• Each heartbeat recorded on an EKG
is part of one cardiac
cycle. The cardiac cycle consists of a relaxation phase (dias-
tole) and a contraction phase (systole) for each of the heart
chambers. During diastole, the chamber fills with blood.
During systole, the atria contract first, followed by the ven-
tricles. The electrical conduction system coordinates each
contraction and relaxation phase. Specific pressure changes
occur within each phase of the cardiac cycle.
• The work done by the heart, called the cardiac output (CO),
is the volume of blood pumped by the heart in one minute.
Cardiac output is the product of the heart rate and stroke
volume—factors affected by the brain's cardiovascular cen-
ter working through the autonomic nervous system. Sympa-
thetic outflow to the heart through the cardiac accelerator
nerve increases the heart rate and force of contraction (con-
tractility), while parasympathetic outflow through the vagus
nerve decreases the heart rate. Sympathetic outflow to the
blood vessels causes vasoconstriction, which increases
venous return and stroke volume. These mechanisms strive
to keep cardiac output in a steady state but also allow a
shift from one steady state to another to adjust to changing
Blood Vessels Are the Body's Plumbing 320
• Arteries and narrower arterioles are thick-walled vessels
that have three layers: an inner endothelial layer, a middle
smooth muscle layer, and an outer layer of elastic fibers. The
smooth muscles of arteries and arterioles contract and relax
to control the diameter of the opening (lumen). These ves-
How arterioles control blood flow into capillaries •
sels, as shown, carry blood away from the heart and toward
the tissues. Precapillary sphincters can restrict the blood
flow into the tissue capillaries.
• Capillaries are thin-walled vessels where the exchange of
gases, nutrients, and wastes occurs. Diffusion drives the
movements of substances across the thin capillary walls.
The movement of fluid is determined by the balance of
hydrostatic pressure, which tends to move fluid out of the
capillary (filtration), and osmotic pressure, which tends to
move fluid into the capillary (absorption). At the arteriolar
end, net fluid flow is out of the capillary, while at the venule
end, net fluid flow is into the capillary. Excess filtrate will
later be returned to the blood circulation via the lymphatic
• Veins and venules have structures similar to those of arter-
ies and arterioles, but with larger lumens, thinner walls, and
less smooth muscle. Many veins have valves in them that
prevent the backflow of blood. These vessels are very elas-
tic and hold almost two-thirds of the blood at any moment
(blood reservoirs). Veins and venules carry blood from the
tissues back to the heart. The pressure in the venous system
is low, so skeletal muscles help pump venous blood back by
squeezing the veins (skeletal muscle pump). Similarly, pres-
sure changes in the abdominal cavity due to the breathing
process help move venous blood along (respiratory pump).
• In the systemic circulation, oxygen-rich blood flows from the
heart through arteries and arterioles to the tissue capillar-
ies, and venules and veins carry deoxygenated blood back
to the heart. However, there are two exceptions to this rule.
In the pulmonary circulation, pulmonary arteries carry de-
oxygenated blood, and pulmonary veins carry oxygenated
blood. In the hepatic portal circulation, blood comes into
the liver from the veins of another vascular bed, the gastro-
intestinal tract, through the hepatic portal vein. This venous
blood is rich in substances absorbed from the gut. The liver
also receives oxygenated blood through the hepatic artery.
All blood leaves the liver to join the vena cava through the
Maintaining Blood Pressure Is Critical for
• In the cardiovascular system, pressures drive blood through
blood vessels that offer resistance to its flow, as shown. Be-
cause the cardiovascular system is closed, blood flow is con-
stant, so the changes in pressure shown are directly related
to changes in resistance. The heart increases the pressure in
the ventricles on the blood, which then flows away.
338 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation