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Control of Ventilation
• Respiratory control center– Receives neural and humoral input
• Feedback from muscles
• CO2 level in the blood
– Regulates respiratory rate
Neural Input to the Respiratory Control Center
• motor cortex - impulses from cortex may “spill over” when passing through medulla on way to heart and muscles
• afferent - from GTO, muscle spindles or joint pressure receptors
• mechanoreceptors in the heart relay changes in Q
Humoral Input to the Respiratory Control Center
• central chemoreceptors - respond to changes in CO2 or H+ in CSF
• peripheral chemoreceptors - aortic bodies and carotid bodies – both similar to central receptors, carotids also
respond to increases in K+ and decreases in PO2
Ventilatory Control During Exercise
• Submaximal exercise– Linear increase due to:
• Central command
• Humoral chemoreceptors
• Neural feedback
• Heavy exercise– Exponential rise above Tvent
• Increasing blood H+
Respiratory Control during Exercise
• Central commmand initially responsible for increase in VE at onset
• combination of neural and humoral feedback from muscles and circulatory system fine-tune VE
• Ventilatory threshold may be result of lactate or CO2 accumulation (H+) as well as K+ and other minor contributors
Effect of Training on Ventilation
• Ventilation is lower at same work rate following training– May be due to lower blood acidity– Results in less feedback to stimulate breathing
Final Note
• the pulmonary system is not thought to be a limiting factor to exercise in healthy individuals
• the exception is elite endurance athletes who can succumb to hypoxemia during intense near maximal exercise
Acids and Bases
• Acid - compound that can loose an H+ and lower the pH of a solution – lactic acid, sulphuric acid
• Base - compound that can accept free H+ and raise the pH of a solution– bicarbonate (HCO3
-)
• Buffer - compound that resists changes in pH– bicarbonate (sorry)
pH
• pH = -log10 [H+]
– pH goes up, acidity goes down
• pH of pure water = 7.0 (neutral)
• pH of blood = 7.4 (slightly basic)
• pH of muscle = 7.0
Acid Production during Exercise
• CO2 - volatile because gas can be eliminated by lungs– CO2 + H2O <--> H2CO3 <--> H+ + HCO3
-
• The next point is erroneous
• Lactic acid and acetoacetic acid - CHO and fat metabolism respectively– termed organic acids– at rest converted to CO2 and eliminated, but during
intense exercise major load on acid-base balance
• Sulphuric and Phosphoric acids - produced by oxidation of proteins and membranes or DNA– called fixed because not easily eliminated– minor contribution to acid accumulation
Buffers
• maintain pH of blood and tissues
• accept H+ when they accumulate
• release H+ when pH increases
Extracellular Buffers
• bicarbonate - most important buffer in bodyremember the reactionhemoglobin - important buffer when deoxygenatedpicks up H+ when CO2 is being dumped into bloodproteins - not important due to low conc.
Respiration and Acid-Base Balance
• CO2 has a strong influence on blood pH
• as CO2 increases pH decreases (acidosis) CO2 + H2O > H+ + HCO3
-
• as CO2 decreases pH increases (alkalosis)
• so, by blowing off excess CO2 can reduce acidity of blood