Inspiration decreases the pressure in the central veins and thus increases the pressure gradient between the peripheral and central veins thus augmenting venous return. Skeletal muscle contraction also milks the venous flow towards the heart and the valves in the veins prevent gravitational return backwards.
Myocardial contractility is mostly dependent on the level of sympathetic nerve activity and is also increased by circulating catecholamines and inotropic drugs like dopamine and dobutamine.Contractility also increases with the fibre length.