Cardiac Cycle
| Home |Chapter: HAP - Cardiovascular System
The cardiac cycle is the sequence of events that occurs from the beginning of one heartbeat to the beginning of the next.
CARDIAC CYCLE
The
cardiac cycle is the sequence of events that occurs from the beginning of one
heartbeat to the beginning of the next.
It
consists of two phases:
- Systole
– contraction of ventricles
- Diastole
– relaxation of ventricles
A
normal heartbeat lasts about 0.8 seconds, with a rate of 60–80 beats
per minute.
Duration
of Cardiac Cycle:
|
PHASE |
DURATION |
|
Ventricular systole |
0.3 sec |
|
Ventricular diastole |
0.5 sec |
|
Atrial systole |
0.1 sec |
|
Atrial diastole |
0.7 sec |
Heart
Sounds
Contraction
and relaxation of the heart produce sounds.
|
SOUND |
CAUSE |
|
First sound (Lub) |
Closure of AV valves |
|
Second sound (Dub) |
Closure of semilunar valves |
|
Third sound (S3) |
Rapid ventricular filling |
|
Fourth sound (S4) |
Atrial systole (abnormal) |
Electrical
Changes in the Heart (ECG)
Electrical
activity of the heart is recorded using an electrocardiograph, producing
an electrocardiogram (ECG).
ECG
Waves
|
WAVE |
MEANING |
|
P wave |
Atrial depolarization |
|
QRS complex |
Ventricular depolarization |
|
T wave |
Ventricular repolarization |
Cardiac
Output
Cardiac
output is the volume of blood pumped by each ventricle per minute.
Formula:
Cardiac
Output = Stroke Volume × Heart Rate
In
a normal adult:
- Stroke
volume ≈ 70 ml
- Heart
rate ≈ 72 bpm
- Cardiac
output ≈ 5 litres/minute
Stroke
Volume
Stroke
volume is the amount of blood pumped by the left ventricle in one beat.
Formula:
Stroke
Volume = End Diastolic Volume − End Systolic Volume
Factors
affecting stroke volume:
- Preload
- Contractility
- Afterload
