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What do you want to know about breathing? Answered in our newsletter



Lecture Notes of a Knowledgeable Respiratory Specialist

HM 273 Lectures 7 & 8 Outline

I chose these because they made the most sense to me based on the way I think of breathing. mgw


1. External vs. internal respiration

2. Pulmonary ventilation

3. Pulmonary diffusion

4. Transport of O2 and CO2

5. Gas exchange at the muscles

6. Regulation of pulmonary ventilation

7. Ventilation & Energy metabolism

8. Respiratory regulation of acid-base balance

9. Respiratory adaptations during exercise and training

1. External vs. Internal Respiration

External Respiration processes that move gases from outside body to the lungs and into the blood

i) Pulmonary ventilation - breathing

ii) Pulmonary diffusion - exchange of O2 and CO2 between lungs and blood

Internal Respiration process of gas exchange between blood and tissues

i) Capillary gas exchange - exchange of O2 and CO2 between capillaries and metabolically active tissues

* NOTE: internal and external respiration

Figure 8.1 pg. 247 Wilmore & Costill (Ed.2)

(Figure 9.1 pg. 192 " (Ed.1))

2. Pulmonary Ventilation

Inspiration active process; requires external intercostal muscles and diaphragm

during heavy exercise additional muscles involved

Expiration a passive process in resting state

during forced breathing or heavy exercise an active process is involved requiring internal intercostal muscles

** NOTE: work of breathing during exercise


VV = VI or VE / min

Vt =

VC =


FEV1 =


3. Pulmonary Diffusion

2 Major Functions:

1. replenish blood’s O2 supply

2. remove CO2 returning from tissues

Respiratory Membrane: where gas exchanges between the air in alveoli and blood in pulmonary capillaries

3 components:

1. alveolar wall

2. capillary wall

3. basement membranes

Figure 8.3 pg. 250 Wilmore & Costill (Ed.2)

(Figure 9.4 pg. 196 " (Ed.1))

FICK’S Law of Diffusion: amount of gas that moves across a membrane is proportional to the surface area but inversely proportional to the thickness

\ blood gas barrier at alveoli is very thin and has a total area ~ 50-100 m2

Partial Pressure of Gases

individual pressure from each gas in a mixture

DALTON’S LAW: the total pressure of a mixture of gases equals the sum of the partial pressures of the individual gases in the mixture

air we breathe:

Table 1. Partial Pressures in Room Air



Standard Atmospheric Pressure





760 mmHg

159.1 mmHg



760 mmHg

0.2 mmHg



760 mmHg

600.7 mmHg

Gas Exchange in Alveoli

Fig. 8.4 pg. 252 Wilmore & Costill (Ed.2)

(Fig. 9.5 pg.197 " (ED.1))

4. Transport of O2 and CO2

O2 Transport

O2 is transported in blood by Hb (98%) or dissolved in plasma (2%)

only 3 ml O2 dissolved per L plasma

x 3-5 L plasma = 9-15 ml dissolved O2

at rest we need 250 ml O2/min

require O2 bound to Hb

Table 2. Hemoglobin O2 binding



[Hb] (g/100ml blood)



O2 content of blood (100% sat.)



O2 content of blood (98% sat.)



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))


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:

i) dissolved in plasma (10%)

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




in Lungs (alveolar capillaries)

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


                             carbonic      CO2 expired


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

PCO2 muscle > PCO2 venous blood > PCO2 alveolar

\ ventilation (to keep alveoli 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 brain activate neural input to rate and depth of breathing

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

change in PCO2 is 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, PCO2 and 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+), PCO2 and temperature

3. recall: EPOC

Breathing Abnormalities During Exercise


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



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 weight lifters

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 pressurised in lungs

2. restriction of venous due to collapse of great veins

3. if over extended periods of time will cardiac output

7. 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))


thought to occur when O2 requirements > O2 delivery (> VO2max)

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 VE/VO2

2. in VE/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

Q. 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?

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