As blood flows from the arterioles into the capillaries the haemoglobin (Hb) concentration falls by approximately 50% to a level of 7 g/dL in the smallest capillaries due to a progressive increase in the relative amount of plasma in blood flowing through vessels < 300µ in diameter (the Fåhraeus effects). The effects reverse on the venous side of the capillaries – dynamical effects on this reversal process have not been described hitherto. Blood sampled from the human finger pulp comes predominantly from venules and small veins < 300µ and represents the physiological space where the Fåhraeus effects are reversed. The Hb content of blood from the finger pulp has been observed to vary in relation to the Hb content of venous blood in the same individuals with sex and venous Hb levels(1,2,3). We have now studied the relationship between venous and finger pulp Hb levels within individuals over time. 465 blood donors with hereditary haemochromatosis, 360 males and 105 females, underwent repeated 450 ml venesections at a single clinic over a 3 year period. The mean age at first venesection was 50 years (range 24 to 71 years) for men and 52 years (range 21 to 70 years) for women. Men had a mean of 6.3 venesections over the period of the study, range 1 – 26; females a mean of 4.88 venesections, range 1 – 28. The difference between finger pulp and venous Hb levels for each donation was plotted against the finger pulp Hb level for the entire cohort of donors and for each individual donor. For each 1g/dL change in finger pulp Hb level, the mean difference between venous and finger pulp levels changed by -0.694 g/dL (Figure 1a). This relationship is significantly different from zero (p<0.001), the value expected for a 1:1 relationship between the Hb levels in the venous and finger pulp vascular spaces. Correcting for donor-specific effects and time-correlation of Hb levels at each donation date, finger pulp Hb levels recovered on average 0.496 mg/dL/day, (p<0.001), while venous Hb levels recovered slower: 0.113 mg/dL/day on average (p=0.019). The rate of finger pulp Hb recovery between venesections was 4.39 times faster than venous Hb recovery (t(4798)=2.743, p=0.006). These rates were not affected by sex or season (Figure 1b). These findings indicate that the Hb level of finger pulp blood affects the reversal of the Fåhraeus effects in blood flowing from smaller to larger vessels on the venous side of the circulation, consistent with vasodilation induced by deoxygenated red cells in the venous microvasculature. A sex difference has been observed before in the relative size of the gap between venous and finger pulp Hb levels per unit Hb level in venous blood (2,3); the net effect of the deoxyHb-associated effect may be to allow red cells in excess of resting requirements to be stored in a low pressure slow moving reserve on the venular side.