Which of the following describes how the cardiovascular system supports high-intensity interval training (HIIT)?

• A. Slower heart rate response and reduced cardiac output; no change in capillarisation.
• B. Rapid heart rate and cardiac output responses with repeated bouts; improved stroke volume and capillarisation; shorter recovery periods rely on efficient oxygen delivery and clearance.
• C. Only respiratory rate increases; cardiovascular response is minimal.
• D. Submaximal stroke volume decreases and capillarisation reduces.

Answer: (B)

High-intensity interval training pushes the cardiovascular system to respond quickly and repeatedly. The heart must ramp up its output with each burst of effort to supply working muscles with oxygen and to remove waste like carbon dioxide and lactate. This means heart rate rises rapidly and cardiac output increases as the body meets the high metabolic demands.

With training, the heart becomes more efficient at delivering this support. Stroke volume can improve, so more blood is ejected with each beat. This, combined with faster heart rate responses, means the system can sustain repeated high-intensity bouts more effectively. At the same time, capillarisation—the development of more capillaries in muscle tissue—enhances oxygen delivery to muscles and the removal of metabolic byproducts. That improved microcirculation means intervals can be performed with shorter recoveries because oxygen delivery and clearance are more efficient, helping the body restore energy systems more quickly between efforts.

So, the best description is rapid heart rate and cardiac output responses with repeated bouts, along with improved stroke volume and capillarisation, and shorter recovery periods that rely on efficient oxygen delivery and clearance.