The distensibility of single capillaries and venules of the cat mesentery has been determined using a simple technique in vivo . Selected vessels were occluded under microscopic control and red cell oscillation in the occluded segment was monitored using cinemicrography. The cells were seen to oscillate in synchrony with local vascular pressure which was recorded simultaneously using the servo-null technique. Less than 10% of the oscillation could be accounted for by filtration/reabsorption so the bulk of the movement was thought to be a consequence of distension of the vessel during systole and relaxation during diastole. Changes in radius would be amplified by the factor 2¢l/r (where r is radius and ¢l the length of the occluded vessel) to give the observed movement of red cells. Mean pulse pressure was 3.2 mm Hg in capillaries and 2.3 mm Hg in venules which gave rise to increases in radius of 0.03 and 0.028 μm, respectively. The nature of the structure supporting the vessels was considered and it seemed likely to be the basement membrane which would require a Young's modulus of 1.8 × 10 6 N m −2 for the capillary and 5.4 × 10 6 N m −2 for the venule although pericytes might contribute to support of the latter. It is considered likely that some of the higher values quoted in the literature are due to measurements being made at different ends of a nonlinear stress-strain curve, the present data being obtained at the low stresses normally found in mammalian microvessels.
