What is defined as the amount of blood returned to the heart by the veins?

The correct answer is venous return, which refers specifically to the volume of blood that returns to the heart via the veins. This concept is critical in understanding cardiovascular dynamics because it plays a vital role in determining the overall efficiency of the circulatory system.

Venous return affects the filling of the heart's chambers during diastole (the relaxation phase), which in turn influences cardiac output—the amount of blood the heart pumps in a given time frame. A higher venous return typically leads to a higher stroke volume because the heart has more blood to pump, which is described by the Frank-Starling law of the heart.

In the context of the choices, cardiac output is the total volume of blood the heart pumps per minute, which is dependent on both the stroke volume and heart rate, rather than just the volume of blood returning to the heart. Blood pressure measures the force of blood against the walls of arteries, and stroke volume refers to the amount of blood pumped by the heart with each beat. While these concepts are interconnected, the specific definition of venous return directly relates to the amount of blood coming back to the heart from the veins.