Final answer:
Blood returns to the heart in the venous system due to the low pressure in the atria during diastole and the assistance of skeletal muscle and respiratory pumps, which create a pressure gradient favoring venous return. Vasoconstriction also helps by maintaining vascular tone.
Step-by-step explanation:
The return of blood to the heart in the venous system is aided by a pressure gradient that exists between the veins and the atria of the heart. This is crucial because, during atrial diastole, the atrial pressure is very low, often approaching zero, which enables blood to flow back into the heart.
Two primary physiological mechanisms assist in maintaining the necessary pressure gradient: the skeletal muscle pump and the respiratory pump. These pumps, along with venous one-way valves, facilitate the increase of blood flow velocity as the pressure decreases when blood moves from venules to veins.
Moreover, vasoconstriction helps in enhancing blood flow by maintaining vascular tone and preventing the veins from distending, which would otherwise impede the flow. The entire venous system is critical for returning deoxygenated blood to the heart, particularly the right atrium, to ensure that blood can once again be oxygenated via the lungs and circulated throughout the body.