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Biology 12
10.3 The Human Heart
The heart is a muscular organ about the size of a fist.
• Myocardium: the major portion of the heart; consists of cardiac muscle tissue
• Pericardium: a thick membrane within which the heart lies; secretes lubricating liquid
• Endocardium: the inner surface of the heart; a membrane composed of connective and endothelial
tissue
Structure of the Heart
The septum separates the heart into a right side and a left side.
The heart has four chambers:
• Two upper, thin-walled atria
o Atria fill with blood returning from the body and lungs
• Two lower, thick-walled ventricles
o Ventricles receive blood from atria and pump it out to body and lungs
The heart has four valves that direct the flow of blood and prevent its backward movement.
• Two atrioventricular valves that are supported by strong fibrous strings called chordae tendineae
o tricuspid valve: the valve on the right side
o bicuspid (mitral valve): the valve on the left side
• Two semilunar valves
o pulmonary semilunar valve: lies between right ventricle and pulmonary trunk
o aortic semilunar valve: lies between the left ventricle and aorta
Biology 12
Path of Blood through the Heart
1. The superior vena cava and the inferior vena cava, which carry
O2-poor blood, enter the right atrium.
2. The right atrium sends blood through the tricuspid valve to the
right ventricle.
3. The right ventricle sends blood through the pulmonary
semilunar valve into the pulmonary trunk and through the two
pulmonary arteries to the lungs.
4. Four pulmonary veins, which carry O2-rich blood, enter the left
atrium.
5. The left atrium sends blood through the bicuspid valve to the
left ventricle.
6. The left ventricle sends blood through the aortic semilunar
valve into the aorta and on to the rest of the body.
Biology 12
• The heart is a double pump.
o Right ventricle sends blood into the lungs
o Left ventricle sends blood into the rest of the body
− A stronger pump, since it has to pump blood to the body
• Cardiac output is the volume of blood that the left ventricle pumps per minute.
o Cardiac output is about 5.25 L of blood per minute in a person with an average heart rate of 70
beats per minute
o The pulse is a wave effect that passes down the walls of the arteries when the aorta expands
and recoils with each ventricular contraction; can be used to determine heart rate
The Heartbeat
Each heartbeat is called a cardiac cycle. When a heart beats:
• The two atria contract at the same time, and the ventricles are relaxed and fill with blood
• Then, the two ventricles contract at the same time
• Then, all the chambers relax
The heart beats about 70 times a minute; each heartbeat is
0.85 s.
• Systole is the contraction of the heart muscle
• Diastole is the relaxation of the heart muscle
Intrinsic Control of Heartbeat
The heart is able to contract and relax rhythmically due to the presence
of nodal tissue, a type of cardiac muscle.
Nodal tissue is located in two areas::
• SA (sinoatrial) node: initiates the heartbeat and sends out an
impulse every 0.85 s; also called the pacemaker
• AV (atrioventricular) node: transmits an impulse through
specialized cardiac muscle fibres called the atrioventricular
bundle (AV bundle), which send the signal to Purkinje fibres
Extrinsic Control of Heartbeat
The body also has extrinsic ways to regulate the heartbeat.
Medulla oblongata (portion of the brain that controls internal organs)
• Can alter heartbeat by the autonomic nervous system
o Parasympathetic: decreases SA and AV nodal activity when inactive
o Sympathetic: increases SA and AV nodal activity when active
Epinephrine and norepinephrine (hormones)
• Released by the adrenal medulla
• Heart pumps faster and stronger due to sympathetic stimulation and release of epinephrine and
norepinephrine
Biology 12
The Electrocardiogram
An electrocardiogram (ECG) is a recording of the electrical changes that
occur in the myocardium during a cardiac cycle.
Different types of abnormalities, known as arrhythmias can be detected
by an ECG
• Atrial fibrillation (AF): multiple, chaotic impulses are generated
from the AV node, causing an irregular, fast heartbeat
• Ventricular fibrillation (VF): uncoordinated contraction of the ventricles; can occur after a heart attack,
injury, or drug overdose
o Heart in VF is not pumping blood and must be defibrillated by applying an electrical current to
reestablish heartbeat
o Automatic external defibrillators(AEDs) can be used to administer an electrical current to the
chest
Figure 10.14 Conduction
system of the heart.
b. A normal ECG .
c. Ventricular fibrillation
produces an irregular
electrocardiogram due to
irregular stimulation of the
ventricles.