As you do, the expansion of your chest reduces the pressure in the thorax below atmospheric pressure.
This has two effects on the heart:
- Reduced pressure on the Vena Cava, means more venous return to the right side of the heart
- Increased capacity of the pulmonary arteries (as they are pulled open), reduced venous return to the left side of the heart
This imbalance of heart filling compromises left ventricular capacity. The resultant reduction in stroke volume causes a reduction in systolic blood pressure by <10mmHg.
Cardiac Output = Stroke Volume x Heart Rate
Cardiac output is maintained by a baroreceptor-mediated increase in sympathetic outflow to the heart resulting in an increased pulse rate.