Control of Ventilation

• Respiratory control center– Receives neural and humoral input

• Feedback from muscles

• CO2 level in the blood

– Regulates respiratory rate

Location of Respiratory Control Centers

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

Ventilation vs. Increasing PCO2

Ventilation vs. Decreasing 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+

Respiration Control during Submaximal Exercise

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

Training Reduces Ventilatory Response to Exercise

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

Acid-Base Balance

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

Acidosis and Alkalosis

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

Sources of H+

Buffers

• maintain pH of blood and tissues

• accept H+ when they accumulate

• release H+ when pH increases

Intracellular Buffers

• proteins

• phosphates

• PC

• bicarbonate

Insert table 11.1

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.

Buffering Capacity of Muscles vs. Blood

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

Changes in Lactate, Bicarb and pH vs. Work Rate

Lines of Defense against pH Change during Intense Exercise