Reflex changes in ventilation help to defend blood pH. By changing the Pco2 and, hence, [H2CO3] of the blood, the respiratory system can rapidly and profoundly affect blood pH. As discussed in Chapter 22, a fall in blood pH stimulates ventilation, primarily by acting on peripheral chemoreceptors. An elevated arterial blood Pco2 is a powerful stimulus to increase ventilation,- it acts on both peripheral and central chemoreceptors, but primarily on the latter. CO2 diffuses into brain interstitial and cerebrospinal fluids, where it causes a fall in pH that stimulates chemoreceptors in the medulla oblongata. When ventilation is stimulated, the lungs blow off more CO2, making the blood less acidic. Conversely, a rise in blood pH inhibits ventilation,- the consequent rise in blood [H2CO3] reduces the alkaline shift in blood pH. Respiratory responses to disturbed blood pH begin within minutes and are maximal in about 12 to 24 hours.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.