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The Circulatory System
The Structure and Function of the Heart
and Blood Vessels
Blood Vessels
• Arteries: thick-walled blood vessels that
ALWAYS carry blood AWAY from the heart.
• Veins: thin-walled blood vessels that
ALWAYS carry blood TOWARD the heart.
Arteries  Arterioles  Capillaries  Venules  Veins
Blood Vessels
Arteries
• Transports blood under pressure.
• Blood moves in a pulse-like wave throughout
the circulatory system.
• Arterioles have smaller diameter than arteries
and are less elastic, BUT the contraction and
relaxation of arterioles is the major determinant
of the overall blood pressure.
Capillaries
• Narrowest of all blood vessels.
• RBCs travel in single file.
• Branching of the capillaries increases the
surface area available for diffusion.
• Connects the arterial & venous systems.
Veins
• Thinner walls, larger diameters & less muscle
than arteries.
• Contains 70% of total blood volume.
• Most veins must work against gravity.
– Valves allow one-way flow of blood.
– Contraction of skeletal muscles pushes blood
toward heart.
– If veins are constantly stretched, they will lose
their elasticity & varicose veins will form.
The Heart
• Size of your fist.
• Hardest-working muscle in the
body.
• Contains four chambers:
– Left and right atria (receiving
chambers).
– Left and right ventricles (delivery
chambers).
• Left and right sides of the
heart are separated by a
muscular septum.
The Heart
• Protected by the ribcage, sternum & spine.
Pathways of Blood
• Pulmonary Circuit
– Right side of heart.
– Low-pressure system.
• Systemic Circuit
– Left side of heart.
– High-pressure system.
Pulmonary Circuit
• The right atrium receives oxygen-poor blood
from the:
– Superior vena cava.
– Inferior vena cava.
• Blood enters the right
atrium & flows through
the tricuspid valve or
the right atrioventricular
valve (AV-valve).
Pulmonary Circuit
• The blood passes through the pulmonary semilunar valve & enters the pulmonary trunk, which
divides into the left & right pulmonary arteries.
• The pulmonary arteries divide into capillaries at the
lungs where external gas exchange occurs.
• Oxygenated blood travels from
the lung capillaries to the left
& right pulmonary veins,
which return the blood to the
left atrium.
Systemic Circuit
• Blood passes from the left
atrium through the left AVvalve or bicuspid (mitral)
valve to the left ventricle.
– Left ventricular walls are two
times thicker than in the right
ventricle.
• Blood travels through the
aortic semi-lunar valve to the
aorta.
Systemic Circuit
• The aorta branches into smaller systemic
arteries, which branch into arterioles and
then capillaries.
• At the capillaries, internal gas exchange
occurs with the body cells.
• Capillaries rejoin as venules & then as
veins.
• Deoxygenated blood returns to the right
atrium via the superior & inferior vena cava.
Systemic
Circuit
Coronary Circulation
• Delivers oxygenated blood directly to the heart
muscle.
• Consists of the left & right coronary artery.
Cardiac Contractions
• Sinoatrial (SA) node or “pacemaker” (in the right
atrium) maintains the heart’s intrinsic pumping
rhythm.
– Nerves influence the rate & strength of the heart’s
contractions.
• This signal travels to the atrioventricular (AV) node,
where it is delayed for 0.1 seconds.
• It travels to the ventricles via the Purkinje fibers and the
Bundle of His.
• The delay at the AV node causes the atria to contract
simultaneously before the ventricles.
Pacemaker
Specialization of Cardiac Muscle
• The rhythmic contractions of the heart are
due to special characteristics of the cardiac
muscle cells.
The Heart Cycle
• The cardiac cycle is a continuous cycle
of contraction and relaxation.
– Systole
• Heart contraction.
– Diastole
• Heart relaxation.
Systole
1. Atria contract to further fill the ventricles.
2. Tricuspid & bicuspid valves forced closed due to an
increase in ventricular pressure.
3. Ventricles contract to force blood from the heart.
4. Right ventricle  (open pulmonary semi-lunar valve) 
pulmonary trunk & pulmonary arteries; and
5. Left ventricle  (open aortic semi-lunar valve)  aorta.
Blood pressure is increased (120 mm Hg).
Diastole
Blood enters all four chambers:
1. Pulmonary veins  Left atrium; and
2. Vena cava (superior & inferior)  Right atrium.
3. Tricuspid & bicuspid valves (AV valves) open, allowing
blood to flow into the left and right ventricles.
4. Pulmonary & aortic semi-lunar valves close due to a
decrease in ventricular pressure.
Blood pressure is reduced (80 mm Hg).
Systole & Diastole
Heart Valves & Heartbeat Sounds
• Heart valves open and
close at different times
to ensure blood flows in
the proper direction.
• “Lub”: tricuspid and
bicuspid valves close
(beginning of systole).
• “Dub”: pulmonary and
aortic semi-lunar valves
close (end of systole).
Blood Pressure