Essentials of Anatomy and Physiology

Right Ventricle—has relatively thin walls

1. Pumps blood to the lungs through the pulmonary
   artery.


2. The pulmonary semilunar valve prevents backflow
   of blood from the pulmonary artery to the right
   ventricle when the right ventricle relaxes.


3. Papillary muscles and chordae tendineae prevent
   inversion of the right AV valve when the right ven-
   tricle contracts.

Left Ventricle—has thicker walls than does
the right ventricle

1. Pumps blood to the body through the aorta.


2. The aortic semilunar valve prevents backflow of
   blood from the aorta to the left ventricle when the
   left ventricle relaxes.


3. Papillary muscles and chordae tendineae prevent
   inversion of the left AV valve when the left ventri-
   cle contracts.


4. The heart is a double pump: The right side of the
   heart receives deoxygenated blood from the body
   and pumps it to the lungs; the left side of the heart
   receives oxygenated blood from the lungs and
   pumps it to the body. Both sides of the heart work
   simultaneously.

Coronary Vessels (see Fig. 12–4)

1. Pathway: ascending aorta to right and left coronary
   arteries, to smaller arteries, to capillaries, to coro-
   nary veins, to the coronary sinus, to the right
   atrium.


2. Coronary circulation supplies oxygenated blood to
   the myocardium.


3. Obstruction of a coronary artery causes a myocar-
   dial infarction: death of an area of myocardium due
   to lack of oxygen.

Cardiac Cycle—the sequence of events in one
heartbeat (see Fig. 12–5)

1. The atria continually receive blood from the veins;
   as pressure within the atria increases, the AV valves
   are opened.


2. Two-thirds of the atrial blood flows passively
   into the ventricles; atrial contraction pumps the
   remaining blood into the ventricles; the atria then
   relax.


3. The ventricles contract, which closes the AV valves
   and opens the aortic and pulmonary semilunar
   valves.
   4. Ventricular contraction pumps all blood into the
   arteries. The ventricles then relax. Meanwhile,
   blood is filling the atria, and the cycle begins again.
   5. Systole means contraction; diastole means relax-
   ation. In the cardiac cycle, atrial systole is followed
   by ventricular systole. When the ventricles are in
   systole, the atria are in diastole.
   6. The mechanical events of the cardiac cycle keep
   blood moving from the veins through the heart and
   into the arteries.

Heart Sounds—two sounds per heartbeat:

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1. The first sound is created by closure of the AV
   valves during ventricular systole.


2. The second sound is created by closure of the aor-
   tic and pulmonary semilunar valves.


3. Improper closing of a valve results in a heart mur-
   mur.

Cardiac Conduction Pathway—the pathway

of impulses during the cardiac cycle (see Fig.

12–6)

1. The SA node in the wall of the right atrium initi-
   ates each heartbeat; the cells of the SA node are
   more permeable to Naions and depolarize more
   rapidly than any other part of the myocardium.


2. The AV node is in the lower interatrial septum.
   Depolarization of the SA node spreads to the AV
   node and to the atrial myocardium and brings
   about atrial systole.


3. The AV bundle (bundle of His) is in the upper
   interventricular septum; the first part of the ventri-
   cles to depolarize.


4. The right and left bundle branches in the interven-
   tricular septum transmit impulses to the Purkinje
   fibers in the ventricular myocardium, which com-
   plete ventricular systole.


5. An electrocardiogram (ECG) depicts the electrical
   activity of the heart (see Fig. 12–6).


6. If part of the conduction pathway does not function
   properly, the next part will initiate contraction, but
   at a slower rate.


7. Arrhythmias are irregular heartbeats; their effects
   range from harmless to life-threatening.

Heart Rate

1. Healthy adult: 60 to 80 beats per minute (heart rate
   equals pulse); children and infants have faster

The Heart 287