Respiratory System Response to Exercise - Exercise Responses

Composition of Air

• oxygen: 20.93%

• nitrogen: 79.04%

• carbon dioxide: 0.03%

Barometric Pressure at Sea Level

760 mm Hg

• PO2: 159 mm Hg

• PCO2: 0.3 mm Hg

• PN2: 600.7 mm Hg

Dalton’s Law of Partial Pressure

each gas contributes to the total pressure in proportion to it’s number of molecules

Partial Pressure

= total pressure x fraction of gas

Partial Pressure Inside Alveoli - Tracheal Air

becomes saturated with water vapor as it passes down the conductive zone

Water Molecules Disperse Gas Molecules

• increase the total volume of air

• decrease the gas pressure for a given volume of air

Factors Affecting Gas Exchange

1. partial pressure gradient across barrier (change in PO2)

2. diffusion capacity → “solubility” of the gas

3. characterisitcs of the barrier (surface area and thickness)

What Factors Increase Diffusion

• increased surface area

• increased pressure differences

• increase difussion coefficient

What Factor Decreases Difussion

tissue thickness

• thicker membrane = slower diffusion

Is CO2 or O2 More Soluble

CO2 is more soluble

Fick’s Law of Diffusion Equation

V gas = A/T x D x (P1 - P2)

Gas Exchange in Lungs

• alveoli: PO2 = 104 mm Hg; PCO2 = 40 mm Hg

• deoxygenated blood: PO2 = 40 mm Hg; PCO2 = 46 mm Hg

• oxygenated blood: PO2 = 95 mm Hg; PCO2 = 40 mm Hg

Ventilation-Perfusion Ratio

ratio of alveolar ventilation to pulmonary BF

Ideal Ventilation-Perfusion Ratio is

1

Efficient Gas Exchange Requires

BF to match ventilation

Average V:P Equals

0.84

• high values = too much VE

• low values = to much BF

Ventilation-Perfusion Ratio - During Exercise

• light exercise: V:P remains 0.8 L

• intense exercise: V:P of 5.0 L

Oxygen Transport in the Blood

99% of O2 is transported bound to Hgb

• amount transported depends on Hgb

Hemoglobin Equals

g/100 mL or “g%”

Normal Hemoglobin

15 g

• higher in men than women

Each Gram of Hgb Can Transport

1.34 mL of O2

Hemoglobin in Males and Females

• males: 15 g Hgn/100 mL blood x 1.34 mL O2/g Hb x 0.97 = 19.5 mL O2/100 mL

• females: 13 g Hgb/100 mL blood x 1.34 mL O2/g Hb x 0.97 = 16.9 mL O2/100 mL

Arteriole Blood Is

almost fully saturated

What Shifts Curve Right

• ↑ temperature 

• ↓ pH (more acidic)

• ↑ 2,3-DPG

What Shifts Curve Left

• ↓ temperature 

• ↑ pH (less acidic)

• ↓ 2,3-DPG

Effects of pH - During Exercise

• ↑ H+ weakens O2 and Hgb bond → unloading

• ↓ blood pH (more acidic) → right shift

Effect of Temperature

• ↑ temperature weakens O2 and Hgb bond → ↑ O2 unloading

• right shift

Effects of 2,3-DPG

• RBCs dont use O2

• anaerobic energy (2,3-DPG)

• can bind to Hgb & reduce Hgb’s O2 affinity

2,3-DPG Are Only a Factor During

exercise at altitude or with low Hgb

Oxygen Exchange - Myoglobin

facilitates O2 transfer to the mitochondria

• cellular PO2 decreases rapidly

• myoglobin retains high O2 saturation

Myoglobin More Readily

binds/retains O2 at low PO2

Greatest Quantity of O2 Releases From

MbO2 when tissue PO2 decrease below 5 mm Hg

What Dont Affect Myoglobin’s O2 Affinity

acidity, CO2 and temperature

System is Well-Designed To

• “offload” at the tissues (when needed)

• “load” O2 at the lungs

• opposite for CO2

Arteriovenous O2 Difference

difference between the O2 content of arterial blood and mixed venous-blood

• averages 4-5 mL of O2/100 mL of blood

Active Muscle Has Huge Capacity To

use available O2

O2 Supply Limits

aerobic exercise capacity → not muscle O2 use

CO2 Transport in Blood

• bicarbonate: 70%

• bound to hemoglobin: 20%

• dissolved in plasma: 10%

CO2 Transport in Blood - At the Tissue

• H+ binds to Hgb

• HCO3- diffuses out of RBC into plasma

• Cl- diffuses into RBC (chloride shift)

CO2 Transport in Blood - At the Lung

• O2 binds to Hgb (drives off H+)

• reaction reverses to release CO2

Pulmonary Ventilation

removes H+ from blood by the HCO3- reaction

Increase VE Results in CO2 Exhalation

↓ CO2 and H+ → ↑ pH (less acidic)

Decrease VE Results in Buildup of CO2

↑ CO2 and H+ → ↓ pH (more acidic)

At Onset of Constant-Load Submaximal Exercise

• VE increase rapidly initially

• slower rise toward steady state

During Exercise PO2 and PCO2

are relatively unchanged

• slight ↓ in PO2 and slight ↑ in PCO2

• increase alveolar ventilation is slower than increase metabolism

VE/VO2

• linear relationship during light/moderate exercise

• 25 L for healthy adults during submaximal exercise

VE/VCO2

• remains constant during steady state exercise

• VE removes CO2

Prolonged Exercise in Heat

• increase VE

• increase blood temperature affects respiratory control centre

Non-Steady State Exercise

• VE increase in proportion to VO2

• as exercise intensity increases, VE increases disproportionately to VO2

Incremental Exercise: Untrained Subject - VE

• linear increase up to 50-75% VO2 max

• expotential increase beyond this point

Ventilatory Threshold (VT)

inflection point where VE increase exponentially

Ventilatory Threshold is Due to

CO2 release from lactic acid buffering

Incremental Exercise: Untrained Subject - PO2

maintained within 10-12 mmHg of resting value

Incremental Exercise: Elite Athletes - VE

VT occurs at higher VO2 max

Incremental Exercise: Elite Athlete - PO2

decrease of 30-40 mmHg near maximal (hypoxemia)

• due to ventilation/perfusion mistach

• due to short RBC transit time in pulmonary capillary due to high CO