Fig. 25.5 ). The blood-brain barrier is composed of endothelial cells, smooth muscle, and the pial and arachnoid membranes, and it regulates the movement of ions between blood and CSF. In addition, the choroid plexus also determines the ionic composition of CSF by transporting ions into and out of CSF. The blood-brain barrier is relatively impermeable by H+ and HCO3 − ions, but it is very permeable by CO2. Thus the PCO2 in CSF parallels the arterial PCO2 tension. CO2 is also produced by cells of the brain as a product of metabolism. As a consequence, the PCO2 in CSF is usually a few millimeters of mercury higher than that in arterial blood, and so the pH is slightly more acidic (7.33) in CSF than in plasma (
Table 25.1 •Fig. 25.5 Carbon Dioxide and the Blood-Brain Barrier. PaCO2crossestheblood-brainbarrierandrapidlyequilibrateswithCO2incerebrospinalfluid(CSF).H+ andHCO3− ionscrossthebarrierslowly.Thepartialpressureofarterialcarbondioxide(PaCO2)combineswithCO2generatedbymetabolismtodilatethesmoothmuscle.Incomparisonwitharterialblood,thepHofCSFislowerandthePCO2ishigher,withlittleproteinbuffering.
PCO2,partialpressureofcarbondioxide.
The carotid and aortic bodies are peripheral chemoreceptors that respond to changes in PaO2 (not the O2 content), PaCO2, and pH, and they transmit afferent information to the central respiratory control center. The peripheral chemoreceptors are the only chemoreceptors that respond to changes in PaO2. The peripheral chemoreceptors are also responsible for approximately 40% of the ventilatory response to PaCO2. These chemoreceptors are small, highly vascularized structures. They consist of type I (glomus) cells that are rich in mitochondria and endoplasmic reticulum. They also have several types of cytoplasmic granules (synaptic vesicles) that contain various neurotransmitters, including dopamine, acetylcholine, norepinephrine, and neuropeptides. Afferent nerve fibers synapse with type I cells, and they transmit information to the brainstem through the carotid sinus nerve (carotid body) and vagus
TheHenderson-Hasselbalch equation relatesthepHofCSFtotheconcentrationofbicarbonate([HCO3−]): whereα isthesolubilitycoefficient(0.03mmol/L/mmHg)andpKisthenegativelogarithmofthedissociationconstantforcarbonicacid(6.1).TheHenderson-HasselbalchequationdemonstratesthatanincreaseinCSFPCO2causesthepHofCSFtodecreaseatanygiven[HCO3−].ThedecreaseinpHstimulatesthecentralchemoreceptorsandtherebyincreasesventilation.ThusCO2inbloodregulatesventilationbyitseffectonthepHofCSF.TheresultinghyperventilationreducesthePaCO2,andthereforethePCO2ofCSF,andreturnsthepHofCSFtowardanormalvalue.Furthermore,cerebralvasodilationaccompaniesanincreaseinPaCO2,andthisenhancesthediffusionofCO2intoCSF.Incontrast,anincreaseinCSF[HCO3−]causesanincreaseinthepHofCSFatanygivenPaCO2.