Page 203 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Cardiac Cycle tion phase), a large portion of the stroke
volume (SV) is rapidly expelled (! A4) and the
The resting heart rate is 60–80 beats per blood flow rate reaches a maximum (! A5).
minute. A cardiac cycle (! A) therefore takes Myocardial excitation subsequently decreases
roughly 1 s. It can be divided into four distinct (T wave of the ECG, ! A1) and ventricular pres-
phases: (I) contraction phase and (II) ejection sure decreases (the remaining SV fraction is
phase, both occurring in systole; (III) relaxation slowly ejected, phase IIb) until it falls below
phase and filling phase (IV), both occurring in that of the aorta or pulmonary artery, respec-
diastole. At the end of phase IV, the atria con- tively. This leads to closing of the semilunar
tract (phase IVc). Electrical excitation of the valves, producing the second heart sound
atria and ventricles precedes their contraction. (! A6). The mean SV at rest is about 80 mL or,
The cardiac valves determine the direction more precisely, 47 mL/m body surface area.
2
of blood flow within the heart, e.g., from the The corresponding mean ejection fraction (SV/
Cardiovascular System (phase II). All cardiac valves are closed during this point is about 40 mL (! A4).
EDV) at rest is about 0.67. The end-systolic
atria to the ventricles (phase IV) or from the
volume (ESV) remaining in the ventricles at
ventricles to the aorta or pulmonary artery
phases I and III (! A, top). Opening and closing
The first phase of ventricular diastole or
of the valves is controlled by the pressures
isovolumetric relaxation now begins (phase
exerted on the two sides of the valves.
III; ca. 60 ms). The atria have meanwhile re-
Cardiac cycle. Near the end of ventricular
filled, mainly due to the suction effect created
tion. As a result, the central venous pressure
trical impulse, marking to the beginning of the
8 diastole, the sinoatrial (SA) node emits an elec- by the lowering of the valve plane during ejec-
(CVP) decreases (! A3, falls from c to x). The
P wave of the ECG (phase IVc, ! A1
and
p. 196ff.). This results in atrial contraction ventricular pressure now drops rapidly, caus-
(! A4) and is followed by ventricular excitation ing the atrioventricular valves to open again
(QRS complex of the ECG). The ventricular when it falls short of atrial pressure.
pressure then starts to rise ( ! A2, blue line) The filling phase now begins (phase IV; ca.
until it exceeds the atrial pressure, causing the 500 ms at rest). The blood passes rapidly from
atrioventricular valves (mitral and tricuspid the atria into the ventricles, resulting in a drop
valves) to close. This marks the end of diastole. in CVP (! A3, point y). Since the ventricles are
The mean end-diastolic volume (EDV) in the 80% full by the first quarter of diastole, this is
ventricle is now about 120 mL (! A4) or, more referred to as rapid ventricular filling (phase
2
precisely, 70 mL/m body surface area. IVa; ! A4). Ventricular filling slows down
The isovolumetric contraction phase now (phase IVb), and the atrial systole (phase IVc)
begins (phase I, ca. 50 ms). With all valves are and the awave of CVP follows (! A2,3). At a
closed, the ventricles now contract, producing normal heart rate, the atrial contraction con-
the first heart sound (! A6), and the ventricu- tributes about 15% to ventricular filling. When
lar pressure increases rapidly. The slope of this the heart rate increases, the duration of the
ascending pressure curve indicates the maxi- cardiac cycle decreases mainly at the expense
mum rate of pressure developed (maximum of diastole, and the contribution of atrial con-
dP/dt). The semilunar valves (aortic and pul- traction to ventricular filling increases.
monary valves) now open because the pres- The heart beats produce a pulse wave (pres-
sure in the left ventricle (! A2, blue line) sure wave) that travels through the arteries at
exceeds that in the aorta (black broken curve) a specific pulse wave velocity (PWV): the PWV
at about 80 mmHg, and the pressure in the of the aorta is 3–5 m/s, and that of the radial
right ventricle exceeds that in the pulmonary artery is 5–12 m/s. PWV is much higher than
.
artery at about 10 mmHg. the blood flow velocity (V), which peaks at
The ejection phase (now begins phase II; ca. 1 m/s in the aorta and increases proportionally
210 ms at rest). During this period, the pres- to (a) decreases in the compliance of aortic and
sure in the left ventricle and aorta reaches a arterial walls and (b) increases in blood pres-
190 maximum of ca. 120 mmHg (systolic pressure). sure.
In the early phase of ejection (IIa or rapid ejec-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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