Describe and explain the effect of an increase in muscular activity on the heart rate.

An increase in muscular activity increases the rate of aerobic respiration. As a result, more carbon dioxide is formed in the Krebs cycle and the pH is lowered. This reduction in pH is detected by chemoreceptors found in the wall of the caratoid arteries (the arteries that serve the brain), which increase the frequency of impulses that they send to the medulla oblongata in the brain. The centre of the medulla oblongata that speeds up heart rate then, in turn, increases the number of impulses to the sinoatrial node via the sympathetic nervous system. Therefore, the sinoatrial node produces electrical waves at a faster rate and the heart rate increases.