## 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.

### Volume

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

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

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