continual rate of breathing by causing diaphragmatic breathing. The VRG is involved in forceful breathing by stimulating the accessory muscles in forced inspiration and expiration to contract. In the pons, there is a second respiratory center, the pontine respiratory group. This has both an apneustic center (to stimulate the depth of breathing in deep breathing) and the pneumotaxic center (that allows relaxation after inspiration, controlling the respiratory rate). Each of these brain centers responds to systemic stimuli in a feedback loop. Increased stimulation from the periphery causes forced breathing. It is the CO2 level that primarily controls respiration (not the O2 level). There are both central chemoreceptors and peripheral receptors (in the carotid arteries and aortic arch) that measure the concentration of hydrogen ions, CO2, and O2. Increased CO2 (and hydrogen ions) in the brain will cause the central chemoreceptors to trigger contraction of the intercostals and the diaphragm, increasing the rate and depth of respiration. This cause homeostasis and normalization of the CO2 and Hydrogen ion concentrations. Low CO2 levels decrease the rate and depth of respirations. Increased CO2 means increased hydrogen ions. The same is true of increased exercise that leads to lactic acid buildup. When the blood is more acidic, the ventilatory rate and depth will increase to remove CO2, normalizing the blood pH. If the O2 level is low (at 60 mm Hg or less), the peripheral chemoreceptors will increase the breathing rate. It takes a significant drop in O2 levels to cause this change in breathing. Increased body temperature will increase the respiratory rate. The hypothalamus and limbic system make one excited or trigger the fight-or-flight response, which will increase the respiratory rate as well. Pain and emotions will increase the breathing rate.
GAS EXCHANGE Gas exchange is the major function of the lungs; this happens at the respiratory membrane, which is the fusion of the alveolar wall and the capillary wall. The atmosphere we breathe in consists of mainly oxygen, carbon dioxide, and nitrogen—each of which has a partial pressure that adds up to a total air pressure. The partial pressure is the pressure of one type of gas in a gaseous mixture. In order of decreasing partial pressures, there is nitrogen (78 percent of air), oxygen (21 percent of air), water (0.4 percent of air), and carbon dioxide (0.004 percent of air). 285
