Cardiorespiratory Function and Control During Exercise

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 capacity of the gases is relatively low, but during exercise the lungs have greater perfusion and faster blood flow allowing superior oxygen diffusion capacity (Wilmore).

Once in the bloodstream, oxygen is transported almost exclusively via hemoglobin. Due to low solubility capabilities, only 2% of oxygen can be supported in the dissolved state (Wilmore).

4O2 + Hb в†" nH+ + Hb(O2)4

(deoxyhemo- (oxyhemo-

globin) globin)

Thus, oxygen binds to hemoglobin, making oxygen capacity 70 times greater. As seen by the equation, the reaction is affected by oxygen and H+ concentration. Exhibiting the effects described by Le ChÐ"Ñžtelier’s Principle, a high pH and PO2 will yield the greatest saturation of oxyhemoglobin, the form of hemoglobin when it is combined with oxygen. Conversely, a low pH and low PO2 will result in a low concentration of oxyhemoglobin. Therefore, a relatively high pH of about 7.4 is desired throughout most of the bloodstream to maximize carrying capacity. On the other end, in the delivery of oxygen to metabolically active tissue, a lower pH is desired. Thus the generation of acid during cellular respiration is beneficial to the unloading of oxyhemoglobin in the surrounding area. Like all previous oxygen transfers, delivery of oxygen from the blood to metabolically active tissues occurs because of partial pressure differences. The aerobic activity constantly occurring in the tissues uses the oxygen and decreases its partial pressure creating large pressure differences. The last major contributor to the transfer of oxygen is the structure of hemoglobin. The molecule is a tetrameric hemeprotein, which allows for one allosteric bonding site in each of the four monomeric parts. This means that oxygen does not bond at the active site of the protein, rather a heme group that contains a central iron ion, Fe2+ (Lefers). The advantages of hemoglobin are seen in the allosteric properties stemming from its quaternary structure, the arrangement of its subunits. As each oxygen molecule bonds to hemoglobin, the allosteric properties change hemoglobin’s structure meaning oxygen acts as a homotropic effector that increases hemoglobin’s affinity for other oxygen molecules. This gives the hemoglobin an extra push to bond with oxygen as it reaches saturation in the pulmonary capillaries. As hemoglobin releases oxygen in the active tissues, oxygen is released more readily (King).

Carbon dioxide is transported in a similar but reverse system to oxygen. Instead of being transported by hemoglobin, however, most carbon dioxide is carried in the form of the bicarbonate ion.

CO2 + H2O в†' H2CO3 в†' H+ + HCO3-