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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
Arterial System
• Transports blood under pressure.
• Blood moves in a pulse-like wave.
• Contraction & 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.
Venous System
• Thinner walls, larger diameters & less muscle
than arteries.
• Contains 70% of total blood volume.
• Most veins work against gravity.
• Valves allow one-way flow.
• Contraction of skeletal muscles pushes blood
toward heart.
Blood Vessels
The Heart
• Size of fist.
• Hardest-working muscle.
• Contains four chambers:
• Left & right atria (receiving chambers).
• Left & right ventricles (delivery chambers).
• Septum separates left &
right sides.
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
• Right atrium receives oxygen-poor blood from:
• Superior vena cava.
• Inferior vena cava.
• Blood flows through the tricuspid valve or the right
atrioventricular (AV) valve
to the right ventricle.
Pulmonary Circuit
• Blood flows through the pulmonary semi-lunar
valve & enters the pulmonary trunk, which divides
into the left & right pulmonary arteries.
• Divides into capillaries where external gas exchange occurs.
• Oxygenated blood enters the left
& right pulmonary veins, returning
blood to the left atrium.
Systemic Circuit
• Blood flows through the left AV-valve 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
• Aorta branches into smaller systemic
arteries, which branch into arterioles & then
capillaries.
• Internal gas exchange occurs.
• Capillaries rejoin as venules & then veins.
• Deoxygenated blood returns to the right
atrium via the superior & inferior vena cava.
Systemic Circuit
Coronary Circulation
• Delivers oxygenated blood 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 Purkinje fibers &
Bundle of His.
• The delay at the AV node causes the atria to contract
simultaneously before the ventricles.
Cardiac Contractions
The Heart Cycle
• The cardiac cycle is a continuous cycle
of relaxation & contraction.
• Diastole
• Heart relaxation
• Systole
• Heart contraction.
Diastole (80 mm Hg)
• Blood enters all four chambers.
• Pulmonary veins  left atrium.
• Inferior & superior vena cava  right
atrium.
• Left & right AV valves are open.
• Pulmonary & aortic semi-lunar valves are
closed.
Systole (120 mm Hg)
• Aria contract to further fill the ventricles.
• Tricuspid & bicuspid valves forced closed
due to an increase in ventricular pressure.
• Ventricles contract to force blood from the
heart.
• Pulmonary & aortic semi-lunar valves are
open.
Diastole & Systole
Blood Pressure
• Pressure exerted on the arterial walls.
• The elastic property of vessels allows
for continuous blood flow throughout the
circulatory system.
• The difference between your systolic
pressure & your diastolic pressure is
called your pulse pressure.
Blood Pressure
• Highest in the aorta.
• Blood pressure & velocity both decline as
blood enters the arterioles.
• The drop in pressure results from the
resistance (friction) to blood flow.
• Low blood pressure reduces your capacity to
transport blood.
• High blood pressure weakens & ruptures
arterial walls.
Blood Pressure
Heart Valves & Heart Sounds
• “Lub”: Tricuspid & bicuspid valves
close (beginning of systole).
• “Dub”: Pulmonary and aortic semilunar valves close (end of systole).
Heart Valves & Heart Sounds