Skeletal muscle capillary function: contemporary observations and novel hypotheses.

Area of Science:

• Physiology
• Microcirculation
• Skeletal Muscle Biology

Background:

• The capillary bed is crucial for O2, substrate, and metabolite exchange.
• Chronic diseases impair capillary function, affecting muscle energetics and contraction.
• A disconnect exists between microcirculation specialists and physiologists regarding capillary function interpretation.

Purpose of the Study:

• To challenge the traditional 'capillary recruitment' hypothesis using modern techniques.
• To re-evaluate blood-myocyte exchange mechanisms in contracting skeletal muscle.
• To propose a revised theory of capillary function during muscle contraction.

Main Methods:

• Intravital microscopy of contracting muscle.
• Magnetic resonance imaging (MRI).
• Near-infrared spectroscopy (NIRS) and phosphorescence quenching.

Main Results:

• Direct observations contradict the 'capillary recruitment' theory.
• Most capillaries exhibit red blood cell (RBC) flux at rest.
• Contractions increase capillary exchange via longitudinal recruitment, not de novo recruitment.

Conclusions:

• The 'Kroghian' capillary recruitment hypothesis requires radical revision.
• Capillary surface area is recruited longitudinally along existing flowing capillaries.
• A new understanding of blood-myocyte exchange is needed for health and disease states.