Using your knowledge of the Circulatory System, describe & explain how and why an increase in intensity of exercise leads to an increased heart rate.

Firstly we must understand what exercise does to the body and the Circulatory system. We know that muscles need ATP to contract, this is demanded much more during a more strenuous exercise. ATP is created during aerobic &anaerobic respiration. Aerobic respiration will cause more CO2 to be made, anaerobic respiration will cause lactic acid to be made- a poisonous acid that lowers the PH of blood and when in large quantities will cause cells to die as enzymes are denatured as they cannot work in the optimum PH. C02 on the other hand can form carbonic acid which dissociates to form H+ ions which will have a similar effect. In order to prevent such an event happening, our body has chemoreceptors such as in the carotid body or aortic body that detect these, higher CO2 concentrations or acidity. These receptors will send signals to the medulla oblongata causing it to, by the sympathetic nerve, send impulses to the SAN node in the heart. The SAN node will increase in rate of depolarisation, thus causing a faster rate of atrial systole and hence the AVN will also depolarise at a faster rate. THis in turn means there are a greater frequency of impulses that travelalong the bundle of his and purkyne fibres to make the ventricles contract faster and harder. A greater heart rate means CO2 is transported to the lungs faster, allowing a steeper concentration gradient to occur and allow more O2 to enter the blood, thus oxidising lactic acid to pyruvic acid, a less toxic acid, increasing the blood PH to its norm. Of course, we must remember that cells respiring faster require more oxygen for aerobic respiration and glucose. A faster heart rate means such nutrients can be transported around the body faster.