This video will examine the adjustments made by the respiratory system, in response to a single bout of exercise. We will also address other considerations related to lung function during exercise. The three major roles of the respiratory system during exercise are, one, to ensure that the partial pressure of oxygen in our arteries is well-maintained allowing for adequate oxygen delivery to the exercising muscles. And two, to eliminate both metabolic and non-metabolic carbon dioxide, thereby maintaining the partial pressure of carbon dioxide in our arteries. And three, to assist in the buffering of metabolic acids produced during intense exercise. Between inspiring oxygen and ambient air and it's eventually used in mitochondria, of working muscles, are the various components of oxygen transport. At sea level, the partial pressure of oxygen in the inspired air is 159 millimeters of mercury. By the time it reaches the alveolar sacs in the lungs, where gas exchange between the lungs and the blood vessels occurs, this partial pressure has dropped to 105 millimeters of mercury. The oxygen then diffuses down its concentration gradient into the lung capillaries, where it binds the hemoglobin located within our red blood cells. Over 98% of the oxygen transported in blood, is bound to hemoglobin. From there, the oxygen-rich blood is transported to the heart, where it can be pumped out via blood vessels to the exercising muscles. As covered in a previous video, the oxygen will be consumed in the muscle mitochondria for ATP production. During exercise, a decrease in pH or metabolic acidosis, plus an increase in body temperature will facilitate the unloading of oxygen off hemoglobin into muscle. Also, during exercise, the partial pressure of oxygen in the working muscles can drop to 20 millimeters of mercury, resulting in an even greater unloading of oxygen from hemoglobin into muscle. Not surprisingly, increasing red blood cell numbers will improve the oxygen carrying capacity of blood. This occurs to some extent as a result of endurance training. Further, this is the concept behind the performance enhancing technique of blood doping, which will be discussed in module three. A second important role for the respiratory system during exercise, is that of carbon dioxide removal. Accumulation of carbon dioxide in blood and tissues would be toxic. Thus, the respiratory system protects against this accumulation. Sources for carbon dioxide production can be both from metabolic and non-metabolic pathways. The metabolic sources come from the oxidative breakdown of our macronutrients for ATP production. The carbon dioxide produced in this way in muscle, then diffuses into the blood and is transported to the lungs for removal. As the name implies, non-metabolic carbon dioxide does not come from the metabolic breakdown of our macronutrients, but instead, comes from the buffering of acids produced during a high-intensity exercise. To help prevent the muscles and blood from becoming too acidic, the hydrogen ions are buffered by bicarbonate, which is then converted to carbon dioxide. This non-metabolic carbon dioxide is also transported to the lungs for removal. In this way, the respiratory system fulfills its third major role, that of acid-base regulation during exercise. Obviously, when we go from rest to exercise, our ventilatory rate increases. This refers to the mechanical process of moving air into and out of the lungs. This increase in ventilation will be dependent upon the exercise intensity, and the need to get oxygen into and carbon dioxide out of the body. The two ways to increase ventilation, are by increasing the tidal volume and breathing frequency. Notice that in this example when going from rest to maximal exercise, there is a 32 fold increase in ventilation, measured in liters of air exchanged by the lungs per minute. This is accomplished by increasing both tidal volume, measured in liters of air per breath and the breathing frequency measured in breaths per minute. However, please take note that it is more efficient from an oxygen exchange standpoint, to increase the tidal volume more than that of the breathing frequency. This allows for greater time for gas exchange, as well as less repeated exchanges of air and the physiological dead space in the lungs clamming. Increasing breathing frequency also contributes to the increase in ventilation during exercise, but not nearly to the same extent as that for the increase in tidal volume. The increase in ventilation during exercise, is controlled by both neural and humoral or chemical mechanisms. At the onset of exercise, the large and rapid increase in ventilation is primarily controlled by neural pathways in the brain stimulating muscles involved in breathing. During submaximal steady state exercise, the fine-tuning of ventilation is accomplished by blood borne substances such as carbon dioxide, Oxygen and pH levels. During graded exercise, the generation of non-metabolic carbon dioxide will result in a ventilatory threshold shown here. Basically, the exponential increase in lactic acid production results in a large increase in non-metabolic carbon dioxide production from the buffering of the acid. This non-metabolic carbon dioxide further stimulates the ventilatory response, resulting in this exponential increase or threshold in ventilation. For this reason, researchers and clinicians frequently measure the ventilatory threshold via indirect calorimetry to estimate the lactate threshold. Now let's discuss the few applied situations related to the respiratory system. I'm sure you've all seen athletes wearing these nasal strips. Do they really improve performance. Conceptually, their purpose is to reduce airway resistance in the nostrils which may increase airflow into the lungs. In reality, there is no scientific evidence that they improve performance. Because of the tremendous capacity of the lungs to increase ventilation even during maximal exercise, ventilation is not considered to be a limiting factor in performance. Thus, marginally reducing airway resistance in the nostrils will very likely have no effect on performance. The only potential benefit nasal strips may have would be psychological in nature or the placebo effect. Next, if you've ever visited the weight room, you may have heard a lot of grunting and groaning, as people are straining to lift very heavy weights. They are executing the Valsalva maneuver shown here. Basically, they are performing a forced exertion with their mouth closed while holding their breath. This results in a closed glottis which is part of the larynx. This maneuver creates compressive forces that increases the intrathoracic pressure collapsing the inferior vena cava. This can drastically reduce venous blood return to the heart, thereby decreasing the amount of blood pumped by the heart. During a prolonged Valsalva maneuver, this can lower blood flow to the brain resulting in dizziness and fainting. While young, healthy individuals can generally tolerate this state for a brief period of time, older individuals, or those with heart conditions can actually suffer a cardiac event, which can be lethal. So care must be taken in these populations to avoid the Valsalva maneuver when lifting heavy weights. The final situation I would like to discuss pertains to exercise-induced asthma. This is more common than you may think, affecting up to 20% of the general population, and 70 to 80% of individuals suffering from persistent asthma. Symptoms include coughing, wheezing and shortness of breath. Constriction of the smooth muscles surrounding the airways in the lungs, known as a bronchospasm, along with inflammation and buildup of mucus in the lungs, are the main mechanisms underlying exercise-induced asthma. Cold dry air is a common culprit, but air pollution, high pollen counts and other airborne chemicals can contribute to exercise-induced asthma. Susceptible individuals should take the proper precautions before exercising outdoors. In summary, adjustments to all components of oxygen transport are essential during exercise to ensure adequate oxygen delivery to muscles. A decrease in pH and an increase in temperature, allow for more oxygen to be unloaded from hemoglobin into muscles during exercise. Ventilation also assists with exercise-induced metabolic acidosis. Neural mechanisms are primarily responsible for the initial rapid rise in ventilation, while humoral mechanisms do the fine tuning during submaximal steady-state exercise. Care should be taken to avoid performing the Valsalva maneuver while lifting heavy weights. Also, individuals susceptible to exercise-induced asthma, should monitor environmental conditions, and take the proper precautions.
