Overview
Most chemistry in the body happens in solution, meaning substances dissolved in water. Understanding how solutions are described and measured lets you make sense of lab values, intravenous (IV) fluids, and the arithmetic behind medication amounts. This lesson builds from the basics of dissolving up to the calculation methods used in clinical settings, framed here as educational practice rather than as instructions for treating anyone.
Solute, Solvent, and Solubility
A solution has two parts: the solute (the substance being dissolved, present in the smaller amount) and the solvent (the dissolving medium, present in the larger amount). In the body, the solvent is almost always water, which is why water is called the universal biological solvent.
Solubility is the maximum amount of solute that will dissolve in a given amount of solvent at a specific temperature. Several factors influence it:
- Temperature — most solids dissolve better in warm water; gases dissolve better in cold.
- Polarity — “like dissolves like.” Polar and ionic solutes dissolve in polar solvents (water); nonpolar solutes dissolve in nonpolar solvents.
- Pressure — mainly affects dissolved gases, such as oxygen and carbon dioxide in blood.
Ways to Express Concentration
Concentration tells you how much solute is present. Medicine uses several units, and being fluent in converting among them prevents errors.
| Unit | Meaning | Example |
|---|---|---|
| Percent (% w/v) | Grams of solute per 100 mL of solution | 0.9% saline = 0.9 g per 100 mL |
| mg/mL | Milligrams of solute per milliliter | A drug labeled 10 mg/mL |
| Molarity (M) | Moles of solute per liter of solution | 1 M glucose = 180 g per liter |
| mEq/L | Milliequivalents per liter (for ions) | Serum sodium ~140 mEq/L |
Note that a percent weight/volume figure is simply grams per 100 mL. This makes it quick to scale: 0.9% saline is 0.9 g per 100 mL, which is 9 g per liter.
Dilution: C1V1 = C2V2
Concentrated stock solutions are often diluted to a working strength. The relationship is:
C₁V₁ = C₂V₂
where C₁ and V₁ are the concentration and volume of the starting (stock) solution, and C₂ and V₂ are those of the final diluted solution. Because the amount of solute does not change when you add solvent, the products stay equal.
Worked example. How much 10% stock is needed to make 500 mL of 2% solution?
- Solve for V₁: V₁ = (C₂ × V₂) / C₁
- V₁ = (2% × 500 mL) / 10% = 100 mL
So you would measure 100 mL of stock and add solvent up to a total of 500 mL.
Dosage-Style Math (Educational Practice)
A common method for medication arithmetic is the “desired over have” approach:
Volume to give = (Desired dose ÷ Concentration on hand) × Volume of that concentration
Worked example. A practice problem asks: a solution is labeled 250 mg per 5 mL, and the ordered dose is 100 mg. How many milliliters contain 100 mg?
- Concentration on hand = 250 mg per 5 mL = 50 mg/mL
- Volume = 100 mg ÷ 50 mg/mL = 2 mL
Working through the units (dimensional analysis) is the safest habit: milligrams cancel, leaving milliliters.
Educational note: These calculations are provided to teach the underlying chemistry and arithmetic method only. They are not clinical orders or dosing guidance. Real medication administration is governed by professional training, verified references, and institutional protocols.
Tonicity and IV Fluids
Tonicity compares a solution’s effective solute concentration with the inside of a cell and predicts which way water will move by osmosis.
| Solution | Compared to cell | Effect on a red blood cell |
|---|---|---|
| Isotonic | Equal solute | No net water movement; cell stable |
| Hypotonic | Less solute | Water enters; cell swells, may burst |
| Hypertonic | More solute | Water leaves; cell shrinks |
0.9% sodium chloride, called normal saline, is isotonic with blood plasma, so it expands circulating volume without shifting water into or out of cells. This is why it is such a common IV fluid.
Clinical relevance
Concentration and tonicity are everyday concerns in patient care. Giving fluid that is accidentally hypotonic can drive water into cells and cause them to swell, while overly hypertonic fluid pulls water out and shrinks them; both can be harmful, which is why isotonic fluids like normal saline are the default for simple volume replacement. Lab reports express electrolytes such as sodium and potassium in concentration units (mEq/L), and interpreting whether a value is high or low depends on understanding what those units mean. Mastering the arithmetic of solutions, dilution, and unit conversion builds the numerical fluency that clinicians rely on to read labs, prepare fluids, and reason carefully about the substances moving through the body.