How is breathing initiated? How are breathing rate and depth adjusted, and which blood gas is most tightly regulated?

Breathing, also known as ventilation, is initiated by the respiratory control center located in the medulla oblongata of the brainstem. Neurons within this control center are responsible for generating rhythmic and spontaneous electrical impulses that stimulate the diaphragm and intercostal muscles to contract and relax, causing inhalation and exhalation respectively.

The rate and depth of breathing are adjusted by the respiratory control center through the input of chemoreceptors. These chemoreceptors are sensitive to changes in blood acidity, levels of oxygen (O2) and levels of carbon dioxide (CO2). There are peripheral chemoreceptors, which are located in the carotid bodies and the aortic arch, and central chemoreceptors, which are located within the medulla oblongata. The peripheral chemoreceptors primarily detect changes in O2 levels, while the central chemoreceptors primarily detect changes in CO2 levels and pH of the cerebrospinal fluid. When blood CO2 levels rise, they lower the blood pH, triggering the chemoreceptors to send signals to the respiratory control center. In response, the respiratory control center will increase both the rate and depth of breathing in order to eliminate the excess CO2 from the body and return the blood pH to normal. Conversely, if blood CO2 levels fall, the respiratory control center will decrease the rate and depth of breathing to retain CO2 and maintain normal blood pH. Similarly, when blood O2 levels fall, peripheral chemoreceptors will signal the respiratory control center, which will then increase the rate and depth of breathing in order to take in more oxygen and eliminate carbon dioxide.

The most tightly regulated blood gas is carbon dioxide (CO2). As mentioned earlier, chemoreceptors are sensitive to changes in blood CO2 levels and pH. CO2 levels in the blood have a direct impact on pH (higher CO2 levels lower the blood pH). Since the body needs to maintain a stable pH for physiological processes to function properly, it is essential that CO2 levels are tightly regulated. This is achieved through the feedback mechanism between chemoreceptors and the respiratory control center, which can quickly adjust the rate and depth of breathing to maintain proper blood CO2 levels and pH.