Measurement of the limb balance for any given substrate or hormone is simply obtained from the measurement of flow (F) to the limb or, preferably, directly to skeletal muscle multiplied by the arterial minus venous concentration difference.
It is important to know whether the substrate under study partitions only to the plasma compartment or to the erythrocyte
water and how quickly this occurs. For example, in humans the balance for glucose requires use of the product of blood flow
and arterial minus venous concentration difference in whole blood, while for FFAs the calculation of limb balance is the product
of plasma flow and the plasma FFA concentration difference between the artery and vein. (A high-quality color representation
of this figure is available in the online issue.)