Nuclear Magnetic Relaxation in Blood

The nuclear magnetic relaxation time T
1
of protons in human blood has been measured as a function of frequency, pH, and hematocrit. For whole blood at 25°C, T
1
is approximately 0.1 s at 20 kHz, increasing to approximately 1 s at 50 MHz. T
1
of whole blood is analyzed in terms of the exchange of water molecules between plasma and erythrocyte cytoplasm. A cellular residence time of 19 ms provides the best fit to the data. The T
1
values for plasma and cytoplasm are explained in terms of their protein content, using the well-established theory of nuclear relaxation in macromolecular solutions. The plasma and cytoplasm data are compared with previous T
1
results for apotransferrin and hemoglobin solutions, respectively, and qualitative agreement is found. The T
1
values increased with decreasing pH, as is expected from existing data on hemoglobin solutions.