Oxygen Transport Capacity: Rest vs. Exercise

Comparing Oxygen Transport Capacity: Rest and Exercise

Oxygen transport by blood is essential for proper cellular metabolism in all tissues of the organism. O₂ is transported in two forms:

• A small percentage circulates dissolved in the plasma; its solubility is very low (0.3 ml of O₂ in 100 ml of blood).
• The remaining 97% is carried by hemoglobin through reversible binding.

Under normal conditions, O₂ is transported to tissues almost entirely by hemoglobin. The resting oxygen consumption of a normal individual is about 250 ml/min, and intense exercise can increase this more than 10 times. Atmospheric oxygen is the source of oxygen consumed at the mitochondrial level and reaches the alveoli through ventilation. From there, it diffuses into the pulmonary capillary blood and is transported to cells by the circulatory system. While some oxygen is physically dissolved in plasma, more than 98% of the gas is transported in chemical combination with hemoglobin (Hb) in red blood cells.

The oxygen that diffuses from the alveolar spaces into pulmonary capillary blood is transported to cells by the circulatory system. The dissolved oxygen is only a small fraction of the total oxygen content of blood and is directly related to the partial pressure of oxygen. For each mmHg of partial pressure of oxygen, 0.003 ml O₂ dissolves per 100 ml of blood. Thus, under normal conditions, with P_aO₂ near 100 mmHg, this value is 0.3 ml/100 ml blood volume—totally inadequate for metabolic requirements.

However, dissolved oxygen has considerable physiological importance since its pressure determines both the degree of hemoglobin saturation and the diffusion or movement of oxygen from blood to tissues.

Key Factors Influencing Energy Cost and Oxygen Consumption During Exercise

The key factors involved in the energy cost and consumption of oxygen during exercise include:

1. Essential Features of the Effort:
   • Power: Higher power leads to higher O₂ consumption.
   • Muscle Mass: Greater muscle mass affected by the effort results in greater O₂ consumption.
   • Execution Speed: Speed determines O₂ consumption; there is an optimum speed that minimizes O₂ consumption. Above and below this optimum, energy expenditure and O₂ consumption are higher.
2. Duration of Activity: Oxygen uptake and energy expenditure increase over the duration of the activity, even without power variation, due to the gradual onset of fatigue.
3. Constraints and Systems: Work done under good ergonomic conditions means less energy waste and lower oxygen consumption. Hence the importance of biomechanical knowledge of the movement and its potential for improvement to reduce metabolic cost. Using materials that optimize effort can lead to significant energy savings.
4. Level of Training: Training increases energy efficiency, which means that it decreases metabolic cost and oxygen consumption. However, training cannot increase genetically determined aerobic fitness beyond 30%.
5. Climatic and Environmental Factors: Exercise performed under adverse conditions of temperature, humidity, unfavorable wind, or high levels of air pollution requires a higher O₂ supply.
6. Non-Genetic Factors: Age, sex, and motivation also play a role.