Porth's Essentials of Pathophysiology, 4e

391

Control of Cardiovascular Function

C h a p t e r 1 7

120

Pressure (mm Hg)

Aortic valve closes

100

80

Aortic pressure

60

Aortic valve opens

40

Left ventricular pressure

20

Atrial pressure

Atrial contraction

100

Ventricular volume (mL)

80

40

R

R

P

T

P

ECG

Q

Q

1st

2nd 3rd

4th

Hear sounds

Diastole

Systole

Time (sec)

Left atrium

Right atrium

Left ventricle

Right ventricle

Isovolumetric contraction

Ventricular ejection

Isovolumetric relaxation

Ventricular filling

Atrial contraction

C

B

A

FIGURE 17-15. (Top) Events in the left side of the heart, showing changes in aortic pressure, left ventricular pressure, atrial pressure, left ventricular volume, the electrocardiogram (ECG), and heart sounds during the cardiac cycle. (Bottom) Position of the atrioventricular valves during (A) the isovolumetric contraction and ejection phases of ventricular systole, (B) the isovolumetric relaxation and ventricular filling phases during early diastole, and (C) atrial contraction.

During this time, which is called the isovolumetric relaxation period, ventricular volume remains the same but ventricular pressure drops until it becomes less than atrial pressure (see Fig. 17-15B). As this occurs, the AV valves open, and the blood that has been accumulat- ing in the atria during systole flows into the ventricles. Most of ventricular filling occurs during the first third of diastole, which is called the rapid filling period. During the middle third of diastole, inflow into the ventricles is almost at a standstill. The last third of diastole is marked by atrial contraction, which gives an additional thrust to ventricular filling. When audible, the third heart sound is heard at the end of the rapid filling period

due to vibrations that are caused by the abrupt cessa- tion of ventricular distention and by the deceleration of blood entering the ventricles. A third heart sound is sometimes heard in children with thin chest walls and in persons with a distended or noncompliant ventricle. A fourth heart sound, when present, occurs during the last third of diastole as the atria contract (see Fig. 17-15C). During diastole, the ventricles increase their volume to approximately 120 mL (i.e., the end-diastolic vol- ume ), and at the end of systole, approximately 50 mL of blood (i.e., the end-systolic volume ) remains in the ventricles. The difference between the end-diastolic and end-systolic volumes (approximately 70 mL) is called the