Background: Measurements of relative blood volume changes (⌬RBV) during hemodialysis (HD) are based on hemoconcentration and assume uniform mixing of erythrocytes and plasma throughout the circulation. However, whole-body hematocrit (Ht) is lower than systemic Ht. During HD, a change in the ratio between whole-body to systemic Ht (F cell ratio) is likely to occur as a result of a net shift of low Ht blood from the microcirculation to the macrocirculation. Hence, ⌬RBV may differ significantly from total blood volume changes (⌬TBV). Therefore, this study compared ⌬RBV and ⌬TBV during HD.Design, setting, participants, and measurements: Plasma and erythrocyte volumes were measured using 125 I-and 123 Iradioiodinated albumin and 51 Cr-labeled erythrocytes, respectively. After validation of the standardized method in two patients on a nondialysis day, seven patients completed the protocol during HD.125 I-albumin and 51 Cr-labeled erythrocytes were administered 20 min before the start of HD.123 I-albumin was administered at 160 min into the HD session to quantify and correct for 125 I-albumin leakage. ⌬RBV was measured continuously throughout HD. The F cell ratio was derived from whole-body and systemic Ht.Results: Total ultrafiltration volume was 2450 ؎ 770 ml. TBV declined from 5905 ؎ 824 to 4877 ؎ 722 ml during HD. Thus, TBV declined 17.3 ؎ 4.4%, whereas the RBV decline was only 8.2 ؎ 3.7% (P ؍ 0.001). The F cell ratio increased from 0.896 ؎ 0.036 to 0.993 ؎ 0.049 during HD (P ؍ 0.002).Conclusions: ⌬RBV significantly underestimates ⌬TBV during HD. The rise in F cell ratio strongly suggests that during HD, blood translocates from the microcirculation to the macrocirculation, probably as a cardiovascular compensatory mechanism in response to hypovolemia.