Introduction:
The duration of the cardiac cycle is the time it takes for the heart to complete one full cycle of contraction and relaxation. It is usually measured from the beginning of one heartbeat to the beginning of the next. In this case, the total time of the cardiac cycle is given as 1.6 seconds.

Breakdown of the Cardiac Cycle:
To understand why the duration of the cardiac cycle is 0.8 seconds, we need to examine the different phases and durations of the cycle:

1. Atrial Systole:
- Duration: 0.1 seconds
- Atrial systole is the contraction phase of the atria, during which blood is pushed into the ventricles. This phase is relatively short and contributes to the overall duration of the cardiac cycle.

2. Atrial Diastole:
- Duration: 0.7 seconds
- Atrial diastole is the relaxation phase of the atria, during which the atria fill with blood from the veins. This phase is longer than atrial systole and allows sufficient time for blood to passively enter the ventricles.

3. Ventricular Systole:
- Duration: 0.3 seconds
- Ventricular systole is the contraction phase of the ventricles, during which blood is pumped out of the heart into the arteries. This phase is relatively short but crucial for efficient blood circulation.

4. Ventricular Diastole:
- Duration: 0.4 seconds
- Ventricular diastole is the relaxation phase of the ventricles, during which the ventricles fill with blood from the atria. This phase is longer than ventricular systole and allows time for optimal filling of the ventricles.

Calculation of Cardiac Cycle Duration:
To determine the duration of the cardiac cycle, we add up the durations of all the phases:

Atrial systol + Atrial diastol + Ventricular systol + Ventricular diastol = 0.1 + 0.7 + 0.3 + 0.4 = 1.5 seconds

However, the given total time of the cardiac cycle is 1.6 seconds. This discrepancy may be due to a small error in the provided values or an additional phase or interval not accounted for in the given information.

Conclusion:
In conclusion, the duration of the cardiac cycle is determined by the durations of the different phases of the heartbeat. The total time of the cycle is the sum of the durations of these phases. However, in the given information, the calculated duration of the cardiac cycle does not match the provided total time. Additional clarification or correction of the values is required to reconcile this discrepancy.