Why is it so that less blood comes in heart while expiration and more blood comes in heart while inspiration?
Because venous return increases during inspiration.
With inspiration there is more negative intra-thoracic pressure and also simultaneous pulmonary vasoconstriction- so more blood flow.
In expiration, high intra-thoracic pressure constricts the veins drawing blood to the heart and also there is peripheral pooling in the reservoirs - so low venous return is there.
During inspiration… pulmonary vessels constricts so more blood flow to the R. heart… more venous return.
During expiration pulmonary vessels dilates so pooling of blood less blood goes to heart…
That’s the reason for S2 Split in Heart Auscultation