ChangesinPaCO2thatresultfromalterationsinpHactivatehomeostaticmechanismsthatreturnthepHbacktowardanormalvalue.Theblood-brainbarrierregulatesthepHofCSFbyadjustingtheioniccompositionand[HCO3 −]ofCSF.ThesechangesinCSF[HCO3−],however,occurslowly,overaperiodofseveralhours,whereaschangesinCSFPCO2canoccurwithinminutes.ThuscompensationforchangesinthepHofCSFrequireshourstodevelopfully.
Several reflexes that arise from the chest wall and lungs affect ventilation and ventilatory patterns (
Table 25.2 ). The Hering-Breuer inspiratory-inhibitory reflex is stimulated by increases in lung volume, especially those associated with an increase in both ventilatory rate and tidal volume. This stretch reflex is mediated by vagal fibers, and when nerve (aortic body). Type I cells are the cells primarily responsible for sensing PaO2, PaCO2, and pH. In response to even small decreases in PaO2, there is an increase in chemoreceptor discharge, which enhances respiration. The response is robust when PaO2 decreases below 75 mm Hg. Thus ventilation is regulated by changes in arterial and CSF pH through effects on peripheral and central chemoreceptors (
Fig. 25.6 •Fig. 25.6 The Ventilatory Response to Partial Pressure of Arterial Carbon Dioxide (PaCO2) is Affected by the Concentration of Hydrogen ([H+]) in Cerebrospinal Fluid (CSF) and Brainstem Interstitial Fluid. Duringchronicmetabolicacidosis(e.g.,diabeticketoacidosis),the[H+]inCSFisincreased,andtheventilatoryresponsetoinspiredPaCO2isincreased(steeperslope).Conversely,duringchronicmetabolicalkalosis(arelativelyuncommoncondition),the[H+]inCSFisdecreasedandtheventilatoryresponsetoinspiredPaCO2isdecreased(reducedslope).Thepositionsoftheresponselinesarealsoshifted,whichindicatesalteredthresholds.
ImagineflyingfromNewYorkCitytoDenver.ThebarometricpressureinNewYorkisapproximately760mmHg,whereasinthemountainssurroundingDenver,Colorado,itis600mmHg.Atsealevel,thePaO2isapproximately95mmHgandPAO2= [(760− 47)× 0.21]− [40/0.8]= 100mmHg(accordingtothealveolarairequation;see ).Ifthealveolar-arterialPO2difference[AaDO2]is5mmHg,PaO2= 100mmHg− 5mmHg= 95mmHg.IntheCSF,pHwouldbeapproximately7.33,PaCO2wouldbe44mmHg(PaCO2+ CO2producedbymetabolismofbraincells),andHCO3− wouldbeapproximately22mEq/L.