Respiration during Exercise

Dalton's Law

Total pressure of a gas mixture equals the sum of the partial pressures of each gas

Partial pressure

Fractional concentration × barometric pressure

Atmospheric pressure at sea level

760 mmHg

Percentage oxygen in air

20.93%

Percentage nitrogen in air

79.04%

Percentage carbon dioxide in air

0.03%

PO2 at sea level

159 mmHg

Reason altitude affects breathing

Lower barometric pressure lowers PO2

Oxygen percentage at altitude

Same as sea level

What changes at altitude

Barometric pressure and PO2 decrease

Pulmonary circulation

Movement of blood between heart and lungs

Pulmonary circulation pressure

Lower than systemic circulation

Right cardiac output equals left cardiac output

True

Blood flow distribution in upright lungs

Greatest at lung base

Exercise effect on pulmonary blood flow

Increases blood flow throughout lungs

Ventilation-Perfusion Ratio (V/Q)

Matching of ventilation to blood flow

Ideal V/Q ratio

Approximately 1.0

Acceptable resting V/Q ratio

Approximately 0.5

Apex V/Q ratio

Greater than 1.0

Base V/Q ratio

Less than 1.0

Light exercise effect on V/Q

Improves matching

Heavy exercise effect on V/Q

Can create inequality

Oxygen transport in blood

99% bound to hemoglobin, 1% dissolved

Hemoglobin

Oxygen carrying protein in red blood cells

Oxyhemoglobin

Hemoglobin with oxygen attached

Deoxyhemoglobin

Hemoglobin without oxygen attached

Oxygen molecules per hemoglobin

4

Oxygen carrying capacity of hemoglobin

1.34 mL O2 per gram Hb

Average male oxygen carrying capacity

200 mL O2/L blood

Average female oxygen carrying capacity

174 mL O2/L blood

CO2 transport dissolved

10%

CO2 transport as carbaminohemoglobin

20%

CO2 transport as bicarbonate

70%

Most common form of CO2 transport

Bicarbonate

Chloride shift

Exchange of bicarbonate leaving RBC and chloride entering

Respiratory control centers

Medulla oblongata and pons

Medullary respiratory center

Controls breathing rhythm

Pontine respiratory center

Modifies breathing pattern

Neural control of ventilation

Central command and afferent feedback

Central command

The motor cortex stimulates ventilation

Mechanoreceptors

Muscle spindles, Golgi tendon organs, joint receptors

Chemoreceptors in muscle

Respond to H+ and extracellular K+

Central chemoreceptors

Located in medulla; sensitive to increased PCO2

Peripheral chemoreceptors

Located in carotid and aortic bodies

Carotid body stimuli

Increased PCO2, decreased pH, decreased PO2, increased K+, NE, temperature

Aortic body stimuli

Increased PCO2 and decreased pH

Most powerful ventilatory stimulus

Increase in PCO2

Effect of small increases in PCO2

Large increase in ventilation

Effect of decreased PO2 at sea level

Little effect on ventilation

Initial exercise ventilation increase

Caused primarily by neural input

Goal of ventilatory regulation

Maintain stable PCO2

Transition to steady-state exercise

Ventilation rises rapidly then gradually reaches steady state

PO2 during exercise onset

Temporarily decreases

PCO2 during exercise onset

Temporarily increases

Steady-state exercise ventilation

Becomes constant

Ventilatory drift

Gradual increase in ventilation during prolonged exercise

Cause of ventilatory drift

Increased temperature and norepinephrine

Incremental exercise test ventilation

Increases linearly then exponentially

Ventilatory threshold (VT)

Point where ventilation increases exponentially

VT in trained individuals

Occurs at higher exercise intensities

Cause of VT

Increased H+, lactate, K+, temperature, NE, central command

Lactate threshold and VT

Related but do not always occur at same workload

PO2 during incremental exercise

Decreases slightly

Typical PO2 decrease in untrained

10-12 mmHg

Hypoxemia

Large drop in arterial PO2 during intense exercise

Hypoxemia prevalence

40-50% of elite athletes

Hypoxemia more common in

Females

Cause of hypoxemia

V/Q mismatch and diffusion limitation

Pulmonary adaptation to training

Very little structural adaptation

Why lungs don't adapt much

Normal lungs already exceed gas transport demands

Reason ventilation decreases after training

Muscles produce less H+ due to improved oxidative capacity

Pulmonary limitation during exercise

Rare during submaximal exercise

When lungs may limit performance

Respiratory muscle fatigue or hypoxemia in elite athletes