ventricular contraction, and therefore stroke
volume, is regulated by mechanisms intrinsic
to the heart, by autonomic nerves and hor-
mones (see Chapters 3, 4, and 6).
The heart has other important functions
besides pumping blood. The heart synthesizes
several hormones. One of these hormones,
atrial natriuretic peptide, plays an impor-
tant role in the regulation of blood volume
and blood pressure (see Chapter 6). Sensory
nerve receptors associated with the heart play
a role in regulating the release of antidiuretic
hormone from the posterior pituitary, which
regulates water loss by the kidneys.
Vascular System
Blood vessels constrict and dilate to regulate
arterial blood pressure, alter blood flow within
and distribute blood volume within the body.
Changes in vascular diameters are brought
about by activation of vascular smooth muscle
within the vascular wall by autonomic nerves,
metabolic and biochemical signals from out-
side of the blood vessel, and vasoactive sub-
stances released by endothelial cells that line
the blood vessels (see Chapters 3, 5, and 6).
Blood vessels have other functions besides
distribution of blood flow and exchange. The
endothelium lining blood vessels produces
substances that modulate hemostasis (blood
clotting) and inflammatory responses (see
Chapter 3).
Interdependence of Circulatory
and Organ Function
Cardiovascular function is closely linked to
the function of other organs. For example, the
brain not only receives blood flow to support
its metabolism but also acts as a control center
for regulating cardiovascular function. A sec-
ond example of the interdependence between
organ function and the circulation is the kid-
ney. The kidneys excrete varying amounts of
sodium, water, and other molecules to main-
tain fluid and electrolyte homeostasis. Blood
passing through the kidneys is filtered, and
the kidneys then modify the composition of
the filtrate to form urine. Reduced blood flow
to the kidneys can have detrimental effects on
kidney function and therefore on fluid and
electrolyte balance in the body. Furthermore,
renal dysfunction can lead to large increases
in blood volume, which can precipitate cardi-
ovascular changes that can lead to hyperten-
sion or exacerbate heart failure. In summary,
organ function is dependent on the circula-
tion of blood, and cardiovascular function is
dependent on the function of organs.
The cardiovascular system must be able to
adapt to changing conditions and demands of
the body. For example, when a person exer-
cises, increased metabolic activity of contract-
ing skeletal muscle requires large increases
in nutrient supply (particularly oxygen) and
enhanced removal of metabolic by-products
(e.g., carbon dioxide, lactic acid). To meet this
demand, blood vessels within the exercising
muscle dilate to increase blood flow; however,
blood flow can only be increased if the arterial
pressure is maintained. Arterial pressure is
maintained during exercise by increasing car-
diac output and by constricting blood vessels
in other organs of the body (see Chapter 9).
If these changes were not to occur, arterial
blood pressure would fall precipitously dur-
ing exercise, thereby limiting organ perfusion
and exercise capacity. Therefore, a coordi-
nated cardiovascular response is required to
permit increased muscle blood flow while a
person exercises. Another example of adapta-
tion occurs when a person stands up. Gravi-
tational forces cause blood to pool in the legs
when a person assumes an upright body pos-
ture (see Chapter 5). In the absence of regu-
latory mechanisms, this pooling will lead to
a fall in cardiac output and arterial pressure,
which can cause a person to faint because
of reduced blood flow to the brain. To pre-
vent this from happening, coordinated reflex
responses increase heart rate and constrict
blood vessels to maintain a normal arterial
blood pressure when a person stands.
It is important to control arterial blood
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