ARCH-B-09: Cardiac Cycle Pressure/Valve Traces, ECG Intervals & Thorax Ventilation Mechanics

Heart valves are passive one-way flaps controlled strictly by pressure differences:
- **At X ($0.10\text{ s}$)**: $P_{\text{LV}}$ rises above $P_{\text{LA}}$ $\implies$ bicuspid (mitral) AV valve closes (producing the first heart sound, lub).
- **At Y ($0.40\text{ s}$)**: $P_{\text{LV}}$ falls below $P_{\text{LA}}$ during diastole $\implies$ bicuspid AV valve opens.
- **At $0.25\text{ s}$**: peak ventricular ejection $\implies$ aortic semilunar valve is open.