The Importance of Key Receptors in the Control of Breathing During Exercise

          The primary function of the respiratory system is to provide gas exchange between the atmosphere and cells within the body. This occurs in four continuous and concurrent processes. The first process is pulmonary respiration (ventilation). Pulmonary respiration occurs when air moves in and out of the lungs (i.e., breathing). Once air enters the lungs and reaches the alveolar sacs, oxygen (O₂) is pulled into the blood and carbon dioxide (CO₂) is shifted out through diffusion. Once O₂ enters the blood, it is transported to cells found throughout the body through a process known as systemic circulation (Powers & Howley, 2018). This circulatory transport process is the third continuous stage. Lastly, the fourth process is known as the systemic gas exchange process which occurs between the blood and cells. Just like O₂ entering the blood via the lungs, cells pull O₂ through the plasma membrane and CO₂ is shifted out and transported back to the lungs through pulmonary circulation and is then released through exhalation where the cycle repeats itself.

            During exercise, there is a demand for oxygen to fuel the brain, muscles, and other vital organs and components (e.g., neurons, etc.) responsible for movement and metabolic regulation. The amount of oxygen needed is dependent on what an individual is performing, nonetheless oxygen utilization is key. Two key structures, the medulla oblongata and pons found within the brain stem, are responsible for regulating the quantity and rate of gas exchange needed for the exercise demand. The following paragraphs will provide further explanation of the five receptors responsible for regulating the rate of gas exchange required dependent on the demands needed during exercise.

Medulla 

            Located within the anterior brain stem is a cone-shaped bundle called the medulla oblongata.  Within the medulla there are a bundle of nerves that are sensitive to changes in pH levels of the cerebrospinal fluid.  As exercise demand increases, CO₂ is released into the blood stream and sent to the brain, stimulating chemoreceptors to send signals to the respiratory control center to increase alveolar ventilation to remove CO₂ from the body and pull O₂ into the blood for energy use (Serna, Mañanas, Hernández, & Rabinovich, 2018; Powers & Howley, 2018). 

Carotid Body

            On the lateral aspect of the neck there is the common carotid artery that houses peripheral chemoreceptors. These receptors are sensitive to arterial PO₂, PCO₂, and pH levels. Just like the chemoreceptors in the medulla. When there is an increase in PCO₂, signals are sent to the respiratory control to stimulate breathing. While sensitive to PCO₂, one difference from the receptors in medulla are that they are also sensitive to pH and PO₂ (Kumar & Prabhakar, 2012). When there is a decrease in pH and PO₂ signals are sent to increase ventilation.

Aortic Body

            Descending the carotid artery passing into the aortic body are a cluster of peripheral chemoreceptors that are also sensitive to arterial pH and PCO₂. On a directional scale, these receptors are located along the aortic body, which is located between the arch of the aorta and the pulmonary artery. These receptors are sensitive to increases in PCO₂ and decreases in pH. As exercise demand increases there is an increase in arterial PCO_{2 ,} prompting cardiorespiratory changes. The receptors within the aortic body are sensitive to this change and send signals to the respiratory control center to increase ventilation and blood pressure on the onset hypoxia (Prabhakar & Peng, 2004).

Skeletal Muscle 

            With over 650 skeletal muscles in the body, there needs to be a detection system to regulate oxygen and carbon dioxide. There are two types of receptors within the muscle that aid in regulation. The skeletal muscles have mechanoreceptors and chemoreceptors, or metaboreceptors. Muscle contraction involves a metabolic process that changes the chemical state of the muscle. As muscle contraction occurs, mechanoreceptors send signals to the respiratory control system to increase breathing.  At the same time this is occurring the chemoreceptors are monitoring the chemical change inside and around the muscles (Joyner & Casey, 2015). This is a process that occurs throughout the body. As exercise increases muscle pH levels decrease, increasing the demand for oxygen. If adequate balance between demand and oxygen utilize occurs acute muscle fatigue sets in and power output starts to decline.

            The human body is a very intricate system that is set up for a continuous cycle of regulatory processes to balance the supply and demand of key elements for exercise. As submaximal exercise increases, so does breathing. This occurs through the communication between both neural and chemoreceptors input to the respiratory control center. The musculoskeletal, neuromuscular, and cardiovascular system are highly adaptable to exercise. Integrating exercise training into an individual’s routine has shown to promote a decrease in ventilation during exercise. Improving ventilation during exercise carries over to steady state life and is paramount if the goal is to reduce pulmonary respiration stress on the body during daily actives outside of exercise.

References

Joyner, M. J., & Casey, D. P. (2015). Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs. Physiological Reviews, 95(2), 549–601.

Kumar, P., & Prabhakar, N. R. (2012). Peripheral chemoreceptors: function and plasticity of the carotid body. Comprehensive Physiology, 2(1), 141–219.

Powers, S. K., & Howley, E. T. (2018). Exercise physiology: Theory and application to fitness and performance (10^th ed.). McGraw-Hill Education.

Prabhakar, N. R., & Peng, Y-J. (2004). Peripheral chemoreceptors in health and disease. Journal of Applied Physiology, 96, 359-366.

Serna, L. Y., Mañanas, M. A., Hernández, A. M., & Rabinovich, R. A. (2018). An Improved  Dynamic Model for the Respiratory Response to Exercise. Frontiers In Physiology, 9, 69.

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