Notas de conferencias del terapeuta respiratorio que parecen ser relevantes para mi forma de pensar sobre la respiración

Note: Hb binds 1.34 ml O₂/g Hb, therefore

15 g Hb/100ml blood x 1.34 ml O₂/g Hb

= 20 ml O₂/100 ml blood

Oxygen-Hb dissociation curve

Figure 8.5 pg. 254 Wilmore & Costill (Ed.2)

(Figure 9.6 pg. 199 " (Ed.1))

Summary:

curvilinear relationship between PO₂ and %Hb saturation

rightshift of curve at low pH and high temperature ® both result in greater offloading of O₂ in muscle during exercise

CO₂ Transport

CO₂ released from cells is carried in blood in 3 forms:

1. i) dissolved in plasma (10%)
2. ii) as bicarbonate ion (60-80%)

iii) bound to Hb (20%)

in Muscle in RBC

CO₂ + H₂O ----------> H₂CO₃ -----------> H⁺ + HCO₃^-

                                         \

           carbonic                       H⁺ buffered by Hb

           anhydrase

in Lungs (alveolar capillaries)

H⁺ + HCO₃^- ---------> H₂CO₃ -----------> CO₂ + H₂O

                                          \

                             carbonic      CO₂ expired

                             anhydrase

Note: direction of the CA reaction is determined by the PCO₂ gradient:

PCO₂ muscle > PCO₂ venous blood > PCO₂ alveolar

\ ­ ventilation (to keep alveolar PCO₂ low) drives removal of CO₂ from tissues

5. Gas Exchange at the Muscles

Review: ® a-vO₂ diff = 4-5 ml/100ml blood at rest

= amount of O₂ taken up by tissues at rest

= proportional to O₂ use for oxidative ATP regeneration

Fig. 8.6 pg. 256 Wilmore & Costill (Ed.2)

(Fig. 9.8 pg. 201 " (Ed.1))

Factors influencing O₂ delivery and uptake:

O₂ content of blood

® not altered by exercise but ¯ by anemic conditions

amount of blood flow

by exercise

local cellular conditions

¯ muscle pH during exercise will ­ muscle O₂ supply

temperature during exercise will ­ muscle O₂ supply

muscle CO₂ will ­ O₂ unloading in muscle

Summary of Respiration

Fig. 8.7 pg. 258 Wilmore & Costill (Ed.2)

(Fig. 9.9 pg. 202 " (Ed.1))

6. Regulation of Pulmonary Ventilation

Goal: to maintain homeostatic balance in blood PO₂, PCO₂ and pH

Mechanisms of Regulation:

respiratory muscles regulated by motor neurons from respiratory centers in medulla oblongata and pons (FEEDFORWARD)

respiration also regulated by changes in chemical environment (FEEDBACK)

i.e. ­ in CO₂ and H⁺ in blood going to the brain activates neural input to ­ rate and depth of breathing

in PCO₂, H⁺ and ¯ PO₂ are sensed by chemoreceptors in the aortic arch and in the carotid artery

change in PCO₂ is the strongest stimulus for regulating breathing

stretch receptors in pleurae, bronchioles and alveoli stimulate expiratory centers to shorten duration of inspiration (Hering-Breuer reflex)

see Fig. 8.8 pg. 260 Wilmore & Costill (Ed.2)

(Fig. 9.10 pg. 203 " (Ed.1))

Regulation of Pulmonary Ventilation During Exercise

Start of Exercise: Two Phase Increase in Ventilation

1. immediate ® feedforward regulation, produced by mechanics of body movement

® motor cortex activated & stimulates inspiratory center

® proprioceptive feedback from skeletal muscle and joints provides input about movement

2. second, gradual phase ® feedback regulation from change in temperature and blood PO₂, PCO₂and pH

® stimulation of inspiratory centers by chemoreceptors

?? chemoreceptors in muscle and LV

see Fig. 8.9 pg. 261 Wilmore & Costill (Ed.2)

(Fig. 9.11 pg. 204 " (Ed.1))

Following Exercise:

1. energy demand drops immediately but pulmonary ventilation decreases at a relatively slow rate
2. slow recovery suggests post-exercise breathing regulated by acid-base balance (H⁺), PCO₂and temperature
3. recall:EPOC

Breathing Abnormalities During Exercise

DYSPNEA

shortness of breath

due to ­ CO₂ and H⁺ which ­ rate & depth of breathing

also due to poor conditioning of respiratory muscles \ respiratory muscles fatigue easily

HYPERVENTILATION

over breathing

due to increase in breathing >> metabolic need for oxygen

results in decreased stimulus to breathe due to ¯ CO₂ and ¯ H⁺

swimmers ® hyperventilate before competition

ADV ® improved mechanics during 1st 8-10s underwater

DISADV. ® alveolar & arterial PO₂ ¯

® may impair muscle oxidation & delivery of O₂ to CNS

® ¯ O₂ delivery AND drive to breathe

Valsalva Maneuver

during lifting of heavy objects; common in weightlifters

due to: 1. closing of glottis

2. ­ intra-abdominal pressure (forcibly contracting diaphragm & abdominal muscles)
3. ­ intrathoracic pressure (forcibly contracting respiratory muscles)

results in: 1. air trapped and pressurized in lungs

2. restriction of venous due to collapse of great veins
3. if over extended periods of time will ¯ cardiac output
4. Ventilation & Energy Metabolism

Ventilatory Equivalent for Oxygen

= ratio between volume of ventilation (VE) and amount of O₂ consumed (VO₂)

= VE/VO₂ (L air/ L O₂/min)

® rest values ~ 23-28 L air/ L O₂/min

® mild exercise: no change

® near max. intensity: ­ to ~ 30 L air/ L O₂/min

** Note: overall VE and VO₂ well matched

\ breathing control systems match body’s need for O₂

Ventilatory Breakpoint

= point where VE ­ disproportionately to ­ VO₂

= breakpoint in the VE vs. WL curve

see Fig. 8.10 pg. 264 Wilmore & Costill (Ed.2)

(Fig. 9.12 pg. 206 " (Ed.1))

Summary:

thought to occur when O₂ requirements > O₂ delivery (> VO_2max)

thought to reflect ­ requirement for glycolytic energy

Lac^- + H⁺ + NaHCO₃ --------> NaLac + H₂CO₃

H₂0 + CO₂

\ ­ inspiratory drive due to ­ VCO₂

Anaerobic Threshold

thought to be the threshold at which glycolytic metabolism ­

results in an ­ VCO₂:VO₂ and \ ­ RER

see Fig. 8.11 pg. 265 Wilmore & Costill (Ed.2)

Fig. 9.13 pg. 207 " (Ed.1))

Criteria for Defining Anaerobic Threshold:

1. ­ in V_E/VO₂
2. ® in V_E/VCO₂

\ ventilation ­ to remove CO₂ but is disproportionate to need for O₂

** Note: anaerobic threshold occurs ~ at lactate threshold BUT NOT ALWAYS THE SAME since they reflect different processes

(Hint: Review notes on Lactate threshold)

8. Respiratory Regulation Acid-Base Balance

Buffering Capacity of Blood

see Table 8.2 pg. 267 Wilmore & Costill (Ed.2)

(Table 9.3 pg. 209 " (Ed.1))

Major regulators of Blood pH

1. chemical buffers
2. pulmonary ventilation
3. kidney function

see Table 8.3 pg. 268 Wilmore & Costill (Ed.2)

(Table 9.4 pg. 211 " (Ed.1)

9. Respiratory Limitations to Performance

at rest respiratory muscles consume ~2% total energy

during heavy exercise energy cost of breathing ­ to 15% total energy

respiratory muscles show glycogen sparing and are fatigue resistant

\ do not likely "fatigue" during high intensity exercise

in average individuals, no ¯ alveolar PO₂ or hypoxemia

BUT in some elite athletes hypoxemia observed near exhaustion

1. Does the ventilation system limit exercise capacity??*

HM273, 1999 Lectures 7-8 Respiration and Ventilation

Wilmore & Costill Ed.2 pg. 245-270

Ed.1 pg. 191-211

*The bigger question is: Does exercise limit the ventilation system?