The heart and blood vessels: Students should be able to use simple compound measures such as rate and carry out rate calculations for blood flow.

The idea

Rate is a compound measure that compares a quantity with time. For blood flow, rate can mean volume of blood passing a point each second, calculated as volume divided by time, or speed of blood flow, calculated as distance divided by time. Use consistent units: cm³/s, dm³/min or m/s. For example, if 300 cm³ of blood flows in 20 s, the rate is 300 ÷ 20 = 15 cm³/s. If blood travels 0.5 m along a vessel in 2.5 s, its speed is 0.5 ÷ 2.5 = 0.2 m/s. Rearranging the formula allows you to find volume, distance or time when the rate is known. Always show the equation, substitute values with units, then state the unit in the answer.

What earns the marks

• Rate is a compound measure linking a quantity such as volume or distance to time.
• Blood flow rate as volume per unit time is calculated using rate = volume ÷ time.
• Blood flow speed is calculated using speed = distance ÷ time.
• Correct units must be used and carried through the calculation, for example cm³/s or m/s.
• Rearranging the equation allows calculation of volume, distance or time from a known rate.
• Answers should show substitution and final unit for full credit.

Common mistakes and how to put them right

• Dividing time by volume instead of volume by time.

  Put it right: Rate = volume ÷ time, so 300 cm³ in 20 s is 15 cm³/s, not 0.067 s/cm³.

• Mixing units, such as cm³ with minutes.

  Put it right: Convert all values to the same time unit before dividing.

• Omitting the unit or writing the wrong unit.

  Put it right: State the compound unit, for example cm³/s or m/s, in the final answer.

Exam tips

• Write the equation first, then substitute numbers with units, then calculate and add the unit.
• Check whether the question asks for volume flow rate or speed, because the formula differs.
• Use unit conversion carefully: 1 dm³ = 1000 cm³ and 1 minute = 60 s.