstitium and transport
it back into the venous circulation by way of
lymphatic ducts that empty into large veins
(subclavian veins) above the right atrium.
It is important to note the overall arrange-
ment of the cardiovascular system. First, the
right and left sides of the heart, which are sepa-
rated by the pulmonary and systemic circula-
tions, are in series with each other (see Fig. 1.1).
Therefore, all of the blood that is pumped from
the right ventricle enters into the pulmonary
circulation and then into the left side of the
heart from where it is pumped into the systemic
circulation before returning to the heart. This
in-series relationship of the two sides of the
heart and the pulmonary and systemic circula-
tions requires that the output (volume of blood
ejected per unit time) of each side of the heart
closely matches the output of the other so that
there are no major blood volume shifts between
the pulmonary and systemic circulations. Sec-
ond, most of the major organ systems of the
body receive their blood from the aorta, and
the blood leaving these organs enters into the
venous system (superior and inferior vena cava)
that returns the blood to the heart. Therefore,
the circulations of most major organ systems are
in parallel as shown in Figure 1.2. One major
exception is the liver, which receives a large frac-
tion of its blood supply from the venous circula-
tion of the intestinal tract that drains into the
■ FIGURE 1.2 Parallel arrangement of organs
w ithin the body. One m ajor exception is the
hepatic (liver) circulation, which receives blood
flow from the hepatic portal veins of the gastroin-
testinal (GI) circulation (series) and from the aorta
via the hepatic artery (parallel).
superior vena
inferior vena cava.
hepatic portal system to supply the liver. The
liver also receives blood from the aorta via the
hepatic artery. Therefore, most of the liver circu-
lation is in series with the intestinal circulation,
while some of the liver circulation is in parallel
with the intestinal circulation (see Chapter 7).
The parallel arrangement has significant
hemodynamic implications as described in
Chapter 5. Briefly,
the parallel arrangement of
major vascular beds prevents blood flow changes
in one organ from significantly affecting blood
flow in other organs.
In contrast, when vascular
beds are in series, blood flow changes in one
vascular bed significantly alter blood flow to
the other vascular bed.
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