How does cardiac output change with increased activity?

Cardiac output refers to the volume of blood pumped by the heart per minute and is a critical factor in determining how well the body can meet its metabolic demands during various activities. With increased physical activity, the body's need for oxygen rises significantly due to the greater demands placed on the muscles and organs. In response to this increased oxygen demand, the heart compensates by increasing cardiac output.

This increase in cardiac output is achieved primarily through two mechanisms: an increase in heart rate and an increase in stroke volume (the amount of blood ejected by the heart with each beat). During exercise or sustained physical activity, the autonomic nervous system stimulates the heart to beat faster while also improving the efficiency of each contraction. This physiological adjustment ensures that enough oxygen-rich blood reaches the working tissues.

In contrast, other responses such as energy conservation or maintaining a constant output do not adequately support the increased demands during heightened activity levels. The body must adapt to ensure that it can sustain the energy requirements of the muscles, thus solidifying why the correct response indicates that cardiac output generally increases to meet the body's oxygen demands during activity.