Courtesy of Dr. Christopher Ellis PhD, Professor and Graduate Chair, Department of Medical Biophysics, Faculty of Medicine, University of Western Ontario, London, Ontario
This video is a good example of why Dr. Krogh chose the cylinder configuration for his model. The camera focuses from a depth of 92 μ to 18 μ through a living rat skeletal muscle. As the picture moves deep to shallow, open capillaries appear in parallel configuration following the muscle fiber orientation. The distance between adjacent capillaries can be measured and used to calculate the radius of the tissue sleeve around each capillary. These radii can then be averaged to calculate the size of the average Krogh cylinder.
In resting muscle the number of open capillaries is relatively small because the need for oxygen delivery and carbon dioxide removal is relatively small. However if the muscle were to begin working the number of open capillaries would increase. This would make each Krogh cylinder smaller and shallower, resulting in much more efficient oxygen delivery and carbon dioxide removal to meet the increased metabolic need. Because this is a resting muscle, this capillary recruitment phenomenon is not seen in this video. However the viewer can see several shunt capillaries that quickly open and close spontaneously in order to balance minor metabolic imbalances within the tissue.
The number of open capillaries within a specific volume of tissue is called perfused capillary density (PCD). PCD normally changes as the metabolic need changes, increasing as metabolic need increases and decreasing as metabolic need decreases.
Perfusion Theory is an educational platform for the Oxygen Pressure Field Theory (OPFT). August Krogh’s theoretical concept of the oxygen pressure field is explained and then applied to clinical applications in perfusion practice.