The Solutions

A solution is a mixture of solute(s) in a solvent. A solvent is generally thought of as the liquid in which something else is dissolved (eg. H₂O is the major solvent in the body). Solutes, then, are substances dissolved to the molecular or ionic level in a solvent. Put a solute in a solvent... and you have a solution.

What is the solvent in plasma; what are the solutes in plasma? (Review with diagram)

8Per Cent Solution: Concentrations of solutions are sometimes given as "percentages." This refers to the mass of solute, in grams, dissolved in a solvent to a volume of 100 ml. "Normal saline" is an aqueous solution of 0.9% NaCl. This means that normal saline can be prepared by measuring out 0.9 g of NaCl and diluting this amount of NaCl to a final volume of 100 ml's in water. This would be the same as diluting 9 g of NaCl to a final volume of 1 liter in water. Normal saline is also known as "physiological saline." Normal saline (0.9% NaCl) is the basis (starting fluid) for intravenous fluid therapy. Normal saline can be directly administered to the bloodstream as it has the same osmolarity as plasma.

8Molarity of a solution: We often use the terms "normal" or "physiological" saline to describe a 0.9% saline (NaCl) solution. The concentration of a solution can also be expressed in terms of the number of molecules in solution. We use a term known as the "mole" (abbreviated mol) to do this. A mole is the "gram atomic mass" (of an atom) or "gram formula mass" of a compound and, incidentally, is the amount of substance containing 6.022 X 10²³ atoms or molecules (Avagadro's number - to see a less serious Avagadro counting particles - Avagadro: 1776-1856).

If we dissolve 1 mole of a substance to a final volume of 1 liter (L) in water, we have produced a 1 molar (1 M) solution. To produce a 1 molar solution of NaCl we would add the amount of NaCl equivalent to its gram formula weight (1 mole of NaCl) to 1 liter of solution. The gram formula weight of sodium chloride (NaCl) is about 58 g. We determined this by looking at a Table of the Elements and adding the atomic masses of Na and Cl together (23 g + 35 g = 58 g). Diluting 58 g of NaCl to 1 L in water gives us a 1 M NaCl solution.

How much NaCl is in physiological saline? Physiological saline is 0.9% NaCl. This means that 100 ml's of physiological saline contains 0.9 g of NaCl. One liter of physiological saline must contain 9 g of NaCl. We can determine the molarity of a physiological saline solution by dividing 9 g by 58 g... since we have 9 g of NaCl in a liter of physiological saline, but we have 58 grams of NaCl in a mole of NaCl. When we divide 9 g by 58 g, we find that physiological saline contains 0.154 moles of NaCl per liter. That means that physiological saline (0.9% NaCl) has a molarity of 0.154 molar. We might express this as 0.154 M... or 154 millimolar (154 mM).

A 0.9% NaCl and a 0.154 M NaCl solution are the same thing. They are physiologic.

But where do I find all of the gram atomic masses for all of the atoms? (Table of Elements)

solid liquid T H E P E R I O D I C T A B L E gas synth

Discoverer

1
H AtomicWeight Melt|Boil(C) 2
He

3
Li 4
Be Shell Isotopes 5
B 6
C 7
N 8
O 9
F 10
Ne

11
Na 12
Mg Orbital SpecificGravity 13
Al 14
Si 15
P 16
S 17
Cl 18
Ar

19
K

20
Ca

21
Sc

22
Ti

23
V

24
Cr

25
Mn

26
Fe

27
Co

28
Ni

29
Cu

30
Zn

31
Ga

32
Ge

33
As

34
Se

35
Br

36
Kr

lanthanons 58
Ce 59
Pr 60
Nd 61
Pm 62
Sm 53
Eu 64
Gd 65
Tb 66
Dy 67
Ho 68
Er 69
Tm 70
Yb 71
Lu

actinons 90
Th 91
Pa 92
U 93
Np 94
Pu 95
Am 96
Cm 97
Bk 98
Cf 99
Es 100
Fm 101
Md 102
No 103
Lr

8Osmolarity of a solution: We can also express the concentrations of solutions in terms of how strongly they will attract water across a membrane. Diffusion of water across a membrane is referred to as "osmosis." The force with which solutes attract water across a membrane is referred to as "osmotic pressure."

If we had 1 mole of some protein molecule in 1 L of water, we would have a 1 molar (1 M) protein solution. As the solution would attract water across a membrane, we could also refer to it as a 1 osmolar solution (or 1 osmole/liter).

NaCl is referred to as an electrolyte. Electrolytes are substances which ionize to give charged particles in solution. As NaCl strongly ionizes (nearly completely) in water, NaCl is referred to as a strong electrolyte. When you dilute 1 mole of NaCl to a final volume of 1 liter in water, you are making a 1 molar (1 M) NaCl solution. However, the NaCl will completely ionize to Na⁺ and Cl^- in water. You now have 1 mole of Na⁺ and 1 mole of Cl^- in solution. The osmolarity generated by all of the individual particles in solution must be added to determine the total osmolarity of the solution. Since diluting 1 mole of NaCl to a final volume of 1 liter in water gives you 1 mole of Na⁺ and 1 mole of Cl^-, you end up with a 2 osmolar (2 osM) NaCl solution. Thus, the concentration of the same NaCl solution can be described as 1 molar or 2 osmolar.

A 1 M solution of NaCl (a strong electrolyte) attracts water across a member just as strongly as a 2 M solution of a substance which does not ionize. Thus, both solutions are described as 2 osmolar (2 osM). A 0.154 M solution of NaCl exerts about 0.308 osmoles/liter or 308 milliosmoles/liter of osmotic pressure. This means that a physiological or normal saline solution could be described as: 0.9% NaCl (zero point nine per cent sodium chloride), 0.154 M NaCl (zero point one five four molar sodium chloride), or as 0.308 osM NaCl... which is approximately 300 milliosmolar NaCl (300 mosM... or three hundred milliosmolar sodium chloride).

Equivalents and solutions: Finally, concentrations of electrolytes in solution are sometimes given in "equivalents." Generally, a millimole (mM) and a milliequivalent (mEq) are the same thing. One mole of NaCl in solution would ionize completely to give 1 mole of Na⁺ and 1 mole of Cl^- (2 osmoles of solute). One mole of NaCl added to water could be referred to as 2 osmoles or 2 equivalents. Thus, 0.154 M NaCl refers to a solution produced by diluting 0.154 moles of NaCl to 1 L in water. The solution could also be referred to as approximately 300 millosmoles/L, 300 milliosmolar (300 mosM) or 300 milliequivalents/L.

150 millimoles of CaCl₂ diluted to 1 L in water would ionize to give 150 milliEq Ca²⁺ and 300 mEq Cl^-, giving 450 milliEq of solute in aqueous solution. This solution would exert an osmotic pressure of 450 milliosmoles/liter (450 mosm/L) or 450 milliosmolar (450 mosM).

Hypotonic - Hypertonic - Isotonic: Solutions of the same concentration are said to be isotonic. A hypertonic solution has a greater concentration than the solution to which it is being compared. A hypotonic solution has a lower concentration than the solution to which it is being compared.

Describe the following as isotonic, hypotonic or hypertonic:
seawater versus plasma
plasma versus ISF
ISF versus ICF

What happens to a red blood cell (RBC) in hypotonic solution? Why? What happens if the cell swells too much? What is this called?

Why are RBC's often used to test the osmolarity of cell or tissue culture media?

Solutions which exert the same osmotic pressure are said to be isosmotic. Solutions of the same total concentration are usually isotonic and isosmotic. That is, they are the same concentration and they exert the same osmolarity.

Exercise: What would you have to do to a 0.9% NaCl solution before infusing it into someone?

We need to check the osmolarity using either a standardized osmometer or by putting some RBC's into a little bit of our solution to see whether the RBC's crenate or lyse. Remember, we want our fluids to be approximately 300 milliosmolar, especially if we are going to infuse more than a few milliliters into the bloodstream.

Can we infuse solutions which are not isotonic with plasma? Yes... but we need to infuse them slowly so that there is sufficient time for such fluids to be diluted in the bloodstream. If we infuse hypertonic or hypotonic solutions too quickly, especially in large volumes, we can damage a lot of circulating cells and dramatically alter plasma volume and blood pressure.

We also need to sterilize our solution before infusion. We can either heat, as in an autoclave, or filter our solution through filter paper with pores smaller than 0.22 microns. Filtration through pores this small will exclude any bacteria. Our fluid containers need to be sealed in a sterile environment.

seawater versus plasma (hypertonic - about 1000 milliosmolar versus about 300 milliosmoles/L)
plasma versus ISF (isotonic - about 300 milliosmoles/L)
ISF versus ICF (isotonic - about 300 milliosmoles/L)
plasma versus ICF (isotonic - about 300 milliosmoles/L)
cytoplasm versus distilled water (cytoplasm is hypertonic to distilled water as distilled water does not contain solutes)

What happens to a red blood cell (RBC) in hypotonic solution? A red blood cell will swell as it takes on water and may burst.

Why? The red blood cell swells as water moves into the cell from the surrounding hypotonic solution. The water is moving into the more concentrated cytoplasm.

What happens if the cell swells too much? If the cell swells too much, it will burst from the hydrostatic pressure inside the cell.

What happens to an RBC in hypertonic solution? The cell will shrivel as water leaves the cell. The cell will become dysfunctional and will likely die.

Why? The water molecules are leaving the cell and moving into an area of higher solute concentration in the surrounding fluid.

Why are RBC's often used to test the osmolarity of cell or tissue culture media?

When RBC's are placed in a solution which is not isotonic with the ICF, the cells will either crenate or rupture. The cells can be observed under a microscope and will begin to swell even in mildly hypotonic solution. Sometimes labs culturing human cells (eg. embryos) will use RBC's as a final check on the osmolarity of culture media, even if the osmolarity of the media has already been checked with an expensive osmometer. This is just an extra precaution that is probably a good idea, as long as the RBC's are handled safely.