Final answer:
The heart rate increases during exercise primarily due to the increased demand for oxygen by muscles, resulting in a higher cardiac output to meet the body's needs.
Step-by-step explanation:
One reason why the heart rate increases during exercise is due to the increased demand for oxygen by muscles. When you exercise, your muscles work harder and require more oxygen to produce energy. In response to this demand, the heart increases the number of beats per minute to pump more oxygen-rich blood to the muscles. This is essential for maintaining the vascular homeostasis and ensuring that the body's tissues receive enough oxygen to function properly.
During exercise, not only does the heart rate increase, but so does the stroke volume (SV), which is the amount of blood pumped out of the heart with each beat. This increase in both heart rate and stroke volume results in a significant rise in cardiac output (CO), which is the total volume of blood the heart pumps per minute. In healthy individuals, cardiac output can increase 4-5 times the resting rate during exercise, while top cardiovascular athletes can achieve even higher levels.