Cardiovascular System—Physiology of the Heart
Lecture 2
This lecture is going to focus on the physiology of the heart, how it
works, how this tremendous pump gets the blood out there, and
how it’s regulated. And again we’ll look at what can go wrong with
the mechanisms.
N
ow we examine the physiology of the heart, starting with the
functioning of the cardiac cycle, in which deoxygenated blood
ows into the heart from the body, is pumped out to the lungs for
oxygenation, and is then returned to the heart for distribution to the body
We also examine the functioning of the heart’s conduction system, the
functioning of the valves (which produce the heart’s distinctive “lub-dub”
sound), and possible complications, notably atherosclerosis.
The heart is a powerful pump. It propels blood (a very viscous liquid) through
50,000 miles of vessels. It pumps 1.3 gallons per minute, or 700,000 gallons
per year at rest [editor’s note: professor correction]. It rests 0.25 second per
beat, and it can increase its output by a factor of 5× to 8× under stress. The
cardiac reserve is the ratio of maximum output to resting output.
Systole and diastole
Systole is the active compression, or squeezing, of the ventricles that pushes
blood outward to the lungs or the body. Diastole is the relaxation of the
ventricles when they are lling with blood. The terms systole and diastole
refer only to the ventricles. Atrial systole and diastole exist but are never
referred to as simply systole or diastole.
Physiology of the cardiac cycle
Each side of the heart has two chambers. The atria are low-pressure
systems, thin walled on both sides. They deliver blood to the ventricles.
They contract during diastole to help ll the ventricles. Much of ventricular
lling is passive; thus, atrial failure decreases cardiac output by only a small
percentage (20–30%).
9
, The ventricles
The left side of the ventricle is four times thicker and more powerful than the
right side. The left (systemic) side is also a high-pressure system. It receives
oxygenated blood from the lungs and pumps it out to the body. The right
(pulmonary) side is a low-pressure system. It receives deoxygenated blood
from the body and pumps it out to the lungs.
The conduction system
During embryologic development, 1% of the muscle mass of the heart is
designated as autorhythmic (self-exciting). These muscles differentiate
to form a conduction system. The conduction system establishes the
fundamental rhythm. Hormones, chemicals, and nerve impulses can alter the
heartbeat strength and heart rate.
The sinoatrial (SA) node initiates the rhythm. It has an inherent rhythm of
60–100 beats per minute. It is located high in the right atrial wall. Impulses
spread from the SA node to both atria, causing atrial contraction.
Lecture 2: Cardiovascular System—Physiology of the Heart
The impulses then spread to the atrioventricular (AV) node, located
at the medial base of the right atrium in the atrial septum above the
ventricles. AV stimulation sends impulses to the AV bundle of His. The
AV bundle of His provides the only electrical connection between the
atria and the ventricles. A bundle of His impulses travel through the
right and left bundle branches to the conduction myo bers of Purkinje,
which conduct impulses to the right and left ventricular muscles, causing
ventricular contraction.
Timing
The AV node bers are small, and this delay allows atrial contraction to be
completed before the next heartbeat. Conduction speeds up in the AV bundle.
The AV node’s inherent rhythm is much slower (40–50 beats per minute).
Disruption of the SA-AV node sequence will result in a slowed heart rate of
less than 60 beats per minute.
Patients with SA-AV node disturbances (bundle branch blocks) can be
tted with pacemakers, which electrically stimulate the heart. Pacemakers
can be programmed for the demands of variable pacing. Valve stenosis (a
10
Lecture 2
This lecture is going to focus on the physiology of the heart, how it
works, how this tremendous pump gets the blood out there, and
how it’s regulated. And again we’ll look at what can go wrong with
the mechanisms.
N
ow we examine the physiology of the heart, starting with the
functioning of the cardiac cycle, in which deoxygenated blood
ows into the heart from the body, is pumped out to the lungs for
oxygenation, and is then returned to the heart for distribution to the body
We also examine the functioning of the heart’s conduction system, the
functioning of the valves (which produce the heart’s distinctive “lub-dub”
sound), and possible complications, notably atherosclerosis.
The heart is a powerful pump. It propels blood (a very viscous liquid) through
50,000 miles of vessels. It pumps 1.3 gallons per minute, or 700,000 gallons
per year at rest [editor’s note: professor correction]. It rests 0.25 second per
beat, and it can increase its output by a factor of 5× to 8× under stress. The
cardiac reserve is the ratio of maximum output to resting output.
Systole and diastole
Systole is the active compression, or squeezing, of the ventricles that pushes
blood outward to the lungs or the body. Diastole is the relaxation of the
ventricles when they are lling with blood. The terms systole and diastole
refer only to the ventricles. Atrial systole and diastole exist but are never
referred to as simply systole or diastole.
Physiology of the cardiac cycle
Each side of the heart has two chambers. The atria are low-pressure
systems, thin walled on both sides. They deliver blood to the ventricles.
They contract during diastole to help ll the ventricles. Much of ventricular
lling is passive; thus, atrial failure decreases cardiac output by only a small
percentage (20–30%).
9
, The ventricles
The left side of the ventricle is four times thicker and more powerful than the
right side. The left (systemic) side is also a high-pressure system. It receives
oxygenated blood from the lungs and pumps it out to the body. The right
(pulmonary) side is a low-pressure system. It receives deoxygenated blood
from the body and pumps it out to the lungs.
The conduction system
During embryologic development, 1% of the muscle mass of the heart is
designated as autorhythmic (self-exciting). These muscles differentiate
to form a conduction system. The conduction system establishes the
fundamental rhythm. Hormones, chemicals, and nerve impulses can alter the
heartbeat strength and heart rate.
The sinoatrial (SA) node initiates the rhythm. It has an inherent rhythm of
60–100 beats per minute. It is located high in the right atrial wall. Impulses
spread from the SA node to both atria, causing atrial contraction.
Lecture 2: Cardiovascular System—Physiology of the Heart
The impulses then spread to the atrioventricular (AV) node, located
at the medial base of the right atrium in the atrial septum above the
ventricles. AV stimulation sends impulses to the AV bundle of His. The
AV bundle of His provides the only electrical connection between the
atria and the ventricles. A bundle of His impulses travel through the
right and left bundle branches to the conduction myo bers of Purkinje,
which conduct impulses to the right and left ventricular muscles, causing
ventricular contraction.
Timing
The AV node bers are small, and this delay allows atrial contraction to be
completed before the next heartbeat. Conduction speeds up in the AV bundle.
The AV node’s inherent rhythm is much slower (40–50 beats per minute).
Disruption of the SA-AV node sequence will result in a slowed heart rate of
less than 60 beats per minute.
Patients with SA-AV node disturbances (bundle branch blocks) can be
tted with pacemakers, which electrically stimulate the heart. Pacemakers
can be programmed for the demands of variable pacing. Valve stenosis (a
10
