Biomedical Engineering Reference
In-Depth Information
contains electrolytes including sodium, potassium, calcium, magnesium, chloride,
bicarbonate, phosphate, and sulfate. Typically, 1 liter of sweat contains 0.02g cal-
cium, 0.05g magnesium, 1.15g sodium, 0.23g potassium, and 1.48g chloride, but
the electrolyte composition of sweat is variable from individual to individual. Ex-
cessive loss of water could lead to dehydration and exhaustion. The loss of essential
electrolytes, which can result in cramping, could lead to reduced nutrient transfer
and water in and out of the cell (known as cellular respiration) because cells require
a precise balance for maximum cellular efficiency. Hence, replacing the lost water
is essential for healthy living.
2.2.2 Concentration
Concentration is the term used to relate the mass of a component in a mixture to the
volume of the mixture. In a flow system, concentration relates the mass flow rate
of the component to the volumetric flow rate. If the concentration is defined by the
mass per unit volume of the mixture (kg/m
3
), then it is called mass concentration.
Concentration can also be defined in moles per unit volume where one mole is the
molecular weight of that component (kgmol/m
3
). This molar concentration defini-
tion is more useful in evaluating systems where reaction is taking place. For charged
molecules, concentration is also defined in equivalents per liter to represent the al-
kalinity or acidity of those components. An equivalent is the amount of a substance
that will combine with or replace one mole of H
ions in a chemical reaction. Thus,
Eq/L can be converted in terms of g/L or mol/L using the valency of the component.
+
EXAMPLE 2.1
A patient has a serum magnesium concentration of 2.7 mEq/L. What is the concentration
in mg/dL?
Solution: From the definition of mEq, mEq
=
mmol
×
valence
mmol
=
2.7 (mEq/L)/2
=
1.35 (mmol/L)
=
1.35 × 24.3 mg/mmol
=
32.8 mg/L or 3.28 mg/dL
The cell membrane is a semipermeable membrane (i.e., it allows some sub-
stances to pass through but not all). Hence, the intracellular fluid has a distinct
ionic composition, different from extracellular fluid [Figure 2.2(b)]. The intracellu-
lar fluid has similar composition in all cell types and contains high levels of K
+
ions
and low levels of Na
+
ions. On the contrary, extracellular fluid contains high levels
of Na
+
ions and low levels of K
+
ions. These differences are important for various
biological functions, which will be described later in this chapter and in Chapter
3. The composition of interstitial fluid is similar to that of blood plasma, except
it contains a smaller concentration of proteins than plasma and thus a somewhat
greater concentration of water. Lymph has almost the same composition as that of
original interstitial fluid.
Differences in concentrations across a barrier are one of the driving forces for
the transport of molecules in biological systems. The second law of thermodynamics
