Explain why heart rate increases to compensate for a physiological need.

When the body's tissues are not receiving enough oxygen (due to conditions like low hemoglobin, strenuous exercise, or high altitude), the cells cannot produce sufficient ATP through aerobic respiration. This state is called hypoxia.

The heart's primary function is to pump oxygenated blood to all body tissues. To compensate for reduced oxygen availability or increased demand, the cardiovascular system must increase the rate of oxygen delivery.

An increase in heart rate directly leads to an increase in cardiac output, which is the volume of blood pumped by the heart per minute. By pumping more blood per minute, more oxygen can be delivered to the tissues, helping to meet their metabolic demands and prevent hypoxia. This is a vital compensatory mechanism to maintain homeostasis.

The final answer is $\boxed{\text{Heart rate increases to increase cardiac output, thereby delivering more oxygenated blood to the tissues to compensate for insufficient oxygen supply or increased oxygen demand.}}$.