Brain stem is a vital part of the human brain that connects it to the spinal cord. The brain stem consists of the medulla, pons, and midbrain, with the first two structures being key to our breathing. Signals for inhalation originate in the dorsal medulla, travel through nerves, reach different groups of breathing muscles that lift the rib cage and move the diaphragm downward, allowing the lungs to expand. At the end of inhalation, the ventral medulla sends out another signal to cease inhalation, relaxes the breathing muscles, and initiates exhalation. The pons is located above medulla and controls the intensity and frequency of the breathing signals released by the medulla. When the breathing is too fast, the pons will cut off signals and slow down inspiration; when breathing becomes shallow, it encourages and prolongs inhalation.
To ensure a smooth transition between each breath, complex regulatory mechanisms exist to transmit signals from the brain stem to the breathing muscles, and send feedback to the brain stem. The primary function of our respiratory activities is to moving air in and out of the lungs. Numerous sensors are located in the windpipe and lungs, primarily monitoring the airflow within the respiratory system. They sense the diameter of the airway, the pressure in the lungs, the size of the lungs, and the speed of airflow. Moreover, these sensors can detect whether there is an excess of fluid in the lungs. All of this sensory information is sent back to the respiratory center of the brain in rea-time to ensure that respiratory activities match the demands of our body.
Another group of sensors exists in skeletal muscles and monitors metabolic byproducts, primarily lactic acid. During exercise, the production of lactic acid increases dramatically. Consequently, these metaboreceptors are activated and signal the respiratory center to increase both the frequency and depth of breathing.
In addition to mechanical and metabolic sensors, chemoreceptors play a crucial role in the regulation of respiratory activities. Each breath serves two purposes: to inhale oxygen and increase the oxygen content in the blood, and to exhale carbon dioxide, a metabolic byproduct. After oxygen enters the blood, a small fraction dissolves in the blood, while the majority binds to hemoglobin, forming oxygenated hemoglobin. Interestingly, cells can only utilize oxygen molecules dissolved in the blood, so oxygenated hemoglobin serves as a reservoir and releases oxygen molecules based on demand. This mechanism is key to maintaining a steady oxygen level in the blood. In certain conditions, such as pneumonia and high altitudes, the oxygen level in the blood decreases. This change can be detected by chemoreceptors located in large blood vessels, including the common carotid arteries and the aorta. Subsequently, the respiratory center is activated, compelling the individual to breathe faster.
When cells release carbon dioxide into the blood, things get complicated. Unlike oxygen, carbon dioxide dissolves much more readily in the blood and produce hydrogen ions, making the blood more acidotic. The same chemoreceptors that monitor oxygen levels also closely monitor the levels of carbon dioxide and hydrogen ions.. Once they detect a higher level of carbon dioxide or hydrogen ions, they signal the respiratory center to increase the depth of breathing.
Chemoreceptors are not only located in the major blood vessels, but also spread throughout the medulla, known as central chemoreceptors. Compared to peripheral receptors, central chemoreceptors primarily respond to changes in hydrogen ion levels within the central nervous system. When carbon dioxide levels rise in the blood, the pH in the central nervous system decreases, leading to faster and deeper breathing, and vice versa. It is important to note that while both peripheral and central chemoreceptors sense oxygen and hydrogen ion levels, central chemoreceptors are more sensitive to changes in hydrogen ion levels than to decreasing oxygen levels. This sensitivity is crucial for individuals with severe chronic obstructive pulmonary disease (COPD).
Since blood acidity and alkalinity play important roles in respiratory activity, you might wonder if certain diets can change how we breathe. The short answer is yes, but it is easier said than done. The key is not just the amount of acid or alkaline in the blood but rather the pH, which represents the balance between acidity and alkalinity.
I wouldn’t be surprised if the mechanisms described above seem confusing. However, let me walk you through several scenarios, and you will realize that it makes sense to have multiple different components monitoring and adjusting breathing.
Imagine you are traveling by air from Maldives, which is 3.5 ft above sea level, to Aspen in Colorado, which is 8,000 ft above sea level. Once you step off the plane, you will notice that your breathing becomes slightly heavier. That is because the oxygen content in the air at Aspen is much lower than that in Maldives. When you take a normal breath, you actually inhale fewer oxygen molecules in Aspen. If your respiratory system doesn’t make any adjustments, the total amount of oxygen you inhale in one minute will be much less. Consequently, the oxygen content in your blood will also decrease, a condition called ‘hypoxemia’. Chemoreceptors monitor changes in oxygen content. When they detect that the oxygen content in your blood falls below a certain threshold, the respiratory center is activated to increase the respiratory rate and strengthen the contraction of breathing muscles. Ultimately, you are able to breathe in more air per minute to supply your body with an adequate amount of oxygen.
In addition, the removal of carbon dioxide depends on exhalation. In general, the more air we exhale, the more carbon dioxide is removed from the blood. When you breathe in a large amount of air to improve the oxygen level in your blood, you also breathe out a higher level of carbon dioxide, which lowers the level of carbon dioxide in the blood and tends to make your blood more alkaline. All of these changes provide signals to suppress the activity of the respiratory center and slow down breathing. This is how the respiratory system maintains a balance to ensure an appropriate response and adapts to a new environment.
Let me give you another example: aerobic exercise. When you engage in aerobic exercise, you breathe faster to obtain more oxygen per minute because your muscles burn more oxygen. Carbon dioxide is the primary byproduct of oxygenation. When you breathe in more oxygen, you also have to produce more carbon dioxide. As explained earlier, carbon dioxide is highly soluble in the blood and makes the blood acidic. Besides carbon dioxide, lactic acid is another common chemical produced by muscles when oxygenation is incomplete, and it also generates more hydrogen ions in the blood. Once the peripheral and central chemoreceptors detect the changes in carbon dioxide and/or hydrogen ions, they signal the respiratory center to increase the rate and depth of breathing. During exercise, this feedback system ensures that you can breathe in enough oxygen to fuel your muscles and exhale carbon dioxide to maintain the pH value of the blood within a normal range.
After you stop exercising, your breathing won’t slow down immediately. For a short period of time, you continue to breathe fast and deep, because your heart and skeletal muscles still maintain at a high level of metabolism. As your heart and skeletal muscle no longer require extra oxygen, the produce of carbon dioxide and lactic acid decreases accordingly, and the amount of hydrogen ions in the blood declines. This information is collected by various sensors at both peripheral and central levels, and instructs the respiratory center to ultimately suppress respiratory activities. Then, your breathing returns to its usual rhythm.
Now you know how our brain controls the breathing. It is crucial to remember that breathing is an unconscious natural process that is influenced by various internal and external factors. Similar to the heartbeat, breathing must be spontaneous and continuous, so that we don’t ‘forget’ to breathe while concentrating on other tasks, sleeping or during a loss of consciousness. However, unlike the heartbeat, it is much easier to change the breathing pattern by adjusting the rate and depth of breathing. This provides an opportunity for us to influence respiratory activities and ultimately improve our breathing.
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