The time to reach the peak fluorescence in the node, was noted for all animals and found to be (, excluding outliers beyond ), in the MB group, and in the IgG-IRDye group. Similarly, time to peak in the vessel was observed to be and in the MB and IgG-IRDye groups, respectively [see Figs. 4(a) and 4(b)]. The differences between these groups were found to be significant with for nodes and in the vessels. We have previously shown that raw intensity measurements on the nodes are highly unreliable and prone to high-signal heterogeneity, attributable to heterogeneity in delivery by the lymphatic vessel,16 and so vessel signal-based normalization improves the quantification of signal in the nodes provided that a single vessel delivers tracer, and tracer has not entered the blood stream. We report vessel-peak normalized node fluorescence in Fig. 4(c) and found that this ratio was in the MB group and in the IgG-IRDye group, revealing a statistically significant difference between groups with . All the above parameters are related to the slower rate of uptake of IgG-IRDye as compared to the much smaller MB molecule at the lymphatic–interstitial interface. If this flow was purely diffusive their relative transport times would be dominated by the Stokes–Einstein diffusion coefficient , () (where is the Bolztmann constant, is the temperature, is the viscocity, and is the particle radius). Given the relative sizes, , of the MB to IgG, their relative diffusion coefficients would be a ratio of 1:7.9. This ratio is interstingly very close to the measured ratio of time to peak [see Fig. 4(b)], 0.6:8, in the afferent vessels, and within the error bounds of the estimates, indicating that diffusive transport dominates this slower passive uptake process, although the ratio in the nodes appears further from this, at 0.8:65, likely due to differing kinetics of tracer exiting the nodes to downstream lymphatics and vasculature, a discussion of which is beyond the scope of this work. The above indicates that as long as the injected agent volume is constant and hence interstitial pressure is constant, diffusion dominates the uptake of molecules into the lymphatic capillaries.