Human Physiology, 14th edition (2016)

420 Chapter 13

isovolumetric contraction of the ventricles (section 13.4). The

“dub,” or second sound, is produced by closing of the semilunar

valves when the pressure in the ventricles falls below the pres-

sure in the arteries. The first sound is thus heard when the ven-

tricles contract at systole, and the second sound is heard when

the ventricles relax at the beginning of diastole. (Systole and

diastole are discussed in section 13.4.)

One-way semilunar valves ( fig.  13.12 ) are located at the

origin of the pulmonary artery and aorta. These valves open dur-

ing ventricular contraction, allowing blood to enter the pulmo-

nary and systemic circulations. During ventricular relaxation,

when the pressure in the arteries is greater than the pressure in

the ventricles, the semilunar valves snap shut, preventing the

backflow of blood into the ventricles.

Heart Sounds

Closing of the AV and semilunar valves produces sounds that

can be heard by listening through a stethoscope placed on the

chest. These sounds are often verbalized as “lub-dub.” The “lub,”

or first sound, is produced by closing of the AV valves during

Figure 13.11 The heart valves. ( a ) A superior view of

the heart valves. ( b ) A sagittal section through the heart, showing

both AV valves and the pulmonary semilunar valve (the aortic

semilunar valve is not visible in this view).

Aortic

semilunar

valve

Pulmonary

semilunar

valve

Tricuspid

valve (into

right ventricle)

Aorta

Superior

vena cava

Right

atrium

Tricuspid

valve

Papillary

muscles

Inferior

vena cava

Bicuspid
valve (into
left ventricle)

Interventricular

septum

Chordae

tendineae

Mitral (bicuspid)

valve

Pulmonary

semilunar valve

Left atrium

Pulmonary

trunk

(a)

(b)

Figure 13.12 Photograph of a sectioned heart

showing the valves. The pulmonic and aortic semilunar valves

are seen toward the top of the photograph. The mitral and

tricuspid AV valves are also visible.

Semilunar

valves

AV valves

CLINICAL APPLICATION

Different auscultatory chest positions allow the closing of

the separate valves to be heard, so that the first and sec-

ond heart sounds may be heard to “split” into their com-

ponents. Closing of the tricuspid valve is best heard when

the stethoscope is placed to either side of the lower ster-

num, just above the xiphoid process, whereas closing of the

mitral valve is best heard at the apex of the heart, in the fifth

left intercostal space ( fig. 13.13 ). Closing of the pulmonary

and aortic semilunar valves is heard best at the second left

and right intercostal spaces, respectively. However, these

auscultatory positions are affected by obesity, pregnancy,

and other conditions.

Heart Murmurs

Murmurs are abnormal heart sounds produced by abnormal pat-

terns of blood flow in the heart. Many murmurs are caused by

defective heart valves. Defective heart valves may be congenital,

or they may occur as a result of rheumatic endocarditis, associated

with rheumatic fever. In this disease, the valves become damaged

by antibodies made in response to an infection caused by strep-

tococcus bacteria (the bacteria that produce strep throat). Many