CARDIAC OUTPUT
The amount of blood pumped by heart per minute is called cardiac output. Cardiac output of humans is approximately 5 litres
per minute. Cardiac output is also the volume of blood flowing through either the systemic or the pulmonary circuit per minute.
It is determined by multiplying the heart rate with the volume of blood ejected by each ventricle during each beat, which is called
the stroke volume. Cardiac output = Heart rate × Stroke volume
= 72 beats/min. × 0.07 litre/beat
= 5.0 litres/min.
These values are within the normal range for a resting, average-sized adult. Coincidentally, total blood volume is also approximately
5 L, so essentially all the blood is pumped around the circuit once each minute. During periods of strenuous exercise in well-trained
athletes, the cardiac output may reach 35 L/min; the entire blood volume is pumped around the circuit seven times a minute. Even
sedentary, untrained individuals can reach cardiac outputs of 20-25 L/min during exercise.
CARDIAC CYCLE
Cardiac cycle is the term referring to the cardiac events that occur from the beginning of one heart beat to the beginning of the next
heart beat. The contraction phase is called as systole while the relaxation phase is called as diastole. The durations of a cardiac
cycle is 0.8 sec. The successive stages of the cardiac cycle are briefly described below.
(b) Beginning of ventricular systole :
The ventricles begin to contract due to a
wave of contraction, stimulated by the AV
node. The bicuspid and tricuspid valves
close immediately producing part of the
first heart sound.(c) Complete ventricular systole : When the
ventricles complete their contraction, the blood
flows into the pulmonary trunk and aorta as the
semilunar valves open.(d) Beginning of ventricular diastole :
The ventricles relax and the semilunar valves
are closed. This causes the second heart
sound.
(e) Complete ventricular diastole :
The tricuspid and bicuspid valves open
when the pressure in the ventricles
falls and blood flows from the atria
into the ventricles. Contraction of the
heart does not cause this blood flow.
It is due to the fact that the pressure
within the relaxed ventricles is less
than that in the atria and veins.
(a) Atrial systole : The atria contract due to a
wave of contraction, stimulated by the SA node. The
blood is forced into the ventricles as the bicuspid and
tricuspid valves are open.HEART SOUNDS
Two sounds are heard normally through a stethoscope (a medical instrument for listening the action of heartbeat) during each
cardiac cycle. The first sound is a low pitched slightly prolonged “lubb”, caused by sudden closure of the mitral and the tricuspid
valves at the start of ventricular systole. The second sound is a shorter, high pitched “dup” caused by vibrations associated with
closure of the aortic and pulmonary semilunar valves just at the end of the ventricular systole. The first sound has a duration of
0.15 seconds and a frequency of 25-45 Hz. The second sound lasts about 0.12 seconds with a frequency of 50 Hz.
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