Effect of postural change and thermoregulatory stress on the capillary microcirculation of the human toe

1. We have applied the technique of television capillary microscopy to make direct measurements of nutritional capillary flow in the toe nailfold (i) at heart level and (ii) in a near standing position, with the foot 96 cm below heart level. By indirect heating we have examined the relationship between posture-related changes and thermoregulation in the capillary circulation of the toe. 2. Capillary blood flow in 14 male subjects with their feet at heart level showed a positive correlation with increasing skin temperature. 3. On quiet standing, capillary blood flow decreased to 11.3% of supine values, achieved both by a reduction in capillary blood velocity and the duration of flow. 4. Indirect heating of the trunk led to a partial release of sympathetic tone and induced a significant increase in capillary blood flow in both the supine and dependent positions. However during heating, the percentage fall in capillary blood flow associated with dependency remained unchanged. 5. In conclusion, it appears that in the dependent foot, compensatory mechanisms exist to limit nutritional capillary blood flow by regulating both erythrocyte velocity and the duration of flow. These mechanisms fulfil a physiological requirement to limit the rate of oedema formation. The preservation of this response during the partial sympathetic release induced by indirect heating suggests a mechanism independent of that controlling flow through arteriovenous shunts. Our results are compatible with the hypothesis that a local vasoconstrictor mechanism is operative in the vascular resistance elements in closest relation with the capillary bed, and distal to that regulating arteriovenous shunt flow.