Respiratory system
From alveolar ventilation to mitochondrial utilisation: how oxygen is taken up, transported and utilised in the tissues. VO₂max as a measure of maximum oxygen uptake, and why the limiting factor depends on one’s health and fitness levels.
Auch bekannt als: Atmungssystem, Lunge, VO2max, Atmung, Gasaustausch, Atemmuskulatur
What is the role of the respiratory system?
The respiratory system renews the air in the alveoli and enables the exchange of oxygen and carbon dioxide. The resulting partial pressure conditions drive the passive diffusion of oxygen into the blood and of carbon dioxide out of the blood.
This function is part of the oxygen transport cascade: Atmospheric oxygen is absorbed, delivered to metabolically active tissue and used mitochondrially.
How oxygen gets from the air to the muscle cell
- Ventilation: The respiratory movement brings oxygen into the alveoli and removes carbon dioxide from the alveolar gas.
- Alveolar-capillary gas exchange: The partial pressures in the alveoli allow the diffusion of oxygen into the blood and of carbon dioxide out of the blood.
- Systemic transport: The blood brings the oxygen it absorbs to the working tissues.
- Deployment in the tissue: Blood circulation and oxygen transport connect the lungs with the metabolically active muscle.
- Mitochondrial usage: The mitochondria of the tissue use the oxygen provided to provide aerobic energy.
The stations form a connected chain. However, it does not follow from their order that the same station determines the individual performance limit for each person.
What VO₂max measures and what can limit it
VO₂max denotes the maximum oxygen uptake. It sets an upper limit for the sustained aerobic energy supply and therefore depends on several stations in the oxygen cascade.
In healthy people, central cardiovascular factors such as cardiac output and stroke volume as well as erythrocyte mass and blood flow to the skeletal muscles usually dominate the systemic oxygen supply.
This is not an unqualified statement against pulmonary limitation. The lungs can also be limiting in elite endurance athletes and those with pulmonary diseases. The relative importance of the individual factors therefore depends on the population.
A review of healthy elite endurance athletes reported that approximately 40 to 50 percent near VO₂max showed a significant decrease in arterial oxygenation. If pronounced exercise-induced arterial hypoxemia, i.e. a decrease in arterial oxygenation during exercise, this drop can limit VO₂max. A ventilation-perfusion imbalance and pulmonary diffusion limitation are discussed as possible contributions. In contrast, arterial oxygenation was mostly preserved in untrained and moderately trained people.
What lung measurements show in athletes
A systematic review evaluated 13 original studies with a total of 193 healthy athletes from various sports. The pooled standardized lung function values were above the respective target values.
- FEV₁: forced expiratory volume in the first second, 111 percent of predicted, 95 percent confidence interval 108 to 113 percent.
- FVC: forced vital capacity, 112 percent of predicted, 95 percent confidence interval 108 to 116 percent.
- TLC: total lung capacity, 106 percent of predicted, 95 percent confidence interval 103 to 108 percent.
- DLCO: Diffusion capacity of the lungs for carbon monoxide as a standardized measure of pulmonary gas exchange, 121 percent of the predicted value, 95 percent confidence interval 120 to 122 percent.
None of the included studies provided sufficient data to examine the association between lung function and VO₂max. This is a lack of data and does not prove that such a relationship does not exist.
What adaptations endurance training actually shows
In a 20-week training intervention, 631 healthy, previously sedentary men and women aged 17 to 65 completed 60 cycling sessions. Training was carried out three times a week, initially with around 55 percent VO₂max for 30 minutes and later with around 75 percent VO₂max for 50 minutes.
At 50 watts, heart rate and cardiac output decreased, while stroke volume and arteriovenous oxygen difference, i.e. the difference in oxygen content between arterial and venous blood, usually increased. Cardiac output fell by 0.6 l/min in the total sample. At 60 percent VO₂max, heart rate decreased while stroke volume, cardiac output and arteriovenous oxygen difference increased.
These measurements describe central responses during submaximal exercise. They do not test a direct pulmonary limit or the adaptation of highly trained endurance athletes or bodybuilders.
A systematic review with meta-regression pooled 5973 participants from 353 articles on mitochondrial adaptation and 131 articles on capillarization. Endurance training, high-intensity training and sprint interval training increased the mitochondrial content of skeletal muscles and VO₂max in the human studies evaluated.
The reported increase in mitochondrial content averaged 23 ± 5 percent after endurance training, 27 ± 5 percent after high-intensity training, and 27 ± 7 percent after sprint interval training. Capillaries per muscle fiber increased by 15 ± 3 percent, 13 ± 4 percent, and 10 ± 11 percent, respectively. Lower baseline fitness was generally associated with greater percentage gains.
These endpoints primarily affect the peripheral muscles. They do not show that a specific form of training changes the structural lungs and do not allow an individual ranking of all stations in the oxygen cascade.
What can be derived for endurance and strength training
A multi-part model is appropriate for interpreting an increasing VO₂max. Central circulatory adaptations and peripheral muscle adaptations are documented, while the available athlete data shows no evaluable connection between lung function values and VO₂max. As a cautious deduction, a change in VO₂max should not automatically be attributed to lung adaptation.
Increased in resistance exercises Valsalva maneuver, i.e. a conscious breathing maneuver with short-term breath holding under tension, increases intra-abdominal pressure, and this pressure increases with load and level of exertion. One review considers it possible to support spinal stability and trunk stiffness. This is not a reliable statement about a higher actual load being moved or about injury prevention.
During resistance exercise, the maneuver is also associated with an increase in blood pressure. The health risks remained unconfirmed in the review. This does not mean that risks are excluded, nor does it make a general statement about long-term safety, because long-term clinical endpoints have not been reliably tested.
Which questions remain unanswered?
For trained bodybuilders, there is no direct human study on whether the lungs limit individual VO₂max or endurance performance. A longitudinal study that records lung function, VO₂max and a relevant performance endpoint together is also not covered by the existing evidence.
- The relationship between standardized lung function metrics and VO₂max could not be examined in the athlete overview due to a lack of sufficient data.
- The prevalence and moderators of exercise-induced hypoxemia at submaximal exercise and at different training statuses remain incompletely understood.
- For heavy basic exercises, there is a lack of common data on trunk stability, actual load moved, risk of injury and blood pressure under standardized Valsalva bracing.
Sources
- Dominelli PB et al. The Oxygen Cascade During Exercise in Health and Disease. Mayo Clin Proc. 2021. PubMed
- Petersson J et al. Gas Exchange in the Lung. Semin Respir Crit Care Med. 2023. PubMed
- Joyner MJ et al. Central cardiovascular system limits to aerobic capacity. Exp Physiol. 2021. PubMed
- Powers SK et al. Exercise-induced hypoxaemia in elite endurance athletes. Incidence, causes and impact on VO2max. Sports Med. 1993. PubMed
- Thomsen RS et al. Standardised lung function metrics in healthy athletes. Scand J Clin Lab Invest. 2025. PubMed
- Wilmore JH et al. Cardiac output and stroke volume changes with endurance training: the HERITAGE Family Study. Med Sci Sports Exerc. 2001. PubMed
- Mølmen KS et al. Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression. Sports Med. 2025. PubMed
- Hackett DA et al. The Valsalva maneuver: its effect on intra-abdominal pressure and safety issues during resistance exercise. J Strength Cond Res. 2013. PubMed
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