Capillary blood transit time in muscles in relation to body size and aerobic capacity.

This study measures the minimum transit time for blood in muscle capillaries and how this correlates with body size and aerobic capacity. The research was conducted on six different species and showed that this transit time was positively correlated with body mass and negatively correlated with oxygen release rate. These findings suggest that blood flow, capillary blood volume and other factors are adjusted to match the oxygen demands of the muscles at peak oxygen consumption.

Research Context

• The researchers organized their study to discover the relationship between body size, aerobic capacity (the maximum amount of oxygen the body can use during intense exercise), and the transit time of blood in muscle capillaries – termed in the study as ‘tc’. This research was performed on a wide range of species, with varying body masses and aerobic capacities, to provide a diverse data set.
• They hypothesized that tc, blood flow, capillary blood volume and other physiological factors would adjust according to the body’s oxygen needs during peak consumption.

Research Methods

• The authors took measurements from six species running on a treadmill at their maximum oxygen consumption rate (VO2max). These species included horses, steers, dogs, goats, foxes, and agouti, all specifically chosen to span a significant range in body mass and aerobic capacity.
• The team measured arterial and mixed venous blood oxygen concentrations, blood hemoglobin concentrations, and oxygen uptake rates. Blood flow to the muscles was assumed to be 85% of cardiac output, calculated using the Fick principle (a calculation used in cardiovascular physiology for cardiac output).
• Total muscle capillary blood volume and total muscle mitochondrial volume were estimated using a morphometry technique, which involves measuring the shapes and sizes of tissues. Through these measurements, the researchers determined the tc by dividing the total capillary blood volume by the blood flow to the muscles.

Findings

• The study found that the tc varied between species, ranging from 0.3-0.5 seconds in 4 kg foxes and agoutis to 0.8-1.0 seconds in 400 kg horses and steers. This suggests a positive correlation between tc and body mass, with larger species having a longer tc.
• Additionally, the research found a negative correlation between tc and the oxygen release rate per unit of capillary length. This means that as the rate of oxygen release increased, the tc decreased.
• A positive correlation was noted between mitochondrial content and VO2max, suggesting a relationship between cellular energy production and aerobic capacity.
• Overall, the data indicated that blood flow, capillary blood volume, maximum hemoglobin flux, and tc are all adjusted together in a way that matches the oxygen needs of muscle tissues at VO2max. This suggests an efficient system of physiological coordination for maximum energy expenditure and output during intense physical activities.