The Metric System in Science and Medicine

Introduction to the Metric System

The metric system is the most widely used system of measurement in science and medicine, including Anatomy & Physiology. It provides a standardized way to measure length, mass, volume, and temperature, ensuring consistency and accuracy in scientific communication and experimentation.

• Unit of Length: meter (m)

• Unit of Mass: gram (g)

• Unit of Volume: liter (l)

• Unit of Temperature: degrees Celsius (°C)

The metric system is based on multiples of ten, making conversions straightforward and logical.

Base Units and Their Relationships

Base Units of Measurement

• Mass: The gram (g) is the base unit. 1 gram ≈ 1/28 ounce.

• Length: The meter (m) is the base unit. 1 meter ≈ 1.094 yards.

• Volume: The liter (l) is the base unit. 1 liter ≈ 1.057 quarts.

• Temperature: Measured in degrees Celsius (°C).

Volume can also be expressed as cubic centimeters (cm3 or cc), where 1 liter = 1000 cm3.

Metric Prefixes and Powers of Ten

Understanding Metric Prefixes

Metric prefixes are used to indicate multiples or fractions of base units, always in powers of ten. This system simplifies calculations and conversions in scientific contexts.

Example: 1 kilometer (km) = 1000 meters (m); 1 milligram (mg) = 0.001 grams (g).

Rules of Metric Notation

Standard Practices

• The unit or abbreviation always follows the amount (e.g., 5 g, 10 mg).

• Decimals are used for fractional units (e.g., 1.5 ml).

• Use a leading zero for values less than one (e.g., 0.5 mg, 0.12 ml).

Converting Between Metric Units

Conversion Principles

• Larger to Smaller Unit: Multiply by the order-of-magnitude difference.

• Smaller to Larger Unit: Divide by the order-of-magnitude difference.

Example 1: 0.5 mg = 0.5 × 1000 μg = 500 μg

Example 2: 130 ml = 130 ÷ 1000 l = 0.13 l

Always check if your answer is logical: more parts of a smaller unit are needed to equal a larger unit, and vice versa.

Common Metric and U.S. Customary Unit Conversions

Length

• 1 kilometer ≈ 0.62 miles

• 1 meter ≈ 1.09 yards ≈ 39.4 inches

• 1 centimeter ≈ 0.4 inches

• 1 inch = 2.54 cm

Volume

• 1 liter ≈ 1.06 quarts

• 1 milliliter (ml) = 1 cubic centimeter (cc)

• 1 teaspoon ≈ 5 ml

• 1 fluid ounce ≈ 30 ml

• 1 gallon ≈ 3.8 l

Mass

• 1 kilogram ≈ 2.2 pounds

• 1 pound ≈ 454 g

• 1 ounce ≈ 28 g

Example: Convert 66 lbs to kg: 66 ÷ 2.2 = 30 kg

Density of Water

Reference Values

• At 4°C, pure water has a density of about 1 g/cm3, 1 g/ml, or 1 kg/liter.

• 1 ml of water weighs approximately 1 g.

Celsius: The Metric Temperature Scale

Definitions and Conversions

• 0°C = 32°F (water freezes)

• 100°C = 212°F (water boils)

• 1°C ≈ 1.8°F

Conversion Formulas:

Example: Convert 35°C to °F:

Thinking in Celsius

• -10°C = frigid (14°F)

• 0°C = cold (32°F)

• 10°C = cool (50°F)

• 20°C = comfortably warm (68°F)

• 30°C = hot (86°F)

• 40°C = very hot (104°F)