650
P A R T 8
Drugs acting on the cardiovascular system
blood then circulates around the alveoli of the lungs,
picking up oxygen and getting rid of carbon dioxide;
flows through pulmonary capillaries (the tiny blood
vessels that connect arteries and veins) into the pulmon
ary veins; and then returns to the left atrium.
Systemic circulation
When the pressure of blood volume in the left atrium is
greater than the pressure in the large left ventricle, this
oxygenated blood flows into the left ventricle. The left
atrium contracts and pushes any remaining blood into
the left ventricle, which is stimulated to contract and
generates tremendous pressure to push the blood out
the aorta, carrying it throughout the body. The aorta
and other large arteries have thick, muscular walls. The
entire arterial system contains muscles in the walls of the
vessels all the way to the terminal branches or arterioles,
which consist of fragments of muscle and endothelial
cells. These muscles offer resistance to the blood that is
sent pumping into the arterial system by the left ventri
cle, generating pressure. The arterial system is referred
to as a
resistance system
. The vessels can either constrict
or dilate, increasing or decreasing resistance, respect
ively, based on the needs of the body. The arterioles are
able to completely shut off blood flow to some areas of
the body; that is, they can shunt blood to another area
where it is needed more. The arterioles, because of their
ability to increase or decrease resistance in the system,
are one of the main regulators of blood pressure.
Blood from the tiny arterioles flows into the
capillary
system, which connects the arterial and venous systems.
These microscopic vessels are composed of loosely con
nected endothelial cells. Oxygen, fluid and nutrients are
able to pass through the arterial end of the capillaries
and enter the interstitial area between tissue cells.
Fluid at the venous end of the capillary, which contains
carbon dioxide and other waste products, is drawn back
into the vessel. This shifting of fluid in the capillaries,
called the capillary fluid shift, is carefully regulated by
a balance between hydrostatic (fluid pressure) forces on
the arterial end of the capillary and
oncotic pressure
(the pulling pressure of the large, vascular proteins) on
the venous end of the capillary. In a normal situation,
the higher pressure at the arterial end of a capillary
forces fluid out of the vessel and into the tissue, and the
now-concentrated proteins (which are too large to leave
the capillary) exert a pull on the fluid at the venous end
of the capillary to pull it back in. A disruption in the
hydrostatic pressure or in the concentration of proteins
in the capillary can lead to fluid being left in the tissue,
a condition referred to as oedema. The capillaries merge
into venules, which merge into veins, the vessels respons
ible for returning the blood to the heart (Figure 42.7).
The veins are thin-walled, very elastic, low-pressure
vessels that can hold large quantities of blood if neces
sary. The venous system is referred to as a
capacitance
system
because the veins have the capacity to hold large
quantities of fluid as they distend with fluid volume.
These capacitance vessels have a great deal of influence
on the venous return to the heart—the amount of blood
that is delivered to the right atrium.
Coronary circulation
The heart muscle requires a constant supply of oxy
genated blood to keep contracting. The myocardium
receives its blood through two main coronary arteries
that branch off the base of the aorta from an area called
the sinuses of Valsalva. These arteries encircle the heart
Hydrostatic pressure (HP)
Driving force of heart
tends to push fluid out of capillary
Oncotic pressure (OP)
Pressure exerted by plasma proteins
tends to pull fluid into capillary
Filtration pressure
Net force on fluid, determined by
the balance between HP and OP
36 mmHg
28 mmHg
8 mmHg
HP > OP
Fluid leaves capillary
Arterial
end
8 mmHg
–7 mmHg
Direction of
blood flow
Capillary
Venous
end
21 mmHg
28 mmHg
–7 mmHg
HP < OP
Fluid enters capillary
FIGURE 42.7
The net shift of fluid out of
and into the capillary is determined by
the balance between the hydrostatic
pressure (HP) and the oncotic pressure
(OP). The HP tends to push fluid out
of the capillary and the OP tends to
pull it back into the capillary. At the
arterial end of the capillary bed, the
blood pressure is higher than at the
venous end. At the arterial end, HP
exceeds OP and fluid filters out. At the
venous end, HP has fallen, and HP is
less than OP; fluid is pulled back into
the capillary from the surrounding
tissue. The lymphatic system also
returns fluids and substances from the
tissues to the circulation.
