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10.4 The Vascular Pathways
The circulatory system includes two circuits.
• Pulmonary circuit: circulates blood through the
lungs
• Systemic circuit: circulates blood to the body
tissues
The Pulmonary Circuit
Pulmonary circuit: circulates blood to lungs
• Blood from all regions of the body first collects
in the right atrium.
• Blood passes into the right ventricle, which
pumps it into the pulmonary trunk.
• Pulmonary trunk divides into right and left
pulmonary arteries, which branch into
arterioles.
• Arterioles take blood to the pulmonary
capillaries, where gas exchange occurs.
• Blood passes through pulmonary venules,
which lead to pulmonary veins that enter the
left atrium.
The Systemic Circuit
Systemic circuit: circulates blood to body tissues.
• The aorta and venae cavae serve as the major
pathways for blood in the systemic circuit
The path of systemic blood to any organ in the body
begins in the left ventricle.
Trace the path of blood to and from the legs:
• left ventricle  aorta  common iliac artery  femoral artery  leg capillaries 
femoral vein  common iliac vein  inferior vena cava  right atrium
Blood Pressure
As blood passes through the blood vessels in the body, it exerts pressure against the vessel
walls (blood pressure).
• Systolic pressure: results from blood being forced into arteries when ventricles contract
(ventricular systole)
• Diastolic pressure: results from pressure in the arteries when ventricles fill with blood
(ventricular diastole)
o Blood pressure can be measured with a sphygmomanometer, which has a
pressure cuff that determines the amount of pressure required to stop the flow
of blood through an artery
o Expressed in millimetres of mercury as a fraction of systolic pressure over
diastolic pressure (e.g., 120/80 mm Hg)
Blood pressure decreases as blood flows from the aorta
into the arteries, arterioles, and capillaries.
• Blood is under minimal pressure in capillaries since
capillaries have a high total cross-sectional area.
Blood pressure in the veins is low.
• Valves prevent the backward flow of blood in veins,
and muscle contraction is sufficient to move blood
toward the heart.
Figure 10.18 Cross section of a valve
in a vein. a. Pressure on the walls of a
vein, exerted by skeletal muscles,
increases blood pressure within the vein
and forces the valve open.
b. When external pressure is no longer
applied to the vein, blood pressure
decreases, and back pressure forces
the valve closed. Closure of the valve
prevents the blood from flowing in the
opposite direction.