Human Physiology, 14th edition (2016)

424 Chapter 13

1

Volume changes

40

80

120

1st 2nd 3rd

Heart sounds

0

20

40

60

80

100

120

0 0.2 0.4

Time (seconds)

Left

ventricle

Artery

0.6 0.8

Systole Diastole

Volume (ml)

Pressure (mmHg)

Pressure changes

1

2

3

4

5

2

3

4

5

Ventricles

contract

AV valves

closed

Atria

relaxed

Isovolumetric

contraction

Systole

Diastole

Rapid filling

Ejection

Atrial

contraction

Atria

relaxed

Atria

relaxed

Atria

relaxed

Atria

contract

Ventricles

contract

Ventricles

relaxed

Ventricles

relaxed

Ventricles

relaxed

Isovolumetric

relaxation

Semilunar valves

closed

reveals an inflection in the descending portion of the arterial pres-
sure graph, which cannot be felt on palpation. This inflection is
called the dicrotic notch and is produced by closing of the aortic
and pulmonic semilunar valves. Closing of these valves produces
the second heart sound and the dicrotic notch during the phase of
isovolumetric relaxation at the beginning of diastole.
An electrocardiogram (ECG) also allows an examiner to
follow the cardiac cycle of systole and diastole (see fig. 13.25 ).
This is because myocardial contraction occurs in response to
the depolarization stimulus of an action potential and myocar-
dial relaxation begins during repolarization. The relationships
between the electrical activity of the heart, the electrocardio-
gram, and the cardiac cycle are described in the next section.

Figure 13.17 Pressure changes in the left ventricle and their
effects during the cardiac cycle. The figure shows the effects of left
ventricular pressure changes on left ventricular volume and arterial pressure
and the correlation of these with the heart sounds. The numbers refer to the
events described in the text.
See the Test Your Quantitative Ability section of the Review Activities at the
end of this chapter.

| CHECKPOINT

8a. Using a drawing or flow chart, describe the sequence

of events that occurs during the cardiac cycle.

Indicate when atrial and ventricular filling occur and

when atrial and ventricular contraction occur.

8b. Describe how the pressure in the left ventricle and in

the systemic arteries varies during the cardiac cycle.

9. Draw a figure to illustrate the pressure variations
   described in question 8b, and indicate in your figure
   when the AV and semilunar valves close.