Cardiorespiratory Function and Control During Exercise

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Running, basketball, soccer, tennis, football: five very different sports, seemingly unrelated in any way. All focus on different skills and abilities and strengthen different parts of the body. Despite their differences, they are unified under several athletic components, most notably cardiorespiratory endurance. Whether we are laying in bed asleep, sitting listening to a teacher, or walking down the hallway, the cardiovascular and respiratory systems work together to regulate oxygen and waste throughout the body. When an activity becomes strenuous for a prolonged period of time, these systems must adapt to increase the capabilities of oxygen and waste management.

The main function of the respiratory system is the exchange of gases with the external environment. In conjunction with the cardiovascular system, the respiratory system forms an efficient method to deliver oxygen and remove carbon dioxide from the body. The transportation involves four separate processes: pulmonary ventilation, pulmonary diffusion, transport via blood, and capillary gas exchange. These processes transition from external respiration to circulatory transportation to internal respiration (Wilmore).

The first step of respiration, pulmonary respiration, is commonly referred to as breathing. In order to create constant partial pressures of gases within the lungs, the internal atmosphere must be exchanged with the air in the surrounding environment. The process is driven by relatively simple concepts; air will move to regions of least pressure until equilibrium is achieved. To accomplish this, the lungs expand and contract in the processes of inspiration and expiration. During inspiration the diaphragm contracts, flattening toward the abdomen while the external intercostal muscles push the ribs and sternum away from the body. This action creates a significantly greater volume of space within the lungs, simultaneously lowering the pressure within. Air rushes into the lungs to reduce the pressure difference. Expiration occurs passively at rest; all the active muscles of inspiration relax, decreasing lung size and increasing pressure. Again, air leaves the lungs to account for pressure differences. During exercise, both of these processes can involve a greater number of muscles allowing more rapid changes in pressure. Overall, pulmonary respiration is an effective method of maintaining gas concentrations within the lungs (Wilmore).

Next, the gases in the lungs and dissolved in the bloodstream must be exchanged. Throughout the lungs are substructures named alveoli that are surrounded by a dense network of capillaries. As the erythrocytes, commonly called red blood cells, pass through the tiny capillary vessels in single file, gases diffuse across the respiratory membrane into and out of the cells. This action is driven by the partial pressure differences between the gases in the blood stream and the gases in the alveoli. Though the partial pressure of oxygen (PO2) at standard atmospheric pressure (760mmHg) is 159mmHg, due to the extra water vapor and exhaled carbon dioxide, PO2 in the alveoli is reduced. The constant mixing with environmental air, however, maintains the PO2 at approximately 105mmHg. Compared to the alveoli’s partial pressure of oxygen, capillary blood generally enters the lungs with a PO2 difference of 60mmHg less than the alveoli’s PO2, or 45mmHg. This drastic pressure difference drives the oxygen to enter the blood stream until it also contains roughly 105mmHg and enters the venous ends of the capillaries, which will return the blood to the heart. Carbon dioxide exchange works in the converse fashion; the partial pressure of carbon dioxide (PCO2) of 45mmHg shifts to equilibrate with the 40mmHg in the alveoli. Though the pressure difference is not as extreme, the far greater solubility of carbon dioxide through the respiratory membrane allows it to diffuse through much more rapidly. At rest the diffusion...
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