the DRG, allowing the individual to relax after inspiration, allowing for control over the rate of respirations. These regions of the brain do not act in a vacuum. They respond to basic systemic stimuli. The greater the stimulus, the greater is the response. The CO2 concentration is the major factor that controls the respiratory drive in the healthy person. There are central chemoreceptors in the brain and brainstem and peripheral chemoreceptors in the aortic arch and carotid arteries. An increased CO2 level will stimulate the respiratory effort, while a decreased CO2 level will inhibit the respiratory effort. Lactic acid will lower the pH and will increase the respiratory rate; lactic acid will increase during intense exercise. The hypothalamus and the cortex are also involved in influencing breathing. The hypothalamus responds to changes in temperature, emotions, and pain signals. These include things like the fight-or-flight response, which will act to increase the respiratory rate in response to stress.
ALVEOLI AND GAS EXCHANGE Gas exchange is the ultimate goal of the respiratory system. Pulmonary ventilation is what it’s called when the body brings air to the alveoli for the process of gas exchange. External respiration is what it is called when the gas exchange occurs at the level of the alveoli. In order to understand this type of respiration, you need to also understand Dalton’s gas law or the law of partial pressures. It indicates that the sum total of a mixed gas pressure is equal to the sum of the partial pressures of each gas in the mixture. Most of air is nitrogen gas at more than 78 percent. Oxygen is next at 21 percent. CO2 makes up just 0.04 percent of the total gas in atmospheric air. Water vapor actually has a higher concentration than CO2 at 0.4 percent of atmospheric air. This partial pressure is important because a gas will move toward from its area of higher partial pressure to its area of lower partial pressure. There is also Henry’s law to consider and that states that the concentration of gas in a liquid medium is proportional to both its partial pressure and its solubility in the liquid. 21
