The Gas Exchange And Transport

The changes in pulmonary ventilation and blood flow are actually regulated by the central nervous system through the respiratory and cardiovascular areas located in the brain. According to Sir Joseph Barcroft in 1934, exercise actually ‘forces’ both cardiovascular and respiratory system to perform at a higher level of function. [1] This helps us to understand better how both respiratory and cardiovascular systems interact with each other to perform well.

Gas Exchange and Transport

Gaseous exchange occurs in the alveoli of the human lungs. Air in the alveolus has a higher partial pressure of oxygen compared to the blood in the pulmonary artery. Therefore, oxygen diffuses into the blood at capillary by dissolving in the moisture on the alveolar surface. On the other hand, air in the alveolus has a lower partial pressure of carbon dioxide compared to the blood in capillaries. Hence, the carbon dioxide diffuses from the blood capillary to alveolus to be exhaled out. [2]

There are several factors which affect the gaseous exchange of carbon dioxide and oxygen between the blood capillaries and alveoli in the lungs. These factors are the surface area available for diffusion, the length of the diffusion pathway, and the hemoglobin concentration in the blood. At rest, not all the capillaries that surround the alveoli are open. During exercise, more alveoli and capillaries are opened which increase the surface area to allow a faster diffusion to occur. [1]

During exercise, there is also movement of fluid from blood into the surrounding cells and tissues. This is termed hemoconcentration. This will increase the concentration of hemoglobin in blood by 5% to 10%. [1] The increase in body temperature that causes the person to sweat will reduce plasma volume. This will produce hemoconcentration as well. This is the reason why during exercise, gas transport per unit volume of blood flow increases. [1]

Oxygen Dissociation Curve

Respiratory system is responsible for the exchange of oxygen and carbon dioxide between our body and the environment. When inspiratory muscles contract, air rushes into the lungs due to the higher pressure of external environment. Air is forced out from the lungs to the environment during expiration when the pressure inside thoracic cavity becomes higher. [2]

During exercises, active tissues such as skeletal muscles need more oxygen to generate ATP. [3] Therefore, they produce more carbon dioxide and the body temperature increase. This carbon dioxide will react with water in the tissue to form carbonic acid which increases acidity. Increase in the acidity of blood will reduce the affinity of hemoglobin to oxygen. As the result, more oxygen is released to these active tissues. [2]

For example, during moderate exercises, skeletal muscles need more oxygen and they produce more carbon dioxide due to the work out. So, the pH is decreased causing the affinity of hemoglobin to oxygen reduce. Hemoglobin with a lower affinity to oxygen has oxygen dissociation curve which is further to the right. Body temperature which increases during exercise will cause the shifting of oxygen dissociation curve to the right as well. [5]

According to Merle L. Foss and Steven J. Keteyian in their book, Physiological Basis for Exercise and Sport, the respiratory system change the rate and depth of ventilation to help regulate the hydrogen ion concentration of our body fluid. When body fluid pH decreases, for example, during exercises, ventilation increases to blow off carbon dioxide. When at rest, ventilation decreases to retain carbon dioxide in body fluid. [1]

Ventilation changes during exercise

Involuntary control of breathing is carried out by the breathing center in the medulla oblongata. [2] This breathing center consists of an inspiratory center and expiratory center. The partial pressure of carbon dioxide which also affects the pH of blood is the most important factor controlling the rate and depth of breathing. The chemoreceptors detect the changes in partial pressure of carbon dioxide of blood and cerebral spinal fluid. These chemoreceptors are the carotid bodies, the aortic bodies, and the medulla [1] that near the breathing center.

During moderate exercises, there is a rapid increase in the partial pressure of carbon dioxide in the blood. This is due to the accumulation of lactic acid in muscles. The increase in the partial pressure of carbon dioxide stimulates the chemoreceptors to transmit impulses to the inspiratory center. Inspiratory center transmits impulses to diaphragm muscles and intercostal muscle for rate and depth breathing. [3]

At the first few second after start the exercise, there is a rapid increase in the ventilation. This is due to the increase in the central command from cortex. The increase in the neural stimuli to medulla because of the activation of muscle or joint receptors may cause the hyperventilation as well. After that, the rapid ventilation start to achieve at steady state or it shows a slower rise. This is because chemoreceptors start to react to increase in the partial pressure of carbon dioxide and decrease in the pH of blood or cerebral spinal fluid. The ventilation continues to increase until the exercise is stop. [1] During normal breathing, a human adult inhales and exhales about 450cm³ of air. This is known as tidal volume. During vigorous activity, tidal volume can increase up to 2000cm³. [2] Oxygen uptake increases linearly as the work rate is increasing. However, above a certain work rate the oxygen uptake reaches a plateau. That’s mean there is a limiting factor to oxygen uptake. [3]

Structure of Human Heart

Human heart consists of 4 chambers, left atrium, right atrium, left ventricle and right ventricle. Both the left and right ventricles have thicker muscular wall compare to left and right atria wall because ventricles need to contract strongly to pump blood out of the heart. Whereas, the wall of left ventricle is 3 to 4 times thicker to right ventricle because left ventricle need to pump blood to all parts of our body except lungs while right ventricle pump blood to lungs only. The intraventricular septum separate left and right side of the heart completely. Left atrium receives oxygenated blood from lungs via pulmonary vein while right atrium received deoxygenated blood from the body through vena cava. [7]

[6]

Control of Heart Beat

Heartbeat is myogenic. This is because beating of the heart is started by cardiac muscles and not by external stimulation. Sino atrial node (SAN) which is also known as the pacemaker for the heart is responsible to originate excitation for starting the heartbeat. SAN have a high permeability to sodium ions. So, SAN cells are depolarized as sodium ions diffuse into these cells continuously. The depolarization will generate electrical impulse that transmitted out from SAN cells to produce contraction of heart. Atrial systole occurs when the wave of excitation is conducted from SAN to walls of both atria. The impulses that generate by SAN is then activates atrioventricular node (AVN). AVN then transmits the impulses to apex of the ventricles via bundle of his. From the apex, impulses are transmitted to ventricular muscles through purkinje fibers. This transmission causes ventricles to contract and hence pump blood into pulmonary artery and aorta. [2]

SAN can be accelerated or slowed down by the autonomic nerve system, endocrine system and some other factors. The amount of blood return to heart actually can induce the increase in the stroke volume and cardiac output of the heart. During exercise, the working skeletal muscles contract strongly and quickly. As a result, a large amount of blood is return to the heart via vena cava. There is stretch receptors (baroreceptors) located within the wall of the vena cava. When large amount of blood return to the heart, the vena cava dilates and this stretches its wall, stimulated the stretch receptors there. The stretch receptors then generate impulses at high frequency to transmit to cardiac accelerator center in the medulla oblongata. The stimulated accelerator center then transmits impulses via the sympathetic nerves to induce a faster and stronger heartbeat. [1]

According to Starling Law, the strength of the heartbeat is related to how much the cardiac muscles are stretched. Therefore, the more the volume of blood returned to the heart, the stronger the ventricle contracts. [4] Stroke volume increases due to the strong ventricular contraction, thus there is high blood pressure in carotid artery and aorta. Stretch receptors are stimulated and transmit impulses to cardiac inhibitory center to slow down heartbeat. This is to prevent the heart from beating too fast. [2]

Distribution of Blood Flow

At rest, majority of the cardiac output is distributed to the visceral organ, the heart and the brain. Only 20% of the total systemic flow is distributed to the muscles. [1] However, during exercise, more active skeletal muscles received a higher proportion of the cardiac output due to the redistribution of the blood flow. The metabolic active skeletal muscles will receive 85 to 90% of the total blood flow during maximal exercise. [1]

The redistribution of the blood flow is caused by the vasoconstriction of the arterioles at visceral organs and non-working skeletal muscles which are less active metabolically during the progress of exercise. The vasodilation of the arterioles which supply blood to the active skeletal muscles is also the reason that causing the redistribution of the blood flow. [1]

The vasoconstriction of the arterioles at non-active tissues in our body during exercises is due to the increase in both neural input and release of noradrenaline to the blood. On the other hand, the vasodilation of arterioles at active skeletal muscles during exercise is mainly due to initial reflex sympathetic nervous system response and chemical changes in the body. Those chemical changes include increase in temperature, partial pressure of carbon dioxide, hydrogen ions in plasma and blood, lactic acid level and decrease in the partial pressure of oxygen. The innermost layer of the arterial blood vessel will also release a vasodilation substance which is nitrous oxide to induce vasodilation of arterioles. [1]

Blood Pressure Regulation

Blood pressure is regulated by coordinating cardiac output and the diameter of the arteries. As cardiac output increases, blood pressure increases as well. Arterioles vasodilation lowers the blood pressure while arterioles vasoconstriction raises the blood pressure. The neurons from the vasomotor center in the medulla innervate the smooth muscles in all arterioles. [2]

During exercises, there is increase in the cardiac output which raises blood pressure and stimulating the stretch receptors in the aortic arch and carotid sinuses. The stretch receptors then transmit impulses to the vasomotor center in the medulla. The vasomotor center then responds by causes the arterioles to vasodilate to decrease the blood pressure. It may cause the cardiac output to decrease also. [2]

Blood pressure also affects by the partial pressure of carbon dioxide. During exercise, the increase in the partial pressure of carbon dioxide will stimulate the chemoreceptors located in the carotid bodies. The chemoreceptors then transmit the impulses to the vasomotor center in medulla that causes the arterioles to vasoconstrict. This can facilitate the carbon dioxide excretion as more blood can be transported to the lungs. [2]

Conclusion

After go through all the topics that we discussed above, we can conclude that all the adjustments make by respiratory and cardiovascular systems (cardiopulmonary) need to be controlled, coordinated and interact with one another well to operate at a higher level of function. Cardiopulmonary system is able to function efficiently because of the control of nervous system which involves both voluntary nervous system and involuntary nervous system.

As both cardiovascular and respiratory systems are interconnected with each other, therefore, the stimulation of one area such as the increase in the partial pressure of carbon dioxide will affect both ventilation and blood flow. As a result, to study physiological changes during moderate exercise, we need to study both cardiovascular and respiratory systems to understand better how they work.