Mass and Weight, Volume, Flow and Flux

Mass and Weight

All substances have mass which is generally expressed as weight. Weight may be expressed as grams (e.g., milligram, kilogram) or by non-metric weight such as pounds, ounces, etc. Molarity is a truer expression of mass since molarity depends upon the actual number of atoms or molecules in a given amount of a substance. One mole of any substance contains Avogadro’s number of atoms or molecules (6.023 x 1023). A mole of substance has the same mass wherever it is measured. A gram of substance will have different mass depending upon the gravitational force acting upon it, hence the same mass weighs less on the moon than on earth.


In physiology, substances of interest are usually dissolved in water, the vital solvent for most physiochemical reactions. The volume of solvent in which a substance (solute) is dissolved is reported in liter (L) or some part of a liter (milliliter = mL; deciliter = dL). Non-metric measures of volume (pints, gallons) are no longer used in medical measurements.


Flow is the term usually used to refer to the movement of a solvent or solution per unit of time. The time units are those that make sense (e.g., L/hour, mL/min). Obvious examples include blood flow rate or urine flow rate. Less obvious is solute clearance, which is also expressed as a flow rate.


Flux is similar to flow, but when applied to dialysis, it usually refers to the movement of a solute a particular distance per unit of time (e.g., cm/sec). Flux measurement is important when describing the way a solute moves through different media (e.g., urea through water versus urea through a dialyzer membrane).


P/N 101819-01 Rev B 02/2021