Circulatory function during exercise: integration of convection and diffusion

Area of Science:

• Physiology
• Comparative Physiology
• Exercise Physiology

Background:

• The cardiovascular system's role in limiting maximal oxygen uptake (VO2 max) is a long-standing hypothesis in mammalian physiology.
• Previous research often focused on the heart's capacity to deliver oxygen as the primary determinant of VO2 max.

Purpose of the Study:

• To critically analyze the factors governing oxygen (O2) transport and determine if a single limiting step for VO2 max exists in mammals.
• To investigate the relationship between circulatory O2 transport capacity and VO2 max under various experimental conditions.

Main Methods:

• Analysis of O2 transport equations and experimental data from studies altering circulatory O2 transport capacity.
• Examination of physiological responses during maximal exercise under hypoxic and hyperoxic conditions.
• Consideration of allometric scaling, adaptive variation, and induced changes in cardiovascular and respiratory structures.

Main Results:

• No direct or absolute relationship exists between circulatory O2 transport capacity and VO2 max.
• Multiple components of the O2 transport system, not just the cardiovascular system, contribute to limiting O2 flux at VO2 max.
• Interdependence of convective and diffusive gas exchangers prevents a single limiting step; PO2 changes required at one step conflict with adjacent steps.

Conclusions:

• The concept of a single limiting step for O2 transport determining VO2 max is not supported by evidence.
• VO2 max is constrained by the interplay of multiple, interdependent components within the O2 transport system.
• While cardiovascular adaptations can increase O2 transport, limitations can arise from other structures (e.g., lungs) or mitochondrial capacity, particularly in species with strong selection for aerobic capacity.