Acute responses of the respiratory system to exercise

In this final week of acute responses to exercise we looked at the respiratory system.

As we start to exercise we breathe more often and more deeply, and these are the two responses I am going to explain.

Increase in breathing rate

When we begin to exercise a number of changes happen to our blood. These are related to the increased demand for energy. In our working muscles, oxygen is combined with nutrients to create ATP through aerobic respiration. One of the by-products of this process is carbon dioxide. Carbon dioxide is a toxic chemical in our blood, as it lowers the pH of it, disrupting homeostasis. We know that once our blood becomes more acidic the rate of reaction of our enzymes slows down, and if this was to continue for too long, our body processes would stop all together and we would die. Therefore, our body must have a mechanism for getting rid of carbon dioxide, and that mechanism is to breathe it out.

During exercise we must breathe this out more rapidly, and therefore our breathing must speed up. This is controlled by chemoreceptors in our aortic arch and carotid bodies. These detect the increase in carbon dioxide, and send signals to the medulla and pons. These, in turn, send neural signals to the respiratory muscles to tell them to increase the rate of breathing. Mechanoreceptors located in the working joints, tendons and muscles also send signals to the medulla and pons as a back up mechanism.

It is often thought that we breathe more rapidly during exercise in order to increase the amount of oxygen that we breathe in. Obviously we do need more oxygen in order to increase the amount of energy we can reproduce, but actually it is the body's sensitivity to elevated levels of carbon dioxide that are responsible for the increase in breathing rate, and not the depletion of oxygen. This is because even during prolonged exercise, our muscles are unable to use oxygen at the rate it is supplied in the blood.