Tolerance Limits: pH

• Human tolerance limit is 7.35 to 7.45
• Carbon dioxide diffuses into red blood cells and transforms into carbonic acid 
• Carbonic acid dissociates into hydrogen ions that reduce blood pH
• pH is above tolerance limits = hypercapnia and respiratory acidosis
• pH is below the tolerance limit = hypocapnia and respiratory alkalosis
• Blood pH is regulated by breathing out carbon dioxide
• Cells produce CO2 as a waste product when they respire aerobically (especially during intense exercise)
• The CO2 produced is excreted by cells into blood by diffusion
• It dissolves into the blood forming carbonic acid which as a weak acid
• Too much CO2 breathed out, there is a decrease in carbonic acid dissolved in the blood and the blood pH would increase, as it is less acidic, this could occur when an individual hyperventilates
• Blood pH is detected by chemoreceptors in the medulla oblongata (in the brain)
• A change in the blood pH would cause nervous messages to be sent to the organs of the respiratory system (the diaphragm and intercostal muscles) to alter the rate of breathing, and therefore, the concentration of CO2 in the blood
• If the blood pH decreases, the rate of breathing would be increased to breathe out more CO2 and increase the pH back to 7.4
• if the blood pH increased, the rate of breathing would be decreased to breathe out less CO2 and decrease pH back to 7.4
• Our breathing rate is controlled by out blood pH and not the amount of oxygen available to cells